- 
 
F THE 
 
 /ERSH7 
 OF 
 
 . 
 

J 
 
 * 
 
ELEMENTS 
 
 OP 
 
 PHYSICS; 
 
 OR, 
 
 NATURAL PHILOSOPHY, 
 
 GENERAL AND MEDICAL: 
 
 WRITTEN FOE UNIVERSAL USE, 
 
 IN 
 
 PLAIN OR NON-TECHNICAL LANGUAGE; 
 
 AND. CONTAINING 
 
 NEf DISQUISITIONS AND PRACTICAL SUGGESTIONS. 
 
 COMPRISED IN FIVE PARTS: 
 
 1. SOMATOLOGY, STATICS AND 
 
 DYNAMICS. 
 
 2. MECHANICS. 
 
 3. PNEUMATICS, HYDRAULICS 
 
 AND ACOUSTICS. 
 
 4. HEAT AND LIGHT. 
 
 5. ANIMAL AND MEDICAL PHYSICS. 
 
 BY NEILL AKNOTT, M. D. , 
 
 OF THE ROTALJ30LLEGE OF PHYSICIANS. 
 
 L Xjpl 
 
 A NEW EDITION, REVISED AND CORRECTED FROM THE LAST ENGLISH EDITION. 
 
 WITH ADDITIONS. 
 BY ISAAC HAYS, M. D. 
 
 COMPLETE IN ONE VOLUME. 
 
 i 
 
 PHILADELPHIA: 
 
 BLANCHARD AND LEA. 
 
 1856. 
 
Entered according to Act of Congress, in the year 1841, by 
 LEA AND BLANCHARD, 
 
 in 'the Clerk's Office of the District Court of the United States, in and for 
 the Eastern District of Pennsylvania. 
 
 GIFT 
 
Al 
 
 ADVERTISEMENT 
 
 AMERICAN PUBLISHERS 
 
 THE very valuable and popular work of Dr. Arnott, has passed 
 through several editions in this country, in the form in which it 
 was originally published by the author, in separate parts. A new 
 edition being now called for, the work has been carefully revised 
 and corrected, and the whole condensed into one volume. In tkis 
 form it cannot fail to be more acceptable to the public, and rendered 
 more convenient and useful for the purposes of instruction in the 
 various Colleges and Seminaries of Learning that have adopted it 
 as a Class Book for their pupils. This volume embraces all that 
 has been prepared or published by the author. 
 
VI INTRODUCTION. 
 
 knowledge. Language thus, at the present moment of the world's existence, 
 may be said to bind the whole human race of uncounted millions, into one 
 gigantic rational being, whose memory reaches to the beginnings of written 
 records, and retains imperishably the important events that have occurred ; 
 whose judgment analyzing the treasures of memory, has discovered many 
 of the sublime and unchanging laws of nature, and has buift on them all 
 the arts of life, and through them piercing far into futurity, sees clearly 
 many of the events that are to come, and whose eyes and ears, and obser- 
 vant mind at this moment, in every corner of the earth, are watching and 
 recording new phenomena, for the purpose of still better comprehending the 
 magnificence and beautiful order of creation, and of more worthily adoring 
 its beneficent author. 
 
 It might be very interesting to show here, in minute detail, how the arts 
 and civilization have progressed in accordance with the gradual increase of 
 man's knowledge of the universe; but to do so would lead too far from the 
 main subject. We deem it right, however, to make evident to the student 
 the arousing truths, that the progress is not yet at an end; that it has been 
 vastly more rapid in recent times than ever; and that it seems still to pro- 
 ceed with increasing celerity : and we know not where the Creator has fixed 
 the limits of the change ! Although there are thousands of years on the 
 records of the world, our BACON, who first taught the true way to investi- 
 gate nature, lived but the other day. NEWTON followed him, and illustrated 
 his precepts by the most sublime discoveries which one man has ever made. 
 HARVEY detected the circulation of the blood only two hundred years ago. 
 ADAM SMITH, DR. BLACK and JAMES WATT were friends, and the last, 
 whose steam-engines are now changing rapidly the condition of empires, may 
 be said to be scarcely cold in his grave. JOHN HUNTER died not long ago ; 
 HERSCHEI/S accounts of newly-discovered planets, and of the sublime struc- 
 ture of the heavens, and DAVY'S account of chemical discoveries not less 
 important to man, are in the late numbers of our scientific journals; illus- 
 trious Britons these, and who have left worthy successors treading in their 
 steps. On the continent of Europe during the same period, a corresponding 
 constellation of genius has shone ; and LAPLACE was lately the bright star 
 shining between the future and the past. 
 
 But there is a change going on in the world, connected closely with the 
 progress of science, yet distinct from it, and more important than a great part 
 of the scientific discoveries; it is the diffusion of existing knowledge among 
 the mass of mankind. Formerly, knowledge was shut up in convents and 
 universities, and in books written in the dead languages or in books which, 
 if in the living languages, were so abtruse and artificial, that only a few per- 
 sons had access to their meaning ; and thus the human race being considered 
 as one great intellectual creature, a smalljfraction only of its intellect was 
 allowed to come into contact with science, and therefore into activity. The 
 progress of science in those times was correspondingly slow, and the evils of 
 general ignorance prevailed. Now, however, the strong barriers which con- 
 fined the stores of wisdom have been thrown down, and a flood is overspread- 
 ing the earth; old establishments are adapting themselves to the spirit of 
 the age ; new establishments are arising ; the inferior schools are introducing 
 improved systems of instruction ; and good books are rendering every man's 
 fireside a school. From all these causes there is growing up an enlightened 
 public opinion, which quickens and directs the progress of every art and 
 science, and through the medium of a free press, although overlooked by 
 many, is now rapidly becoming the governing influence in all the affairs of 
 
INTRODUCTION. VH 
 
 
 
 man. In Great Britain, partly perhaps as a consequence of its insular situa- 
 tion, which lessened among its inhabitants the dread of hostile invasion, and 
 sooner formed them into a united and compact people, the progress of enlight- 
 ened public opinion had been more decided than in any other state. The 
 early consequences were more free political institutions ; and these gradually 
 led to greater and greater improvements, until Britain became an object of 
 admiration among the nation*. A colony of her children, imbued with her 
 spirit, now occupies a magnificent territory in the new world of Columbus ; 
 and although it has been independent as yet for only half a century, it already 
 counts more people than Spain, and will soon be second to no nation on 
 earth. The example of the Anglo-Americans has aided in rendering their 
 western hemisphere the cradle of many other gigantic states, all free, and 
 following, although at a distance, the like steps. In the still more recently 
 discovered continent of Australasia, which is nearly as large as Europe, and 
 is empty of men, colonization is spreading with a rapidity never before wit- 
 nessed ; and that beautiful and rich portion of the earth will soon be covered 
 with the descendants of free-bore and enlightened Englishmen. Thence, 
 still onward, they or their institutions will naturally spread over the vast 
 archipelago of the Pacific Ocean, a tract studded with islands of paradise. 
 Such, then, is the extraordinary moment of revolution, or transit, in which 
 the world at present exists ! And where, we may ask again, has the Creator 
 predestined that the progress shall cease ? Thus far at least we know, that 
 he has made our hearts rejoice to see the world filling with happy human 
 beings, and to observe that the increase of the sciences can make the same 
 spot maintain thousands in comfort and godlike elevation of mind, where 
 with ignorance even hundreds had found but a scanty and degrading supply. 
 
 The progress of knowledge which has thus led from former barbarism to 
 present civilization, has gone on by certain remarkable steps, which it is easy 
 to point out ; and which it is very useful to consider, because we thereby 
 discover the nature of human knowledge, with the relations and importance 
 of its different branches; and we obtain great facilities for studying. science, 
 and for quickening its farther progress. 
 
 The human mind, when originally directed to the almost infinity of objects 
 in the universe around it, must soon have discovered that there were resem- 
 blances among them ; in other words, that the infinity was only a repetition 
 of a certain number of kinds. Among animals, for instance, it would distin- 
 guish the sheep, the dog, the horse ; among vegetables, the oak, the beech, 
 the pine ; among minerals, lime, flint, the metals, and so forth. And becom- 
 ing aware that by studying an examplar of each kind, its limited power of 
 memory might acquire a tolerably correct knowledge of the whole, while 
 this knowledge would enable the possessors more easily to obtain what was 
 useful to them, and to avoid what was hurtful, the desire for such knowledge 
 must have arisen with the first exercise of reason. Accordingly, the pursuit 
 of it has been unremitting, and the labour of ages has at last nearly com- 
 pleted an arrangement of the constituent materials of the universe, under 
 three great classes of MINERALS, VEGETABLES and ANIMALS; commonly 
 called the three kingdoms of Nature, and .of which the minute* description 
 is termed NATURAL HISTORY : and museums of natural history have been 
 formed, which contain a specimen of almost every object included in these 
 classes, so that now, a student within the limits of an ordinary garden, may 
 be said to be able to examine the whole of the material universe. 
 
 While men are examining the forms and other qualities of the bodies 
 around them, they could not avoid noticing also the motions or changes going 
 
Vlll INTRODUCTION. 
 
 
 
 on among bodies ; and here, too, they would soon make the grand discovery 
 that there were resemblances in the multitude. Self-interest, as in the case 
 of the bodies themselves, having prompted to careful classification, in the 
 present day, as the result of countless observations and experiments made 
 through the series of ages, we are enabled to say, that all the motions, or 
 changes, or phenomena (words synonymous here) of the universe, are merely 
 a repetition and mixture of a few simple manners, or kinds of motion or 
 change, which are as constant and regular in every case as where they pro- 
 duce the returns of day and night, and of the seasons. All these phe- 
 nomena are referable to four distinct classes, which we call Physical, Chemi- 
 cal, Vital and Mental. The simple expressions which describe them are 
 denominated General Truths or Laws, of Nature, and as a body of know- 
 ledge, they constitute what is called SCIENCE OF PHILOSOPHY, in contra- 
 distinction of NATURAL HISTORY, already described. Now as man cannot, 
 independently of a supernatural revelation, learn anything but what respects, 
 1st, the momentary state, past or present, of himself and the objects around 
 him; and 2d, the manner in which the states are changed; Natural His- 
 tory and Science, in the sense now explained, make up the whole sum of his 
 knowledge of nature. 
 
 To exemplify the process by which a general truth or law of nature is 
 discovered, we shall take the physical law of gravity or attraction. 1st. It 
 was observed that bodies, in general, if raised from the earth, and left un- 
 supported, fell towards it; while flame, smoke, vapors, &c., if left free, 
 ascended away from the earth. It was held, therefore, to be a very general 
 law, that things had weight; but that there were exceptions in such mat- 
 ters, as now mentioned, which were in their nature light or ascending. 2d. 
 It was discovered that our globe of earth is surrounded by an ocean of air, 
 having nearly fifty miles of altitude or depth, and of which a cubic foot 
 taken near the surface of the earth, weighs about an ounce. It was then 
 perceived that flame, smoke, vapor, &c., rise in the air only as oil rises in 
 water, viz., because not so heavy as the fluid by which they are surrounded ; 
 it followed, therefore, that nothing was known on earth naturally light, in 
 the ancient sense of the 'word. 3d. It was found that bodies floating in 
 water, near to each other, approached and feebly cohered ; that any contigu- 
 ous hanging bodies were drawn towards each other, so as not to hang quite 
 perpendicularly ; and that a plummet suspended near a hill was drawn to- 
 wards the hill with force only so much less than that with which it was 
 drawn towards the earth, viz , the weight of the plummet, as the hill was 
 smaller than the earth. It was then proved that weight itself is only an 
 instance of a more general mutual attraction, operating between all the con- 
 stituent elements of this globe ; and which explains, moreover, the fact of 
 the rotundity of the globe, all the parts being drawn towards a common cen- 
 tre, as* also the form of dew-drops, rain-drops, globules of mercury, and 
 of many other things ; which, still further, is the reason why the distinct 
 particles of which any solid mass, as a stone or piece of metal, is composed, 
 cling together as a mass, but which, when overcome by the repulsion of heat, 
 allows the game particles to assume the form of a liquid or air. 4th. It 
 was further observed, that all the heavenly bodies are round, and must, 
 therefore, consist of material obeying the same law. 5th. And lastly, that 
 these bodies, however distant, attract each other ; for that the tides of our 
 ocean rise in obedience to the attraction of the moon, and become high or 
 spring-tides, when the moon and sun operate in the same direction. Thus 
 the sublime truth was at last made evident, and by the genius of the immor- 
 
INTRODUCTION. IX 
 
 tal Newton, that there is a power of attraction connecting together the bodies 
 of this solar system at least, and probably limited only by the bounds of the 
 universe. 
 
 Acquaintance with the laws of nature has been very slowly obtained, owing 
 to that complexity of ordinary phenomena, which is produced by several 
 laws operating together, and under great variety of circumstance. With 
 respect to many laws of Chemistry and Life, men seem to be yet little far- 
 ther advanced than they were with respect to the physical law of attraction, 
 when they knew only that heavy things fell to the earth. But we have 
 learned enough to perceive that the great universe is as simple and harmo- 
 nious as it is immense ; and that the Creator, instead of interposing sepa- 
 rately, or miraculously, in the common sense of the word, to produce every 
 distinct phenomenon, has willed that all should proceed according to a few 
 general laws. There is nothing in nature so truly miraculous and adorable 
 as that the endless and beneficent variety of results which we see, should 
 spring from such simple elements. In times of ignoranee, men naturally 
 regarded every occurrence which they did not understand, that is to say, 
 which they could not refer to a general law, as arising from a direct inter- 
 ference of supreme power ; and thus, for many ages, among some nations 
 still, eclipses and earthquakes, and many diseases, particularly those of the 
 mind, and the winds and weather, were or are accounted miraculous. Hence 
 arose, among heathens, many ceremonies, and sometimes even barbarous 
 sacrifices, for propitiating or appeasing their offended deities ; but founded 
 on expectations no more reasonable than if we should now pray to have the 
 day of the year made shorter, or to have a coming eclipse averted. They 
 had not yet risen to the sublime conception of the one God, who said, " Let 
 there be light," and the light was; and who gave the whole of nature per- 
 manent laws, which he allows men to discover for the direction of their con- 
 duct in life laws so unchanging, that by them we can calculate eclipses 
 backward or forward for thousands of years, almost without erring, by the 
 time of one beat of a pendulum ; and as our knowledge of nature advances, 
 we can anticipate and explain other events with equal precision. Even the 
 wind and the rain, which, in common speech, are the types of uncertainty 
 and change, obey laws as fixed as those of the sun and moon ; and already, 
 as regards many parts of the earth, man can foretell them without fear of 
 being deceived. He plans his voyages to suit the coming monsoons, and he 
 prepares against the floods of the rainy seasons. 
 
 The general laws of Nature, divisible, as stated above, into the four classes 
 of, 1st, Physics, often called Natural Philosophy ; 2d, of Chemistry ; 3d, 
 of Life, commonly called Physiology ; and 4th, of Mind, may be said to 
 form the pyramid of Science, of which Physics is the base, while the others 
 constitute succeeding layers in the order now mentioned ; the whole having 
 certain mutual relations and dependencies well figured by the parts of a 
 pyramid. We must describe them more particularly, to show these relations. 
 
 Physics. The laws of Physics govern every phenomenon of nature in 
 which there is any sensible change of place, being concerned alone in the 
 greater part of these phenomena, and regulating the remainder which origi- 
 nate from chemical action, and from the action of life. The great physical 
 truths, as comprehended in the present day by man, are reduced to four, and 
 are referred to by the words atom, attraction, repulsion and inertia. It 
 gives an astonishing, but true idea of the nature and importance of methodi- 
 cal Science, to be told that a man, who understands these words, viz., how 
 the ATOMS of matter by mutual ATTRACTION approach and cling together 
 
X INTRODUCTION. 
 
 to form masses, which are solid, liquid, or aeriform, according to the quan- 
 tity or REPULSION of heat among them, and which, owing to their INERTIA 
 or stubbornness, gain and lose motion, in exact proportion to the force of 
 attraction or repulsion acting on then, understands the greater part of the 
 phenomena of nature ; but such is the fact ! Solid bodies existing in con- 
 formity with these truths, exhibit all the phenomena of Mechanics ; Liquids 
 exhibit those of Hydrostatics and Hydraulics ; Airs, those of Pneumatics ; 
 and so forth, as seen in the table of heads given below, at page xii. And 
 the whole of this work is merely a list of the most interesting physical 
 phenomena, arranged in classes under these heads. 
 
 Chemistry. Had there been only one kind of substance or matter in the 
 universe, the laws of Physics would have explained all the phenomena; but 
 there are iron, and sulphur, and charcoal, and about fifty others, which to 
 the present state of science, appear essentially distinct. Now these, when 
 taken singly, obey the laws of Physics ; but when two or more of them are 
 placed in contact, under certain circumstances, they exhibit a new order of 
 phenomena. Iron and sulphur, for instance, brought together and heated, 
 disappear as individuals, and unite into a yellow metallic mass, which, in 
 most of its properties, is unlike to either : under other new circumstances, 
 the two substances will again separate, and assume their original forms. Such 
 changes are called chemical, (from an Arabic word signifying to burn, be- 
 cause so many of them are effected by means of heat,) but during the 
 changes, the substances are not withdrawn from the influence of the physi- 
 cal laws, their weight or inertia, for instance, is not altered ; and indeed 
 the phenomenon is merely a modification of general attraction and repul- 
 sion. Many chemical changes, besides, are only the beginnings of purely 
 mechanical changes, as when the new chemical arrangement produced by 
 heat among the intimate atoms of gunpowder, causes the mechanical or 
 physical motion of the sudden expansion or explosion. And all the mani- 
 pulations of Chemistry, as the transferring of gases from vessel to vessel, 
 the weighing of bodies, pounding, grinding, &c., are directed to Physics 
 alone. Chemistry, then, is truly, as figured above, a superstructure on 
 Physics, and cannot be understood or practised by a person who is ignorant 
 of Physics. The chief departments of study involving the consideration of 
 Chemical in conjunction with Physical laws, are enumerated in the table 
 below, under the head of CHEMISTRY. 
 
 Life. The most complicated state in which matter exists, is where, under 
 the influence of life, it forms bodies with a curious internal structure of tubes 
 and cavities, in which fluids are moving, and producing incessant internal 
 change. These are called Organized Bodies, because of the various distinct 
 parts or organs which they contain ; and they form two remarkable classes, 
 the individuals of one of which are fixed to the soil, and are called Vegeta- 
 bles ; and of the other, are endowed with power of locomotion, and are 
 called Animals. The phenomena of growth, decay, death, sensation, self- 
 motion, and many others, belong to life, but from occurring in material 
 structures which subsist in obedience to the laws of Physics and Chemistry, 
 the life is truly a superstructure on the other two, and cannot be studied 
 independently of them. Indeed the greater part of the phenomena of 
 organic life are merely chemical and physical phenomena, modified by an 
 additional principle. The science of Life is divided into animal and vege* 
 table Physiology (see the table below). 
 
 Mind. The most important part of all science is the knowledge which 
 man has obtained of the laws governing the operations of his own MIND. 
 This department stands eminently distinct from the others, on several 
 
INTRODUCTION. XI 
 
 accounts. Unlike that of organic life, which could not be understood until 
 physics and chemistry had been, previously investigated, this had made 
 extraordinary advances in a very early age, when the others, as methodical 
 sciences, had scarcely begun to exist. In proof of this assertion we need 
 only refer to the writings of the Greek' philosophers. The most brilliant 
 discoveries and applications, however, were reserved for the moderns, as will 
 occur to many readers, on perusing, in the table below, the several divisions 
 of the subject, and recollecting the honoured names which are now associa- 
 ted with each. It is truly admirable to see the modern analysis, deducing 
 from a few simple laws of mind, act the subordinate departments, just as it 
 deduces mechanics, hydrostatics, pneumatics, &c., from the laws of physics : 
 and let us hope that sound opinions on this subject, ensuring human happi- 
 ness, and therefore, beyond comparison, more important than any other 
 knowledge, will soon be widely spread. The crowning science of Mind, 
 although in certain respects independent of the science of Matter, is still 
 closely allied to them in the following ways. The faculties of the mind are 
 originally awakened or called into activity solely by the impressions of mat- 
 ter or external nature ; all the language used in speaking of mind and its 
 operations, is borrowed from matter ; and many mental emotions are entirely 
 dependent on bodily conditions. The science of Mind, therefore, cannot be 
 studied until after knowledge acquired of an external nature; and cannot 
 be studied extensively until that knowledge be extensive. 
 
 Quantity. To express most of the facts an J laws of Physics, Chemistry 
 and Life, terms of QUANTITY are required, as when we speak ( f the magni- 
 tude of a body, or say, that the force of attraction between two bodies 
 diminishes, in a certain proportion, as their distance increases. Hence 
 arises the necessity of having a set of fixed measures or standards, with which 
 to compare all other quantities. Such measures have been adopted; and 
 they are, for NUMBERS, the fingers, Jives and tens; for LENGTH, the human 
 foot, cubit, pace, &c. ; and lately the second' s pendulum and the French metre, 
 (taken from the magnitude of our globe); for SURFACES, the simplest forms 
 of circle, square, triangle, &c., compared among themselves by the lengths 
 of their diameters or other suitable lines; and for SOLID BULK, the corres- 
 ponding simple solids, of globe., cube, pyramid, cone, &c. t similarly compared 
 by the lengths of diameters or of other lines of dimension. The rules for 
 applying these standards to ail possible cases, and for comparing all kinds 
 of quantities with each other, constitute a body of science, called the Science 
 of Quantity, the Mathematics. It may be considered as a subsidiary depart- 
 ment of human science, created by the mind itself, to facilitate the study of 
 the others. 
 
 Supposing description of particulars, or Natural History, to be studied 
 along with the different parts of the System of Science sketched in the table, 
 there will be included in the scheme the whole knowledge of the universe 
 which man can acquire by the exercise of his own powers : that is to say, 
 what he can acquire independently of a supernatural Revelation. And 
 on this knowledge all his arts are founded, some of them on the single 
 part of Physics, as that of the machinist, architect, mariner, carpenter, &c. ; 
 some on Chemistry, (which includes Physics,) as that of the miner, glass- 
 maker, dyer, brewer, &c. ; and some on Physiology, (which includes much of 
 Physics and Chemistry,) as that of the scientific gardener or botanist, agri- 
 culturist, zoologist, &c. The business of teachers of all kinds, and of governors, 
 advocates, linguists, &c., &c., respects chiefly the science of Mind. The art 
 of medicine requires in its professor a comprehensive knowledge of all the 
 departments. 
 
xu 
 
 INTRODUCTION. 
 
 TABLE OF SCIENCE AND ART. 
 1. PHYSICS. 2. CHEMISTRY. 
 
 Mechanics, 
 Hydrostatics, 
 Hydraulics, 
 Pneumatics, 
 Acoustics, 
 Heat, 
 Optics, 
 Electricity, 
 Astronomy, 
 &c. 
 
 Simple substances, 
 Mineralogy, 
 Geology, 
 Pharmacy, 
 Brewing, 
 Dyeing, 
 Tanning, 
 &c. 
 
 3. LIFE. 
 
 Vegetable Physiology. 
 Botany, 
 Horticulture, 
 Agriculture, 
 &c. 
 
 Animal Physiology, 
 Zoology, 
 Anatomy, 
 Pathology, 
 Medicine 
 &c. 
 
 4. MIND. 
 
 Intellect, 
 Logic, 
 
 Mathematics, 
 &c. 
 
 Motives to action, 
 
 Emotions and Passions, 
 
 Morals, 
 
 Government, 
 
 Political Economy, 
 
 Theology, 
 
 Education. 
 
 In the first stages of education, viz., during the years of childhood and 
 youth, the learning acquired is necessarily of the most mixed kind, and 
 much of it is determined by what is called accident; but from the mutual 
 dependence of the different departments of science, as explained in the pre- 
 ceding paragraphs, it follows that with a view to complete erudition', the 
 order exhibited in "The Table/' is that in which they should afterwards be 
 studied, so as to prevent repetitions and anticipations, and to diminish, as 
 much as possible, the labor of acquirement. 
 
 Every man may be said to begin his education, or acquisition of know- 
 ledge, on the day of his birth. Certain objects, repeatedly presented to the 
 infant, are, after a time, recognized and distinguished. The number of 
 objects thus known gradually increases, and from the constitution of the 
 mind, they are soon associated in the recollection, according to their resem- 
 blances, or obvious relations. Thus, sweetmeats, toys, articles of dress, &c., 
 soon form distinct classes in the memory and conceptions. At a later age, 
 but still very early, the child distinguishes readily between a mineral mass, 
 a vegetable, and an animal; and thus his mind has already noted the three 
 great classes of natural bodies, and has acquired a certain degree of acquaint- 
 ance with Natural History. He also soon understands the phrases a a falling 
 body," "the force of a moving body," and has therefore a perception of the 
 
INTRODUCTION. Xlll 
 
 great physical laws of gravity and inertia. Then having seen sugar dissolved 
 in water, and wax melted round the wick of a burning candle he has learned 
 some phenomena of Chemistry. And having obs'erved the conduct of the 
 domestic animals, and of the persons about him, he has begun his acquaint- 
 ance with Physiology, and the science of mind. Lastly, when he has learned 
 to count his fingers and his sugar plums, and to judge of the fairness of the 
 division of a cake between himself and brothers, he has advanced into Arith- 
 metic and Geometry. Thus within a year or two, a child of common sense 
 has made a degree of progress in all the great departments of human science ;' 
 and in addition has learned to name objects, and to express feelings, by the 
 arbitrary sounds of language. Such, then, are the beginnings or founda- 
 tions of knowledge, on* which 'future years of experience, or methodical 
 education, must rear the superstructure of the more considerable attain- 
 ments which befit the various conditions of men in a civilized com- 
 munity. 
 
 In the course of the preceding disquisition, we have seen that Physics, or 
 Natural Philosophy, the subject of the present volume, is fundamental to 
 the other parts, and is therefore that of which a knowledge is indispensable. 
 Bacon truly calls it " the root of the sciences and arts." That its import- 
 ance has not been marked by the place which it has held in common systems 
 of education, is owing chiefly, 1st, to the misconception that a knowledge 
 of technical mathematics was a necessary preliminary ; and, 2d, to an 
 opinion, also erroneous, that the degree of acquaintance with Physics which 
 all persons acquire by common experience, is sufficient for common pur- 
 poses ; now it is true, that the toys of childhood, as the windmill, ball, 
 syphon, tube, and a hundred others, furnish so many exemplifications of 
 the laws of Physics, and may well be called a philosophical apparatus ; but 
 they give information which is exceedingly vague, and not at all such as is 
 absolutely requisite in the practice of many of the arts. If, then, the study 
 of Physics be so easy as now appears, and so important as we shall try still 
 farther to show, there can be no excuse for neglecting it. 
 =- The greatest sum of knowledge acquired with the least trouble is, perhaps, 
 that which comes with the study of the few simple truths of Physics. To 
 the man who understands these, very many phenomena, which, to the unin- 
 formed, appear prodigies, are only beautiful illustrations of his fundamental 
 knowledge, and this he carries about with him, not as an oppressive weight, 
 but as a charm supporting the weight of other knowledge, and enabling him 
 to add to his valuable store every new fact of importance which may offer 
 itself. With such a principle of arrangement, his information, instead of 
 resembling loose stones or rubbish thrown together in confusion, becomes 
 as a noble edifice, of correct proportions and firm contexture, and is acquiring 
 greater strength and consistency with the experience of every day. It has 
 been a common prejudice, that persons thus instructed in general laws, had 
 their attention too much divided, and could know nothing perfectly. But 
 the very reverse is true; for general knowledge renders all particular 
 knowledge more clear and precise. The ignorant man may be said to have 
 charged his hundred books of knowledge, to use a rude simile, with single 
 objects, while the informed man makes each support a long chain, to which 
 thousands of kindred and useful things are attached. The laws of Philosophy 
 may be compared to keys which give admission to the most delightful gar- 
 dens that fancy can picture ; or to a magic power, which unveils the face 
 of the universe, and discloses endless charms of which ignorance never 
 dreams. The informed man, in the world, may be said to be always sur- 
 
xiv INTRODUCTION. 
 
 rounded by what is known and friendly to him, while the ignorant man is 
 as one in a land of strangers and enemies. A man reading a thousand 
 volumes of ordinary botfks as agreeable pastime, will receive only vague 
 impressions; but he who studies the methodized Book of Nature, converts 
 the great universe into a simple and sublime history, which tells of God, 
 and may worthily occupy his attention to the end of his days. 
 
 We have said already, that theJaws of Physics govern the great natural 
 phenomena of Astronomy, the tides, winds, currents, &c. We will now 
 mention some of the artificial purposes to which man's ingenuity has made 
 the same laws subservient. 
 
 Nearly all that the civil engineer accomplishes, ranges under the head of 
 Physics. Let us take, for instance, the admirable specimens scattered over 
 the British Isles : the numerous canals for inland traffic ; the dock to 
 receive the riches of the world, pouring towards us frbm every quarter; the 
 many harbours offering safe retreat to the storm-driven mariner ; the mag- 
 nificent bridges which every where facilitate intercourse ; hills bored through 
 to open ways for commerce by canals, common roads and rail-roads, the 
 canals in some places being supported, like the roads, on arches across valleys 
 or above rivers, so that here and there the singular phenomenon is seen of one 
 vessel sailing directly over another ; vast tracts of swamps or fen-land drained 
 and now serving for agriculture ; the noble light-house, rearing its head 
 amidst the storm, while the dweller within trims his lamp in safety, and 
 guides his endangered fellow-creature through the perils of the night, &c., &c. 
 
 In Holland, great part of the country has been won and is now preserved 
 from the sea, by the same almost creating power, and now rich cities and 
 an extended garden smile, where, as related by Caesar, were formerly only 
 bogs and a dreary waste. 
 
 As a general picture, it is interesting to consider, that in many situations 
 on earth where formerly the rude savage beheld the cataract falling among 
 the rocks, and the wind bending the trees of the forest, and sweeping the 
 clouds along the mountain's brow, or whitening the face of the ocean, and 
 regarding these phenomena with awe and terror, as marking the agency of 
 some great but hidden power, which might destroy him; in the same situa- 
 tions now, his informed son, who works with the laws of nature, can lead 
 the waters of the cataract, by sloping channels, to convenient spots, where 
 they are made to turn his mill-wheel, and to do his multifarious work ; the 
 rushing winds, also, he makes his servant, by rearing in their course the 
 broad-vaned wind-mill, which then performs a thousand offices for its master, 
 man; and the breezes which whiten the ocean are caught in his expanded 
 sails, and are made to waft their lord and his treasures across the deep, for 
 his pleasure or his profit. 
 
 In Architecture, also, Physics in supreme, and has directed the construc- 
 tion of the temples, pyramids, domes and palaces which adorn the earth. 
 
 In respect to machinery, generally, Physics is the guiding light. There 
 are, for instance, the mighty steam-engine; machines for spinning and. 
 weaving, and for moulding other bodies into various shapes, yea, even iron 
 itself, as if it were plastic clay; wind-mills and water-mills, and wheel 
 carriages; the plough and implements of husbandry ; artillery and the fur- 
 niture of war; the balloon, in which man rides triumphantly above the 
 clouds, and the diving-bell, in which he penetrates the secret caverns of the 
 deep ; the implements of the intellectual arts, of printing, drawing, painting, 
 sculpture, &c. ; musical instruments, optical and mathematical instruments, 
 and a thousand others. 
 
INTRODUCTION. XV 
 
 But Physics is also an important foundation of the healing art. The medi- 
 cal man, indeed, is the engineer pre-eminently ; for it is in the animal body 
 that true perfection and the greatest variety of mechanism are found. Where, 
 to illustrate Mechanics, is to be found a system of levers and hinges, and 
 moving parts, like the limbs of an animal body ; where such an hydraulic 
 apparatus, as in the heart and blood-vessels ; such a pneumatic apparatus, 
 as in the breathing chest ; such acoustic instruments, as in the ear and 
 larynx ; such an optical instrument, as in the eye ; in a word, such variety 
 and perfection, as in the whole of the visible anatomy ? All these struc- 
 tures, then, the medical man should understand, as the watchmaker knows 
 the parts of a time-piece about which he is employed. The watchmaker, 
 unless he can discover where a pin is loose, or a wheel injured, or a particle 
 of dust adhering, or oil wanting, &c., would ill succeed in repairing an 
 injury; and so, also, of the ignorant medical man in respect to the human 
 body. Yet will it be believed, that there are many medical men who 
 neither understand mechanics, nor hydraulics, nor pneumatics, nor optics, 
 nor acoustics, beyond the merest routine ; and that systems of medical 
 education are set forth at this day which do not even mention the depart- 
 ment of Physics ! That such is the case, furnishes an illustration of what is 
 stated in the beginning of this essay, viz., that the sciences and arts are 
 progressive, and that perfect methods of education must arise gradually, 
 like all other things of human contrivance. It is within the recollection of 
 persons now living, that political economy was discovered to be a grand 
 foundation of the art of government, indicating means of security against 
 many national misfortunes common in former times, yea, even against famine 
 and war. And the day is not distant, when the members of the medical 
 pro^ssion generally will understand how much the correct knowledge of 
 animal structure and function, and of many remedies, must depend on pre- 
 cise acquaintance with Physics. Besides the more strictly professional 
 matters contained in the medical sections of the present work, there are 
 many others scattered through it which greatly interest the medical man ; 
 such are the subjects of meteorology, climate, ventilation and warming of 
 dwellings, specific gravities, &c., &c. 
 
 The laws of Physics having an influence so extensive as appears from 
 these paragraphs, it need not excite surprise that all classes of society are at 
 last discovering the deep interest they have to understand them. The lawyer 
 finds that in many of the causes tried in his courts, an appeal must be made 
 to Physics, as in cases of disputed inventions; accidents in navigation, or 
 among carriages, steam-engines, and machines generally ; questions arising 
 out of the agency of winds, rains, water-currents, &c. : the statesman is con- 
 stantly listening to discussions respecting bridges, roads, canals, docks, and 
 the mechanical industry of the nation : the clergyman finds ranged among 
 the beauties of nature, the most intelligible and striking proof of God's wis- 
 dom and goodness; the sailor in his ship has to deal with one of the most 
 admirable machines in existence : soldiers, in using their projectiles, in 
 marching where rivers are to be crossed, woods to be cut down, roads to be 
 made, towns to be besieged, &c., are dependent chiefly on their knowledge 
 of Physics : the land-owner, in making improvements on his estates, building, 
 draining, irrigating, road-making, &c. : the farmer equally in these particu- 
 lars, and in all the machinery of agriculture; the manufacturer, of course; 
 the merchant who selects and distributes over the world the products of 
 manufacturing industry all these are interested in Physics ; then also the 
 man of letters, that he may not, in drawing his illustrations from the material 
 
XVI INTRODUCTION. 
 
 world, repeat the scientific heresies and absurdities which have heretofore 
 prevailed, and which, by shocking the now better-informed public, exceed- 
 ingly lower the estimation in which such specimens of the Belles Lettres 
 are held, and lessen their general utility; and lastly, parents of either sex, 
 whose conversation and example have such powerful effect on the character 
 of their children, who when grown up, are to fill all the stations in society; 
 all should study Physics, as one important part of their education. 
 
 And it is for such reasons that Natural Philosophy is becoming daily more 
 and more a part of common education. In our cities now, and even in an 
 ordinary dwelling-house, men are surrounded by prodigies of mechanic art, 
 and cannot submit to use these, regardless of how they are produced, as a 
 horse is regardless of how the corn falls into his manger. A general diffu- 
 sion of knowledge, owing greatly to the increased commercial intercourse of 
 nations, and therefore to the improvements in the physical departments of 
 astronomy, navigation, &c., is changing every where the condition of man, 
 and elevating the human character in all ranks of society. In remote times 
 the inhabitants of the earth were generally divided into small states or socie- 
 ties, which had few relations of amity among themselves, and whose thoughts 
 and interests were confined very much within their own little territories and 
 rude habits. In succeeding ages, men found themselves belonging to larger 
 communities, as where the English heptarchy was united ; but still distant 
 kingdoms and quarters of the world were of no interest to them, and were 
 often totally unknown. Now, however, every one feels that he is a member 
 of one vast civilized society, which covers the face of the earth ; and no part 
 of the earth is indifferent to him. In England, for instance, a man of small 
 fortune may cast his looks around him, and say with truth and exultation, 
 " I am lodged in a house which affords me conveniences and comforts w^ich 
 some centuries ago, even a king could not command. Ships are crossing 
 the seas in every direction, to bring me what is useful to me from all parts 
 of the earth. In China, men are gathering the tea-leaf for me ; in America, 
 they are planting cotton for me ; in the West Indies, they are preparing 
 my sugar and my coffee; in Italy, they are feeding silk-worms for me; in 
 Saxony, they are shearing the sheep to make me clothing; at home, power- 
 ful steam-engines are spinning and weaving for me, and making cutlery for 
 me, and pumping the mines that minerals useful to me may be procured. 
 Although my patrimony was small, I have post-coaches running day and 
 night on all the roads to carry my correspondence ; I have roads and canals, 
 and bridges, to bear the coal for my winter fire ; nay, I have protecting fleets 
 and armies around my happy country, to secure my enjoyments and repose. 
 Then I have editors and printers, who daily send me an account of what is 
 going on throughout the world, among all these people who serve me. And 
 in a corner of my house I have BOOKS ! the miracle of all my possessions, 
 more wonderful than the wishing-cap of the Arabian Tales : for they trans- 
 port me instantly, not only to all places, but to all times. By my books I 
 can conjure up before me, into vivid existence, all the great and good men 
 of antiquity ; and for my individual satisfaction I can make them act over 
 again the most renowned of their exploits, the orators declaim for me ; the 
 historians recite ; the poets sing ; and from the equator to the pole, or from 
 the beginning of time until now, by my books, I can be where I please." 
 This picture is not overcharged, and might be much extended, such being 
 God's goodness and providence, that each individual of the civilized millions 
 dwelling on the earth, may have nearly the same enjoyment as if he were 
 the single lord of all. 
 
INTRODUCTION. Xvij 
 
 Reverting to the importance of Natural Philosophy as a general study, it 
 may be remarked that there is no occupation which so much strengthens and 
 quickens the judgment. This pra ; se has usually been bestowed on the Ma- 
 thematics, although a knowledge of abstract Mathematics existed with all the 
 absurdities of the dark ages ; but a familiarity with Natural Philosophy 
 which comprehends Mathematics, and gives tangible and pleasing illustra- 
 tions of the abstract truths, seems incompatible with the admission of any 
 gross absurdity. A man whose mental faculties have been sharpened by 
 acquaintance with these exact sciences in their combination, and who has 
 been engaged, therefore, in contemplating real relations, is more likely to 
 discover truth in other questions, and can better defend himself against 
 sophistry of every kind. We cannot have clearer evidence of this than in 
 the history of the sciences, since the Baconian method of reasoning by indi- 
 cation took place of the visionary hypotheses of preceding times. Until then, 
 even powerful minds did not recoil from the most absurd theories on all sub- 
 jects. Astronomy was mixed with Astrology ; Chemistry with Alchemy ; 
 Physiology with the singular hypotheses which preceded the discovery of the 
 circulation of the blood ; Politics with the errors of monopolies, prohibitions, 
 balance of trade, &c. Even Religion itself, in various ages and countries, 
 has felt the influence of the state of the public mind as to solid attainments. 
 To a man with the knowledge of nature which we now possess, the fables 
 .and licentious abominations of the Greek and Roman theologies are shocking 
 indeed ; as are the religions of the God of Fire in China, of Vishnoo in 
 India, of Mahomet's imposture and pretended miracles, &c. But the enlight- 
 ened Christian minister earnestly recommends the study of nature ; first 
 because from contemplating the beauty of creation, with the wisdom and 
 benevolent design manifest in all its parts, there spring up in every unde- 
 praved mind those feelings of admiration and gratitude, which constitute the 
 adoration of natural religion, and which form, as shown by many estimable 
 writers on Natural Theology, a fit foundation for the sublime doctrine of 
 immortality, and secondly, because a Revelation being probable only by the 
 miracles occurring at its establishment; to enable men to distinguish between 
 miracles and the usual course of nature, a perfect knowledge of that course, 
 or of Natural Philosophy, is essential : all the false religions of antiquity 
 were founded on, and upheld by pretended miracles. As regards the ques- 
 tion of immortality, even independently of Revelation, no man who con- 
 templates the order and beauty of the material world, and then thinks on the 
 hideous deformities of the moral world where vice so often triumphs, and 
 modest virtue pines and dies can for a moment believe that they are the 
 work of the same author, unless there be a hereafter of retribution ; and 
 feeling thus that eternal justice requires another state for man, he embraces 
 with delight the cheering promises of immortality. There have been, how- 
 ever, at various times, even among Christians, sincere, but weak-minded or 
 ill-informed men, who decried the study of the natural sciences, as inimical 
 to true religion ; as if God's ever-visible and magnificent revelation of his 
 attributes in the structure of the universe could be at variance with any 
 other revelation. But such prejudices are now quickly passing away. 
 Wherever considerable knowledge of nature exists, debasing and gloomy 
 superstition must cease. It is not the abject terror of a slave which is in- 
 spired by contemplating the majesty and power of our God, displayed in his 
 works, but a sentiment akin to the tender regard which leads a favored child 
 to approach with confidence a wise and indulgent parent. 
 
 It remains for the author now only to say a few words with respect to the 
 
 2 
 
XV111 INTRODUCTION. 
 
 present work. He was originally led to the undertaking with the view of 
 supplying the desideratum in medical literature, of a treatise on Medical 
 Physics; but soon perceiving that the preliminary investigation of General 
 Physics, necessary to adapt the work to medical readers, would require to 
 be nearly as extensive as it would for general readers, and reflecting that 
 every person of liberal education must now possess such a book, not to be 
 read once and then thrown aside as a novel is, but to be frequently consulted 
 as a manual, he determined to make his book as complete and as extensively 
 useful as possible. He has been encouraged during his labor, by the belief 
 that the growing light of science, which now exhibits more clearly the na- 
 tural relations of the different departments of study, as attempted to be por- 
 trayed in the preceding pages, might enable him to avoid some of the defects 
 of former elementary treatises, and to add features of novelty and improve- 
 ment to his own. The sections on Animal Physics were, of course, written 
 for medical men ; and a great service will be rendered by the work, if it 
 only awakens them to a just sense of the importance of Physics as one of 
 the foundations of their art. But even for general readers there are few 
 parts of these sections which the author would exclude. There is nothing 
 more admirable in nature than the structure and functions of the human 
 body, and there are many reasons why no liberal mind should be careless of 
 the study. The details here given are not more anatomical than the illus- 
 trations from the animal economy contained in the common treatises on 
 Natural Theology. From the attempt in this work to compress into the 
 smallest possible ppace the greatest possible sum of scientific information, 
 few historical details have been admitted, whether relating to the distin- 
 guished men who have benefitted the world as authors or inventors, or to 
 the history of the progress of science : such details form an interesting, but 
 distinct branch of study. 
 
 The author must not conclude without observing, that no treatise on Na- 
 tural Philosophy can save, to a person desiring full information on the sub- 
 ject, the necessity of attendance on experimental lectures or demonstrations. 
 Things that are seen, and felt, and heard, that is, which operate on the ex- 
 ternal senses, leave on the memory much stronger and more correct impres- 
 sions than where the conceptions are produced merely by verbal description, 
 however vivid. And no man has ever been remarkable for his knowledge 
 of Physics, Chemistry, or Physiology, who has not had practical familiarity 
 with the objects. With reference to this familiarity, persons who take a 
 philanthropic interest in the affairs of the world, must observe, with much 
 pleasure the now daily increasing facilities of acquiring useful knowledge, 
 afforded by the scientific institutions formed and forming, not only through 
 this kingdom, but through most civilized nations. 
 
 Bedford Square, 1st March, 1827. 
 
ELEMENTS 
 
 NATURAL PHILOSOPHY. 
 
 SYNOPSIS, OR GENERAL REVIEW. 
 
 IF it excite our admiration that a varied edifice, or even a magnificent 
 city can be constructed of stone from one quarry, what must our feeling be 
 to learn how few and simple the elements are out of which the sublime fab- 
 ric of the universe, with all its orders of phenomena, has arisen, and is now 
 sustained, These elements are general . facts and laws which human 
 sagacity is able to detect, and then to apply to endless purposes of human 
 advantage. 
 
 Now the four words, atom, attraction, repulsion, inertia, point to four 
 general truths, which explain the greater part of the phenomena of nature. 
 Being so general, they are called physical truths, from the Greek word 
 signifying nature as also " truths of Natural Philosophy," with the same 
 meaning, and sometimes " mechanical truths/' from their close relation to 
 ordinary machinery. These appellations distinguish them from the remain- 
 ing general truths, namely the chemical truths, which regard particular 
 substances, and the vital and mental truths, which have relation only to 
 living beings. And even in the cases where a chemical or vital influence 
 operates, it modifies, but does not destroy, the physical influence. By 
 fixing the attention, then, on these four fundamental truths., the student 
 obtains, as it were, so many keys to unlock, and lights to illuminate the 
 secrets and treasures of nature. 
 
 1st. ATOM. Every material mass in nature is divisible into very 
 minute indestructible and unchangeable particles, as when a piece of any 
 metal is bruised, broken, cut, dissolved, or otherwise transformed, a 
 thousand times, but can always be exhibited again as perfect as at first. 
 This truth is conveniently recalled by giving to the particles the name atom, 
 which is a Greek term, signifying that which cannot be further cut or divided, 
 or an exceeding minute resisting particle. 
 
 2d. ATTRACTION. It is found that the atoms above referred to, whether 
 separate or already joined into masses, tend towards all other atoms or 
 masses, as when the atoms of which any mass is composed are, by an in- 
 visible influence, held together with a certain degree of force ; or when a 
 block of stone is similarly held down to the earth on which it lies; or when 
 the tides on the earth rise towards the moon. These facts are conveniently 
 
20 SYNOPSIS. 
 
 / 
 
 recalled by connecting with them the word Attraction (a drawing together) 
 or gravitation. 
 
 3d. REPULSION. Atoms under certain circumstances, as of heat diffused 
 among them, have their mutual attraction countervailed or resisted, and 
 they tend to separate ; as when ice heated melts into water, or when water 
 heated bursts into steam, or when gunpowder ignited explodes. Such 
 facts are conveniently recalled by the term Repulsion (a thrusting asunder.) 
 
 4th. INERTIA. As a fly-wheel made to revolve, at first offers resistance to 
 the force moving it, but gradually acquires speed proportioned to that force, 
 and then resists, being again stopped, in proportion to its speed, so all bodies 
 or atoms in the universe have about them, in regard to motion, what may 
 be figuratively called a stubb.orness, tending to keep them, in their existing 
 state, whatever it may be in other words, they neither acquire motion, nor 
 lose motion, nor bend their course in motion, but in exact proportion to 
 some force applied. Many of the motions now going on in the universe 
 with such regularity as that turning of the earth which produces the phe- 
 nomena of day and night are motions which began thousands of years 
 ago, and continue unvarying in this way. Such facts are conveniently 
 recalled by the term inertia applied to them. 
 
 A person comprehending fully the import of these four words, that is to 
 say, having present to his mind numerous good types or exemplars of the 
 facts referred to them, may predict or anticipate correctly, and may control 
 very many of the facts and phenomena which the extended experience of a 
 life can display to him ; and such a person is commonly said to know the 
 causes or reasons of things and events. Now it is important here to observe, 
 that when a person gives a reason or explanation of any fact, other than 
 that it is a fact, or than that the Creator has willed it, he is merely, although 
 he may not be aware of this, showing its resemblance to many other facts, 
 no one of which he understands better than itself and what he calls a 
 general truth, or law, or principle, is merely an expression for the observed 
 but unaccountable resemblance of the facts. Thus, when a man says that 
 a stone falls because of attraction or gravitation, he only uses a word which 
 recalls thousands of instances which he has witnessed of one body approach- 
 ing another; but by any cause of the approach, other than that God has 
 willed it, is to him utterly unknown. Should men, in the course of their 
 researches, discover that the phenomena now classed by them under the 
 heads of attraction and repulsion, although apparently opposite, are really 
 as closely allied as they already know the rising of a balloon and the falling 
 of a stone to be (the balloon rises like a cork in water, being pushed up by 
 the fluid air around it, heavier than it, and seeking to descend,) they will 
 not have discovered a new cause, but a new resemblance, (new to them) 
 among phenomena, and will only have advanced one step farther in 
 perceiving the simplicity of creation. In accordance with these views, it 
 will be found that this volume is chiefly an extensive display of the ^rnost 
 important phenomena of nature and art, classified so as to be explained by the 
 f our physical truths, and mutually to illustrate one another. They will be 
 listributed under the following heads or divisions : 
 
SYNOPSIS. 21 
 
 * 
 
 PAKT I. 
 
 CONSTITUTION OP MASSES, MOTIONS AND FORCES. 
 
 The four fundamental truths extensively examined, and used to explain 
 generally, in 
 Section 
 
 1. The nature or constitution of the material masses which compose the 
 universe ; (a department technically called SOMATOLOGY, from Greek 
 words signifying a discourse on body.} 
 
 2. The motions or phenomena going on among the masses; a department 
 including the common divisions of STATICS (things stationary or at 
 rest,) and DYNAMICS (what relates to force or power.} 
 
 PART II. 
 
 PHENOMENA OP SOLIDS. 
 
 The four truths explaining the peculiarities of state and motion among 
 solid bodies : a department called, in a restricted sense, MECHANICS, (from 
 the Greek, and signifying machine.) 
 
 PART III. 
 
 PHENOMENA OP FLUIDS. 
 
 The truths explaining the peculiarities of state and motion among fluid 
 bodies : a department called HYDRODYNAMICS (from Greek words signify- 
 ing water &M& force) 
 Section 
 
 1. HYDROSTATICS (water at rest or in equilibrium.) 
 
 2. PNEUMATICS (air phenomena.) 
 
 3. HYDRAULICS (water or fluid in motion.) 
 
 4. ACOUSTICS (phenomena of sound and hearing.) 
 
 PART IY. 
 
 PHENOMENA OP IMPONDERABLE SUBSTANCES. 
 
 The truths aiding to explain the more recondite phenomena of IMPON- 
 DERABLE SUBSTANCES, under the heads of 
 Section 
 
 1 . HEAT or Caloric. 
 
 2. LIGHT or Optics. 
 
 PART Y. 
 
 ANIMAL AND MEDICAL PHYSICS. 
 
 In this part will be ranged the most interesting illustrations afforded by 
 the animal economy, constituting ANIMAL AND MEDICAL PHYSICS. 
 
 As no man can well understand a subject of which he does not carry a 
 distinct outline in his mind, it is recommended to the reader of this work 
 to study the general synopsis, and the analysis placed at the heads of the 
 chapters and sections, until the memory be well impressed with them. 
 
22 . CONSTITUTION OF MASSES 
 
 PART I. 
 
 THE FOUR FUNDAMENTAL TRUTHS MINUTELY EXAMINED, AND USED TO 
 EXPLAIN GENERALLY, FIRST, THE NATURE OR CONSTITUTION OF THE 
 MATERIAL MASSES WHICH COMPOSE THE UNIVERSE, AND SECONDLY, 
 THE MOTIONS OR PHENOMENA GOING ON AMONG THEM. 
 
 SECTION L THE CONSTITUTION OF MASSES. 
 
 ANALYSIS OF THE SECTION. 
 
 The visible universe is built up of very minute indistructible ATOMS called 
 matter, which by mutual ATTRACTION, cohere or cling together in masses 
 of various form and magnitude. The atoms are more or less approxi- 
 mated, according to the quantity or REPULSION of heat among them, and 
 hence arise the three remarkable forms in the masses, of solid, liquid and 
 air, which mutually change into each other with change in the quantity 
 of heat. Certain modifications of attraction and repulsion produce the 
 subordinate peculiarities of state called crystal, dense, hard, elastic, brittle, 
 malleable, ductile and tenacious. 
 
 " Minute Indestructible ATOMS."* 
 
 THAT the smallest portion of any substance which the human eye can per- 
 ceive, is still a mass of many ultimate atoms or particles, which may be 
 separated from each other, or newly arranged, but which cannot individu- 
 ally be hurt or destroyed, is deduced from such facts as the following : 
 
 A particle of powdered marble, hardly visible to the naked eye, still ap- 
 pears to the microscope a block susceptible of indefinite division ; and, when 
 it is broken by fit instruments, until the microscope can hardly discover the 
 separate particles of fine powder, these may be yet further divided, by 
 solution in an acid; the whole becoming then absolutely invisible, as part 
 of a transparent liquid. 
 
 A small mass of gold may be hammered into thin leaf, or drawn into fine 
 wire, or cut into almost invisible parts, or liquefied in a crucible, or disolved 
 in an acid, or dissipated by intense heat into vapour ; yet, after any and all 
 these changes, the atoms can be collected again to form the original mass 
 of gold, without the slightest diminution or change. And all the substance of 
 
 * The different heads or titles, which appear thus, throughout the work, between in- 
 verted commas, are the successive portions of the Analysis, detached for separate 
 consideration. The reader is particularly requested to re-peruse the analysis at the 
 several interruptions, that he may have constantly before him that clear view of the 
 general relations among the different parts of the subject, which is essentially to a per- 
 fect understanding of it. 
 
CONSTITUTION OF MASSES. 23 
 
 elements of which our globe is composed, may thus be cut, torn, bruised, 
 ground, &c., a thousand and a thousand times, but are always recoverable as 
 perfect as at first. 
 
 And with respect to delicate combinations of these elements, such as exist 
 in animal and vegetable bodies, although it be beyond human art, originally 
 to produce, or even closely to imitate many of them for we cannot build up 
 a feather or a rose still, in their decomposition and apparent destruction, 
 the accomplished chemist of the present day does not lose a single atom. 
 The coal which burns in his apparatus, until only a little ash remains behind, 
 or the wax -taper that seems to vanish altogether in flame, or the portion of 
 animal flesh which putrefies, and gradually dries up and disappears present 
 to us phenomena which are now proved to be only changes of connection 
 and arrangement among the indestructible ultimate atoms ; and the chemist 
 can offer all the elements again, mixed or separate as desired, for any of 
 the useful purposes to which they are severally applicable. When the funeral 
 piles of the ancients, with their charge of human remains, appeared to be 
 wholly consumed, and left the idea with survivors that no base use could be 
 made, in after time, of what had been the material dwelling of a noble or 
 beloved spirit, the flames had only, as it were, scattered the enduring blocks 
 of which a former edifice had been constructed, but which were soon to serve 
 again in new combinations. 
 
 Facts to be stated under the heads of " chemical composition" and " crys- 
 tal," will prove, that the ultimate particles of any substance must be, among 
 themselves, perfectly similar. 
 
 " Minute." (Read the Analysis, page 22.) 
 
 The following are interesting particulars in the arts or in nature, helping the 
 mind to conceive how minute the ultimate atoms of matter must be. 
 
 Goldbeaters, by hammering, reduce gold to leaves so thin, that 360,000 
 must be laid upon one another to produce the thickness of an inch. They 
 are so thin, that if formed into a book, 1,800 would occupy only the space 
 of a single leaf of common paper ; and an octavo volume an inch thick would 
 have as many pages as the books of a well-stocked ordinary library contain- 
 ing 1,800 volumes of 400 pages each ; yet those leaves are perfect, or free 
 from holes, so that one of them laid upon any surface, as in gilding, gives 
 the appearance of solid gold. 
 
 Still thinner than this is the coating of gold, uponithe silver wire of what 
 is called gold lace ; and we know not that such coating is of only one atom 
 thick. If we place a piece of this wire in nitric acid, so as to dissolve the 
 silver within, the gold coating remains as a metallic tube of exquisite 
 tenuity. 
 
 Platinum can be drawn into wire much finer than human hair. 
 
 A grain of blue vitriol or carmine, will tinge a gallon of water, so that in 
 every drop the colour may be perceived. 
 
 A grain of musk will scent a room for twenty years, and will have lost 
 but little of its weight. 
 
 The carrion crow seems to smell its food at a distance of many miles. 
 
 The thread of the silk worm is so small, that many folds have to be 
 twisted together to form our finest sewing thread; but that of the spider is 
 smaller still, for two drachms of it by weight would reach from London to - 
 Edinburgh, or 400 miles. 
 
 In the milk of a cod-fish, or in water in which certain vegetables have 
 
24 CONSTITUTION OF MASSES. 
 
 been infused, the microscope discovers animalcules, of which many thou- 
 sands together do not equal in bulk a grain of sand ; yet these have their 
 blood and other subordinate parts like larger animals ; and, indeed, nature, 
 with a singular prodigality, has supplied many of them with organs as com- 
 plex as those of the whale or elephant. Now the body of an animalcule 
 consists of the same elementary substances, or ultimate atoms, as the body 
 of man himself. In a single pound of matter, it thus appears, that there 
 may be more living creatures than of human beings on the face of this globe, 
 What scenes has the microscope laid open to the admiration of the philoso- 
 phic inquirer. 
 
 Water, mercury, sulphur, or, in general, any substance, when sufficiently 
 heated, rises as invisible vapour or gas ; in other words, is made to assume 
 the aeriform state. Great heat, therefore, would cause the whole of the 
 material universe to disappear, the previously most solid bodies' becoming as 
 invisible and impalpable as the air we breathe. Utter annihilation would 
 seem but one stage beyond this. 
 
 "MaMtrS* 
 
 The inconceivable minuteness of ultimate atoms, as shown above, has led 
 some inquirers to doubt whether there really be matter ; that is to say, 
 whether what we call substance or matter have existence or not. In answer 
 to this it has been usual to adduce, besides the weights of the substances, 
 and the proofs of indestructibility already mentioned, which seems conclu- 
 sive, the fact that every kind or portion of matter obstinately occupies some 
 space to the exclusion of all other matter from that particular space. This 
 occupancy of space is the simplest and most complete idea which we have 
 of material existence. The awkward word impenetrability has been used to 
 express it, with reference of course to the individual atoms. The following 
 are elucidations : 
 
 We cannot push one billiard-ball into the substance of another, and then 
 a second, and then a third, and so on ; or the material of the universe might 
 be absorbed in a point. 
 
 A mass of iron on a support will resist the weight of thousands of pounds 
 laid upon it and pressing to descend into its place ; and although a very 
 great weight might crush or break it into pieces, still one particle would not 
 be annihilated. In a forcing pump, or in Braham's water-press, millions of 
 pounds can not push the piston down, unless the water below it be allowed 
 to escape. 
 
 A weight laid upon bladders full of air, or on the piston handle of a closed 
 air-pump, is supported in the same manner. 
 
 A quantity of air escaping from a vessel under water ascends through the 
 water as a bubble displacing its bulk of water in its way. 
 
 A glass tube, left open at bottom, while the thumb closes the top, if 
 pressed from air into water, is not filled with water, because the air contained 
 in it resists ; but if the air be allowed to escape by removing the thumb 
 from the top, the tube becomes filled immediately to the level of the water 
 around it. In a goblet or basin pushed into water, with the mouth down- 
 wards, the entrance of water is resisted for the like reason ; and if the goblet 
 be inverted over a floating lighted taper, this will continue to float under it, 
 and to burn in the contained air, however deep in the water it may be car- 
 ried exhibiting the curious phenomenon of light below water, and being 
 an emblem of the living inmate of a diving bell, which is merely a larger 
 goblet holding. a man instead of a candle. 
 
GENERAL ATTRACTION. 25 
 
 "Mutual attraction" (See the Analysis, page 22.) 
 
 Any visible mass of matter, then, as of metal, salt, sulphur, &c., we 
 know to be really a collection of dust, or -minute atoms, by some cause made 
 to cohere or cling together; yet there are no hooks connecting them, nor 
 nails, nor glue ; and the connection may be broken a thousand times, by 
 processes of nature or art, but is always ready to take place again ; the cause 
 being no more destroyed in any case by interruption, than the weight of a 
 thing is destroyed by frequent lifting from the ground. Now the cause we 
 know not, but we call it attraction. The phenomena of attraction and its 
 contrary, repulsion, particularly when occurring between bodies at consider- 
 able distances from each other, are as inexplicable as any subjects which 
 the human mind has to contemplate; but the manner or laws of the pheno- 
 mena are now well understood. The general nature and extensive influence 
 of attraction may be judged of from the following facts : 
 
 Logs of wood floating in a pond, or ships in calm water, approach each 
 other, and afterwards remain in contact. When the floating bodies are very 
 small, or can approach very near to each other at the water's edge as glass 
 bulbs in a tea-cup an additional force is called into play, as will be ex- 
 plained unjer the head of " capillary attraction." 
 
 l"he wreck of a ship, in a smooth sea after a storm, is often seen gathered 
 into heaps. 
 
 Two bullets or plummets suspended by strings near to each other, are 
 found by the delicate test of the torsion balance (which will be described 
 afterwards) to attract each other, and therefore not to hang quite perpen- 
 dicularly. 
 
 A plummet suspended near the side of a mountain inclines towards it, in 
 a degree proportioned to its magnitude ; as was ascertained by the well- 
 known trials of Dr. Maskelyne near the mountain Schehallion, in Scotland. 
 
 And the reason why the plummet in such a case tends much more strongly 
 towards the earth than towards the hill, is only that the earth is larger than 
 the hill 
 
 At New South Wales, which is situated on our globe nearly opposite to 
 England, plummets hang and fall towards the centre of the globe, as they 
 do here ; so that in respect to England, they are hanging and falling upwards, 
 and the people there, like flies on the opposite side of a pane of glass, are 
 standing with their feet towards us, hence called our antipodes. Weight, 
 therefore, is merely general attraction acting everywhere. 
 
 But it is owing to this general attraction that our earth itself is a globe : 
 all its parts being drawn towards each other, that is, towards a common 
 centre, the mass assumes the spherical or rounded form. 
 
 And the moon also is round, and all the planets ; nay, the glorious sun, 
 too, so much larger than these, is round ; suggesting the inference that all 
 must at one time have been a certain degree fluid, and that all are subject 
 to the same law. 
 
 Descending again to the earth and observing minuter masses, we have 
 many interesting instances of roundness from the same cause ; as the par- 
 ticles of a mist or fog floating in air these, mutually attracting and coalescing 
 into larger drops, and so forming rain dew-drops water trickling on a 
 duck's wing the tear dropping from the cheek drops of laudanum glob- 
 ules of mercury, like pure silver beads, coalescing when near, and forming 
 larger ones melted lead allowed to rain down from an elevated sieve, and by 
 
26 
 
 CONSTITUTION OP MASSES. 
 
 cooling as it descends so as to retain the form of its liquid drops, becoming 
 the spherical shot-lead of the sportsman, &c. 
 
 The cause of this extraordinary phenomenon which we call attraction, acts 
 at all distances. The moon, though 240,000 miles from the earth, by her 
 attraction, raises the water of our ocean under her, and forms what we call 
 the tide. The sun, still farther off, has a similar influence ; and when the 
 sun and moon act in the same direction, we have the spring tides. The 
 planets, so distant that they appear to us little wandering points in the 
 heavens, yet, by their attraction, affect the motion of our earth in her orbit, 
 quickening it when she is approaching them, retarding it when she is re- 
 ceding. 
 
 The attraction is greater the nearer the bodies are to each other as the light 
 of a taper is more intense near to the taper than at a distance. 
 
 A board of a foot square, represented in fig. 1 by A B at a certain distance 
 from a light supposed at C, just shadows a board of two feet square, as E D, 
 at double distance ; but a board with a side of two feet has four times as 
 much surface as a board with a side of one foot, for it is not only twice as 
 high or long, which would make it double, but twice as broad also, which 
 
 Fig. 1. 
 
 E * 
 
 D 
 
 makes it quadruple as a globe of two feet in diameter requires *just four 
 times as much paper to cover it as a globe of one foot, and the corner, or 
 fourth part E F, of the larger square here shown is just equal to the whole 
 of the smaller square A B. Light, therefore, at double distance from its 
 source, being spread over four times the space, has only one-fourth of the 
 intensity; and for a similar reason, at thrice the distance it has only a ninth 
 part, at four times a sixteenth part, and so on. Now light, heat, attraction, 
 sound, and indeed every influence from a central point are found to decrease 
 in the proportion here illustrated, viz., as the surface of squares which shadow 
 one another increases. The technical expression is, " the intensity is in- 
 versely as the squares of the distance ; (the distances being estimated from 
 the centres of attraction or radiation) or one-fourth part as strong at double 
 distance, four times as strong at half distance, and in a corresponding man- 
 ner for all other distances. 
 
 Accordingly, what weighs 1,000 Ibs. at the sea-shore, weighs five Iba. less 
 at the top of a mountain of a certain height, or when raised in a balloon as 
 is proved experimentally by a spring balance, or other means ; and at the 
 distance of the moon, the weight, or force towards the earth, of 1,000 Ibs., 
 is diminished to five ounces, as is proved by astronomical test. 
 
 ATTRACTION has received different names as it is found acting under differ- 
 ent circumstances. The chief distinctions are Gravitation, Cohesion, 
 Capillary and Chemical attraction. 
 
 Gravitation is the name given to it when acting at sensible distance, as in 
 the cases of the moon lifting the tides the sun and earth attracting each 
 
COHESIVE ATTRACTION. 27 
 
 other a stone falling, &c. Most of the facts enumerated at page 25, 
 belong to this head. 
 
 Cohesion is the name given when it is acting at very short distances, as in 
 keeping the atoms of a mass together., 
 
 It might appear, at first sight, that it cannot be the same cause which 
 draws a piece of iron to the earth with the moderate force called its weight, 
 and which contains the constituent atoms of the iron in such strong cohesion ; 
 but when we recollect that attraction is stronger as the substances are nearer 
 to each other, the difficulty is met. Atoms very nearly in contact may be 
 a million times nearer to each other than when only a quarter of an inch 
 apart, aud therefore, when the heat among the atoms of any mass allows 
 them to approach very near, they should attract mutually with great force. 
 
 If, then, the surfaces of the bodies were not in general so very rough and 
 irregular, that, when applied to each other, they can touch only in a few 
 points of the million, perhaps, which each surface contains, bodies would 
 be invariably sticking together or cohering by any accidental contact. The 
 effect of artificially smoothing the touching surfaces is seen in the following 
 examples : we may remark, however, that besides irregularity of surface, 
 there is another ,reason, explained a little farther on, which prevents the 
 cohesion. 
 
 Similar portions being cut off with a clean knife from two leaden bullets, 
 and the fresh surfaces being brought in contact with a slight turning pres- 
 sure, the bullets cohere, almost as if they had been originally cast inone 
 piece. 
 
 Fresh-cut surfaces of India-rubber or caoutchouc, cohere in a similar way. 
 We may hence make elastic air-tight tubes, by cutting off the edges of a 
 strip of India-rubber and bringing the cut surfaces into contact by winding 
 the strip spirally round any small rod or cylinder, and fixing it there for a 
 time by tape or cord. 
 
 Two pieces of perfectly smooth plate glass or marble, laid upon each other, 
 adhere with great force : and so, indeed, do most well-polished flat surfaces. 
 
 Cohesion between a solid and liquid, and between the particles of a liquid 
 among themselves, is seen in the following instances : 
 
 A flat piece of glass, balanced at the end of a weighing beam, and then 
 allowed to come into contact with water, adheres to the water, and with 
 much more force than the weight of water remaining upon it when again 
 forcibly raised ! If there were not cohesion or attraction of the water par- 
 ticles among themselves, as well as to the glass, the latter could only be 
 held down by the weight of the water which directly adhered to it. In 
 pouring water from a mug or bottle-lip, the water does not at once fall per- 
 pendicular, but runs down along the inclined outside of the vessel ; chiefly 
 in consequence of the attraction between this and the water; hence the dif- 
 ficulty of pouring from a vessel which has not a projecting lip. 
 
 The particles of water cohere among themselves in a degree which causes 
 small needles gently laid on the surface to float : the weight of the needles 
 is not sufficient to overcome the cohesion of the water surface. 
 
 For the same reason, many light insects can walk upon the surface of 
 water without being wetted. 
 
 It is chiefly the different force of the attraction of cohesion in different 
 
28 CONSTITUTION OF MASSES. 
 
 liquids that causes their drops or gutts from the lip of a phial to be of differ- 
 ent magnitude. Sixty drops of water fill the same measure as 10.0 drops 
 of laudanum from a lip of the same size. 
 
 In a larger mass of liquid, the attraction which, if acting alone, would 
 draw the particles into the form of a distinct globe, yields to that which 
 draws them towards the centre of the earth, and therefore the liquid assumes 
 more or less completely, what is called the level surface, that is to say, a 
 surface corresponding with the general surface of the earth. 
 
 Attraction is called capillary when it acts between a liquid and the interior 
 of a solid, which is tubular or porous. 
 
 When an open glass tube is partially immersed in water, the water within 
 it stands above the level of that on the outside ; and the difference of level 
 is greater as the tube is less, because in small tubes, the glass all round 
 being nearer to the raised water, attracts it more powerfully. 
 
 Between the two plates of glass standing near to each other, with their 
 lower edges in water, a similar rising of water will occur ; and if they are 
 closer at one perpenificular edge than at the other, the surface of the sus- 
 pended water will be higher there. The two plates of glass in such a case 
 are found to be drawn towards each other by the interposed waters with a 
 certain force, as happens also to glass beads, or other small bodies, floating 
 in water with their surfaces so near to each other at the water's edge, that 
 the water may rise between them, and the nearer they approach, the 
 higher the water rises, and the more strongly it attracts. 
 
 Water, ink, or oil, coming in contact with the edge of a book, is rapidly 
 absorbed far inwards among the leaves. 
 
 A piece of sponge or a lump of sugar touching water by its lowest corner, 
 soon becomes moistened throughout. 
 
 The wick of a lamp lifts the oil to supply the flame, from two or three 
 inches below it. 
 
 A mass of cotton thread hanging over the e.dge of a glass from the water 
 within it, will empty it as a syphon would. A towel will empty a basin 
 of water in the same way. 
 
 Dry wedges of wood driven into a groove formed round a pillar of stone, 
 on being moistened, will swell so as to rive off the portion from the block. 
 In some portions of Germany, mill-stones are thus cut from the rock. 
 
 An immense weight or mass suspended by a dry rope may be raised a 
 little way, by merely wetting the rope; the moisture imbibed by capillary 
 attraction in the substance of the rope causes it to swell laterally, and to 
 become shorter. 
 
 At one time, the small vessels of vegetables were supposed to raise the 
 sap from the roots, by capillary attraction j but this is known now to be 
 chiefly an action of vegetable life. 
 
 Attraction has received the name of chemical attraction, or affinity, when 
 it unites the atoms of two or more distinct substances into one perfect 
 compound. 
 
 There are about fifty substances in nature which appear, in the present 
 state of science, distinct from each other, and are therefore called kinds of 
 matter ; such as the various metals, sulphur, phosphorus, &c.; but whether 
 these are in truth, originally and essentially different, or only one simple 
 
CAPILLARY ATTRACTION. 29 
 
 primordial matter, modified by circumstances as yet unknown to us, we 
 cannot at present positively determine. Diamond and pure black carbon 
 are the same substance only with different arrangement of atoms; and 
 steel, which in the soft state the graver cuts as it would copper or silver, is 
 exactly the same substance as when, after being tempered by heating and 
 sudden cooling, it has become as hard nearly as diamond itself. Yet these 
 differences are more striking than appear between some substances, which 
 we now account essentially distinct. 
 
 It is found, however, that the atoms of what we call different substances 
 will not cohere and unite indifferently, to form masses, as atoms of the same 
 kind do, there being singular preferences and dislikes among them, if it 
 may be so expressed, or affinities, as the chemists term it : and when atoms 
 of two kinds do combine, the resulting compound generally loses all resem- 
 blance to either of the elements. Thus : 
 
 Sulphuric acid will unite with copper and form a beautiful transcendent 
 blue salt ; with iron it will form a green salt ; and if a piece of iron be thrown 
 into a solution of the copper salt, the acid will immediately let fall the cop- 
 per, and take up or dissolve the iron. Sulphuric acid will not unite with or 
 dissolve gold at all. Quicksilver and sulphur unite in certain proportions 
 and form the paint called vermillion ; in other proportions they form the 
 black mass called Ethiops Mineral, Lead, with oxygen absorbed from the 
 atmosphere or other source, forms what is called red lead, used by painters. 
 Sea-sand, or flint and the substance called soda, when heated together, 
 unite and form that most useful substance called glass. Certain proportions 
 of sulphur and of iron combine and produce those beautiful cubes of pyrites 
 or gold-like metal which are seen in slate. Chemical attraction operating 
 thus, does not, in the slightest degree, interfere with general attraction or 
 gravity, for every chemical compound weighs just as much as its elements 
 taken separately. 
 
 The history and classification of such facts connected with the combina- 
 tions and analysis of different substances, constitute the science of chemistry, 
 so attractive and so useful. It explains how the fifty kinds of matter above 
 alluded to, by variously combining, form the endless diversity of bodies 
 which constitute, as far as it has yet been explored, the mass of our ^lobe. 
 The reasons of these various modifications of attraction are yet much hidden 
 from us. 
 
 It is a remarkable truth, that when different substances combine in the 
 way now described, the proportions of the ingredients are always uniform, 
 and such as to lead to the conclusion, that for every atom present, of one 
 substance, there is exactly one, or two, or three, &c., of the other; so that, 
 if there be ten atoms of one substance, there are exactly ten, or twenty, &c., 
 of the other, but never an intermediate number, as 13 or 23 to 10, for then 
 a particle of the compound would consist of one atom of the first, and of 
 one and three-tenths, or two and three-tenths, &c., of the second substance, 
 an absurdity if the atom be indivisible. For instance, a certain number of 
 atoms of quicksilver, which weigh twenty-five grains, combine with a certain 
 number of atoms of sulphur, weighing two grains, and form a black com- 
 pound called Ethiops Mineral, or black sulphur of mercury ; and if a little 
 more of either ingredients be added, it lies as a foreign mixture in the sul- 
 phuret of mercury; but if just as much more sulphur be added as at first, 
 so that there may be two atoms of it, instead of one, in every particle of 
 the compound, a perfect combination of the whole will take place, and a new 
 substance will appear, which we call vermilion. Many elementary substances 
 
30 CONSTITUTION OF MASSES. 
 
 will only unite with each other in one proportion, so that any two such sub- 
 stances form only one compound ; but others unite in several proportions, 
 so that several distinct compounds arise out of the same two elements. 
 
 It thus appears, that although we do not know the exact number of atoms 
 in a given quantity of any substance, whether, for instance, a grain of 
 sulphuret of mercury has more or less than a million of them ; still, as. we 
 know that in that grain there are just as many atoms of sulphur as of mer- 
 cury, and that the weight of the whole sulphur to that of the whole mercury 
 is as two to twenty-five, we know that the single atoms must have the same 
 relation, or that the atom of mercury is 12 times as heavy as that of 
 sulphur. 
 
 Tables have been formed exhibiting the relative weights of the atoms of 
 different substances ; and the number standing opposite to each substance 
 is called its chemical equivalent, that is to say, the weight of its atom in 
 relation to the weight of the atom of some other substance chosen as a 
 standard. The equivalent of a compound substance depends, of course, 
 both on the equivalents of the ingredients, and on the number of atoms 
 existing in one integrant particle of the compound. 
 
 There is no such thing as an atom of vermilion, or of any other compound, 
 for the ultimate molecule or particle must contain at least one atom of the 
 respective ingredients. 
 
 The facts of the peculiarities and constancy of chemical unions are among 
 the strongest arguments for the existence of similar ultimate atoms. 
 
 Besides the simple cases of attraction now explained, there are two curious 
 modifications, called electrical and magnetical attractions, which from their 
 peculiarities are reserved for consideration in a future division of this work. 
 
 " Atoms are more or less close, according to the quantity or REPULSION of 
 heat among them; hence the forms of solid, fluid, air, &c." (Read the 
 Analysis, p. 22.) 
 
 Were there in the universe only atoms and attraction, as hitherto ex- 
 plained, the whole material of creation would rush into close contact, forming 
 one huge solid mass of stillness and death. But there is also heat or caloric, 
 which counteracts attraction, and singularly modifies the results. It has 
 been described by some as- a most subtle fluid, pervading all things, some- 
 what as water pervades a sponge : others have accounted it merely a vibra- 
 tion among the atoms. The truth is, that we know little more of heat as 
 a cause of repulsion than of gravity as a cause of attraction ; but we can 
 study and classify most accurately the phenomena of both. 
 
 When a continued addition of heat is made to any body, it gradually 
 increases the mutual distance of the constituent atoms, or dilates the. body. 
 A solid thus is first enlarged and softened ; then melted or fused, that is to 
 say, reduced to the state of liquid, as the cohesive attraction is overcome; 
 and lastly, the atoms are repelled to still greater distances, so that the sub- 
 stance is converted into elastic fluid or air. Abstraction of heat from such 
 air causes return of states in the reverse order. 
 
 Thus ice, when heated becomes water, and the water when farther heated 
 becomes steam ; the steam when cooled again becomes water as before, and 
 the water when cooled becomes ice. Ice, water and steam, therefore, are 
 three forms or states of the same substance one of the most common in 
 nature, being the material of the ocean. 
 
LIQUID AND AIR. 31 
 
 Other substances are similarly affected by heat, but as all have different 
 relations to it, some requiring much for liquefaction, and some very little, 
 we have that beautiful variety of solids, liquids and air, which constitutes 
 four external nature. 
 
 Dilitation. A rod of iron, which, when cold, will pass through a certain 
 opening, and will lie lengthwise between two fixed points, when heated, be- 
 comes too thick and to long to do either. For accurate mensuration, there- 
 fore, rods or chains used as the measure, must either be at a given tempera- 
 ture, or due allowance must be made for the difference. 
 
 The walls of a building, under the pressure of a heavy roof, had begun 
 to bulge out so as to threaten its stability. No force tried was sufficient to 
 restore them to perpendicularity, until the idea occurred of using the con- 
 tracting force of cooling iron. The opposite walls were then connected by 
 a number of iron bars, passing through both, and having nuts to screw close 
 to the wall upon their projecting ends, of which bars one-half were heated 
 at a time, viz., every second or alternate bar, by lamps placed under them, 
 and while lengthened in consequence, and projecting farther beyond the 
 wall, their nuts were again screwed close up ; so that on cooling and con- 
 tracting, they pulled the walls in a degree back to its place. The nuts of 
 the second set of bars being then screwed home, the others were again heated, 
 and advanced the object as much as the first; and so on, until the object 
 was accomplished. 
 
 The iron rim of a coach wheel, when heated, goes on loosely and easily, 
 but when afterwards cooled, it binds the wheel most tightly, giving remark- 
 able firmness and strength. 
 
 Iron hoops on masts and casks, are made to bind in .a similar manner. 
 
 The common thermometer for measuring degrees of heat, is a glass bulb, 
 filled with mercury or other fluid, and having a narrow tube rising from it, 
 into which the fluid, on being expanded by heat, ascends, and so marks the 
 degree. 
 
 A bladder not quite full of cold air, on being heated, becomes tense, and 
 if weak, may even be burst. 
 
 Liquid and Air. A piece of gold, lead, pitch, ice, sulphur, or of other 
 thing, if sufficiently heated, melts or becomes liquid ; each substance, how- 
 ever, requiring a different degree of heat gold requires 5,000 degrees, lead 
 600, ice, 32, and so forth ; and if the heating be afterwards continued, most 
 things at certain higher temperatures suddenly expand again to many time 
 the liquid volume, and becomes aeriform fluids. 
 
 The conversion of water into steam is familiarly known to all. One pint 
 of water driven off as steam from the boiler of a low-pressure steam-engine, 
 fills a space of nearly 2,000 pints, and raises the piston through this, with 
 a force of many thousands of pounds : it immediately afterwards appears 
 again in the cold condenser as a pint of water. 
 
 Six times as much heat is required to convert a pint of water into steam, 
 as to raise it from an ordinary temperature to that of boiling but the steam, 
 by occupying nearly 2,000 times the space -of the water, proves that heat 
 merely produces a revulsion among the particles, and by no means fills up 
 the interstices. The steam rising from boiling water does not appear to the 
 thermometer hotter than the water itself; and hence it was that Dr. Black, 
 whose genius shed so much light on this part of knowledge, gave the excess 
 of heat the name of latent heat. 
 
 The latent heat of common air is made sensible in the matcJi syringe. In 
 this, which is close at the bottom, the piston Is driven down quickly and 
 
32 CONSTITUTION OF MASSES. 
 
 strongly, so as to compress very much the air which is underneath it, and 
 the heat then condensed with the air is sufficiently intense to light a small 
 piece of tinder attached to the bottom of the piston. 
 
 Not only are spirits, aethers, oils, &c., convertible, as water is into aerifon* 
 fluid, but also sulphur, phosphorus, mercury, and, indeed, all the metals and 
 elementary substances ; some of them, however, requiring heats of great 
 intensity. 
 
 The varieties of form, then, in the bodies on the face of this earth, may 
 be considered accidental, as dependent on the temperature of the earth, and 
 do not mark the permanent nature ef the substances. 
 
 In the planet Mercury, which is near the sun, resin, tallow, wax and many 
 vegetable substances deemed by us naturally solid, would all be liquid, as oil 
 is with us ; and a certain mixture of tin, zinc and lead, which with us is solid 
 at common temperatures, but melts in boiling water, would there bo always 
 liquid like our quicksilver. Our water, oils, and spirits, would there be in a 
 state of steam or air, and could not be known as liquids, except by cooling 
 processes and compression, such as we have lately teamed to use for reducing 
 our different airs to the form of liquids. 
 
 Again, in the cold planet Herschel, which is nineteen times farther from 
 the sun than our earth is, water, if it exist, can be known only as rock crys- 
 tal, which fire would have to melt as it does glass with us : our oils would 
 be as .butter or resins, and quicksilver might be hammered as lead or silver 
 is with us. 
 
 On our own earth, near the equator, common sealing-wax will not retain 
 impressions ; butter is oil in the day, and a soft solid at night ; and tallow 
 candles cannot be used. And near our pole, in winter, the quicksilver from 
 a broken thermometer is solid metal ; water must be melted by fire for use ; 
 oils are solid, &c. 
 
 To judge, then, of the constitution of nature aright, we must always take 
 extended surveys, and not allow prejudice to mislead us, as it did that Eastern 
 potentate, who put a traveller to death for saying he had visited remote 
 northern countries, where water was sometimes to be seen solid like crystal, 
 and sometimes white and fleecy, like feathers. The ancients believed that 
 there were just four elements concerned informing our globe, with all upon 
 it, viz., earth, water y air and fire. What a contrast between former and 
 present knowledge ! 
 
 Repulsion without sensible Seat. 
 
 As we stated in a former paragraph that besides general attraction, under 
 names gravitation, cohesion, capillary and chemical attraction, there are 
 modifications which have the names of electrical and magnetical attractions ; 
 so we nave now to remark, that, besides the general repulsion of heat just 
 described, there are peculiarities which we call electrical and magnetical re- 
 pulsions. Whether these depend altogether on different causes, or are only 
 modifications of effect from the- same cause, we cannot yet positively decide. 
 
 And it is a curious fact connected with the subject, that there seems to be 
 a film of repulsion, so to express it, covering the general surfaces of all bodies, 
 and preventing their meeting in absolute contact, even when they appear to 
 the human eye so to meet. Were it not for this, things would be constantly 
 approaching so closely to each other, that they would stick or cohere, in a 
 way to disturb the common operations of nature. The following facts illus- 
 trate this superficial repulsion, and the means which art uses to overcome it 
 for particular purposes. 
 
REPULSION OP SURFACES. 33 
 
 Newton found that a ball of glass, or a watch-glass, laid upon a flat surface 
 of glass does not really touch it and cannot be made to touch it by a force 
 of even 1,000 pounds to the inch. 
 
 In like manner, when glass, stone, porcelain, or indeed almost any body 
 is broken, we cannot make the parts cohere again by simply pushing them 
 together in their former position. Where a union, therefore, between sepa- 
 rate masses is desired, we are compelled to have recourse to various artifices 
 
 A few cases in which cohesion is easily affected, were enumerated at page 
 27 : the following are other instances of a different kind. 
 
 Gold leaf laid upon clean steel, and then forciby struck by a hammer, 
 coheres to the steel, and gilds it permanently. 
 
 But iron can be made to cohere to iron, only by rendering both pieces red 
 hot before hammering : the process is called welding. Iron and platinum 
 are the only metals that can be welded. 
 
 Tin and lead, in sheets, pressed together between the strong rollers of a 
 flatting-mill, cohere. 
 
 The other metals require to be melted before the superficial repulsion 
 gives way so as to allow separate quantities to cohere or run into one mass. 
 It is thus, for instance, that gold, silver, lead, &c., are treated. 
 
 In many cases the substances are not such as can be melted, (wood or 
 marble, for instance,) and then it is necessary to use some soft glue or cement. 
 Cements must have strong attraction for both substances, and, when dry or 
 cool, must be tenacious in themselves ; solder, paste, common glue, motar, 
 &c., are the principal substances of this kind. 
 
 " Certain modifications of attraction produce the subordinate states, called 
 crystal, porous, dense, &c" (Read the Analysis, page 22.) 
 
 " It is a remarkable circumstance, that attraction, in causing the atoms to 
 cohere so as to form solid masses, seems not to act equally all around each 
 atom, but between certain sides or parts of one, and corresponding parts of 
 the adjoining one; so that when atoms are allowed to cohere according to 
 their natural tendencies, they always assume a certain regular arrangement 
 and form, which we call crystalline. Because in this circumstance they 
 seem to resemble magnets, which attract each other only by their poles, the 
 fact has been called the polarity of atoms. It is the cause of several of the 
 peculiarities above enumerated, as elasticity, &c. 
 
 " Crystallization" is exemplified in the following particulars : 
 
 Water beginning to freeze, shoots delicate needles across the surface ; 
 these thicken and interweave until the whole mass has become solid, but 
 the crystalline arrangement always remains. In most substances, this 
 arrangement is remarkably proved, by the forms of the surfaces left, when 
 the mass is broken. 
 
 Moisture, freezing on the window-pane in winter, exhibits a beautiful 
 variety of arborescence. 
 
 A flake of snow viewed in the microscope, is seen to be as symmetrically 
 formed as a fern-leaf or a swan's feather. 
 
 If a piece of copper be thrown into a solution of silver in nitric acid, it is 
 preferred by the acid to silver, and is dissolved accordingly : the silver in 
 the mean time, during its precipitation or separation, assumes the form of 
 a singularly beautiful shrub or tree, resting on the remaining copper as its 
 root. This appearance is call the arbor Diande. 
 
 3 
 
34 CONSTITUTION OF MASSES. 
 
 Any metal which has been melted, when allowed to cool again, slowly 
 and at rest, becomes solid first on the outside of the mass. If, before the 
 cooling be completed, the remaining liquid be poured from within, a curious 
 internal crystalline structure, like grotto work, is seen. What is called the 
 grain of a metal is the result of this crystallization. 
 
 Saltpetre, glaubler salt, copperas (to use popular names,) or any other of 
 the many neutral salts, being dissolved in water, and the water being then 
 allowed slowly to evaporate, reappears in beautiful regular crystals, each 
 salt having its peculiar forms, bounded by perfectly plane and polished 
 surfaces. If any such crystal be broken in any part, the broken surface 
 appears to the microscope as if regular layers of particles had been disturbed, 
 (as we see on a larger scale in a broken stack of bricks, or broken pile of 
 shot in a battery yard,) and the defect of the crystal will be exactly filled 
 up by replacing it in the evaporating solution proving that the ultimate 
 particles are all of the same size. 
 
 All the precious stones are crystals, and can be well cut only parallel to 
 their natural surfaces. 
 
 The basaltic pillars of the Giant's Causeway in Ireland, and of the Isle 
 of Staffa, which appears like a garden supported on magnificent columns in 
 the midst of the ocean, are natural crystalline arrangements of particles, 
 equalling in regularity and beauty any human work, and in granduer so far 
 surpassing even the Egyptian pyramaids, that superstitious conjecture 
 naturally supposed them the work of giant architects. 
 
 It would be endless to go on enumerating crystalline masses, for nature's 
 forms generally, in the inanimate creation, as well as in organized bodies, 
 are regular and symmetrical ; and what we see on earth of broken conti- 
 nents, and islands, and rocks, and wild Alpine scenery, are the effects of 
 subsequent convulsions, which have deranged a primitive and natural order. 
 
 Much ingenuity has been employed to account for the specific forms 
 which different crystalline bodies assume; but the subject is not yet reduced 
 to a state fitting it to be a part of this elementary study. A familiarity 
 with the various figures which the exact science of measures treats of, is 
 required in the person who expects to pursue it with pleasure or advantage. 
 The facts are extremely curious, and the scientific investigation of them 
 may ultimately give important information respecting the intimate consti- 
 tution of material nature. 
 
 " Porous." The crossing of the constituent crystalline needles or plates 
 in bodies, causes them to be porous or full of small vacant spaces. In some 
 cases these are visible to the eye, in many more cases, they are visible to the 
 microscope, and in all, they are to be proved in some way. 
 
 Owing to the porosity arising from the new arrangement of atoms of 
 solidifying, water and a very few other substances become more bulky in 
 the change from the liquid to the solid state. Water then dilates with 
 such force as to burst the strongest vessels which art can provide, and in 
 winter to split even rocks, where it has been retained in their crevices ; 
 freezing water thus curiously producing effects which surpass those of 
 exploding gunpowder. This agency of water contributes to the gradual 
 breaking down of our Alpine summits, and the falling of their destructive 
 fragments into the valleys. 
 
 The stone called hydrophane (agate) is opaque, until dipped into water, 
 when it absorbs into its pores one-sixth of its weight of the water, and after- 
 wards gives passage to light. 
 
 Into crystallized sugar, and various stones, much water will enter without 
 increasing the bulk. 
 
DENSITY. 35 
 
 * A kind of sandstone, suitably shaped, forms an excellent filter or strainer 
 for water. 
 
 Pressure will force water through the pores of the most solid gold : as 
 was seen in the famous Florentine experiment, where a hollow, thick, golden 
 ball, being filled with water and squeezed^ to try the compressibility of water, 
 was found to perspire all over. 
 
 The examples of porosity in animal and vegetable bodies, are, however, the 
 most remarkable. 
 
 Bone is a tissue of cells and partitions, as little solid as a heap of empty 
 packing-boxes. 
 
 Wood is a congeries of parallel tubes, like bundles of organ pipes. It 
 has lately been proposed to prepare wood for certain purposes, as for making 
 the great wooden pins or nails used in ship-building, by squeezing it to half 
 its lateral bulk between very strong rollers, and thus making its density 
 approach to that of metal. 
 
 A piece of wood sunk to a great depth in the ocean, and exposed to the 
 pressure there, has its pores soon filled with water, and becomes nearly as 
 heavy as stone. Thus it was with the boat of a whale-fishing ship, which had 
 been dragged far under water by a whale, and which, on being afterwards 
 drawn up, was supposed by the crew to be bringing a piece of rock with it. 
 
 A piece of cork in a strong, close glass vessel, nearly full of water, may 
 be seen floating at the top ; but if more water be then forcibly pumped into 
 the vessel, the cork will be squeezed and reduced in size, until at last it be- 
 comes heavier than water, and sinks. On water being afterwards allowed 
 to escape, the cork will resume its bulk and will rise. A cork sunk 200 
 feet under water will never rise again of itself. 
 
 A bottle of fresh water, corked and let down thirty or forty feet into the 
 sea, often comes up again with the water saltish, although the cork be still 
 in its place: the explanation being, that the cork, when far down, is" so 
 squeezed as to allow the water to pass in or out by its sides, but on rising, 
 resumes its former size. 
 
 " Density" or the quantity of atoms which exist in a given space, is very 
 different in different substances. 
 
 A cubic inch of lead is forty times heavier than the same bulk of cork 
 Mercury is nearly fourteen times heavier than an equal bulk of water. 
 
 The density must depend on, first, the size or weight of the individual 
 atoms ; secondly, the degree of porosity just now explained; and thirdly, the 
 proximity of the atoms in the more solid parts which stand between the pores. 
 
 From many circumstances it appears, that the atoms even of the most 
 solid bodies are nowhere in actual contact, but are retained in their places 
 by a balance between attraction and repulsion thus, 
 
 A body dilates or contracts, according as heat is added or taken away 
 from it. 
 
 A weight placed on any upright rod or pillar, shortens it and lessens its 
 bulk, and if suspended from the bottom, lengthens it and increases its bulk, 
 the .rod in both cases returning to its former dimensions when the weight 
 is removed. 
 
 When a plank or rod is bent, the atoms on the concave side are, for the 
 time, approximated, and those on the convex side are drawn more apart. 
 It is remarkable in solid bodies, not only how precisely the balance between 
 
36 CONSTITUTION OF MASSES. 
 
 attraction and repulsion determines the relative position of the particles, but 
 also how strongly ; for any farther separation of the particles is resisted by 
 all the force which we call the tenacity or cohesion of the substance, and 
 any nearer approach by all the force which we call the hardness or incom- 
 pressibility. 
 
 Tin and copper, when melted together, to form bronze, occupy less space 
 by one-fifteenth than when separate : proving that the atoms of the one are 
 partially received into what were vacant spaces in the other. A similar con- 
 densation is observed in many other mixtures. A pound of water and a 
 pound of salt, when mixed, form two pounds of brine, but which has much 
 less bulk than the ingredients apart. So also of a pound of sugar dissolved 
 in a pound of water. 
 
 Water and liquids generally resist compression very powerfully, but yield 
 enough to show that the particles are not in contact. It is found that at 
 1,000 fathoms down in the sea the water is compressed by the superincum- 
 bent water so as to have bulk about a hundredth part less than it would have 
 at the surface. 
 
 In aeriform masses the atoms are very distant, and hence the masses are 
 more easily compressed. A pint of water, on assuming the aeriform state, 
 in which it is called steam, under ordinary pressure, acquires nearly 2,000 
 times its former bulk. A hundred pints of common air may be compressed 
 into a pint vessel, as in the chamber of an air-gun ; and if the pressure be 
 much farther increased, the atoms will at last collapse and form a liquid. 
 The heat which was contained in such air, and gave it its form, is squeezed 
 out in this operation, and becomes sensible all around. 
 
 From these proofs of the non-contact of the atoms, even in the most solid 
 parts of bodies ; from the very great space obviously occupied by pores 
 the mass often having no more solidity than a heap of empty boxes, of which 
 the apparently solid parts may still be as porous in a second degree, and so 
 on ; and from the great readiness with which light passes in all directions 
 through dense bodies like glass, rock crystal, diamond, &c., it has been 
 argued that there is so exceedingly little of really solid matter, even in the 
 densest mass, that the whole world, if the atoms could be brought into 
 absolute contact, might be received into a nut-shell. We have as yet no 
 means of determining exactly what relation this idea has to truth. 
 
 The comparative weights of equal l>ulk$ of different bodies are called their 
 specific gravities. 
 
 In thus comparing bodies, it was necessary to choqse a standard ; and 
 water, as being the substance most easily procurable at all times and in all 
 places, has been generally adopted. 
 
 The metal called platinum, the heaviest of known substances, is about 
 twenty-two times as heavy as an equal bulk of water, and is therefore said 
 to have specific gravity of 22 gold is nineteen times as heavy mercury 
 thirteen and a half lead eleven iron eight and a half copper eight com- 
 mon stones about two and a half woods from half to one and a half cork 
 one-quarter, &c. 
 
 " Hardness" is not proportioned, as might be expected, to the density of the 
 different bodies, but to the polarity of the atoms in them, that is, to the 
 force with which the atoms hold their places in some particular arrangement. 
 
 Hardness is measured generally by the circumstance of one body being 
 
DENSITY. 37 
 
 capable of scratching another. It is here worthy of notice, however, that 
 the powder or dust of a softer body will often, through an effect of motion 
 to be described below, aid in wearing down or polishing one that is harder. 
 
 Gold, though soft, is four times heavier than the hard diamond; and 
 mercury, which is fluid, is nearly twice as dense as the hardest steel. 
 
 Diamond is the hardest of known substances. It cuts or scratches every 
 other body, and is generally polished by means of its own dust. 
 
 Glass-cutters use a point of diamond as a glass-knife, for dividing and 
 shaping their panes. 
 
 Common flint also cuts glass, as is proved by the frequent scribblings on 
 windows. 
 
 It is remarkable, that the preparation of iron, called steel, may either be 
 soft like pure iron, or from being heated and suddenly cooled, in the process 
 called tempering, may become nearly as hard as diamond. The discovery 
 of this fact is, perhaps, second in importance to few discoveries which man 
 has made ; for it has given him all the edge tools and cutting instruments 
 by which he now moulds every other substance to his wishes. A savage 
 will work for twelve months, with fire and sharp stones, to fell a great tree, 
 and to give it the shape of a canoe ; where a modern carpenter, with his 
 tools, could accomplish the object in a day or two. 
 
 The project has lately been realized, of engraving on plates of soft steel, 
 instead of copper, and afterwards tempering the steel to such hardness, that 
 it may be used as a type or die to make its impression, not on paper, but on 
 other plates of soft steel or of copper ; each of which then is equal in value 
 to an original and distinct engraving. By this means the beautiful produc- 
 tions of art, instead of being limited to a comparatively small number of 
 copies and of persons, may be multiplied almost to infinity, becoming the 
 cheap delight of all. 
 
 " Elasticity" is present in a mass when the atoms, cohering in a particular 
 arrangement only, yield, however, to a certain extent, when force is applied, 
 but move back or regain their natural positions on the force being with- 
 drawn. 
 
 Elastic bodies vary much as to the extent to which they yield without 
 breaking, and as to the degree of perfection with which, after the bending, 
 or displacement of atoms, they regain their former state. India rubber is 
 extensively elastic, for it yields far ; but it is not perfectly elastic, for when 
 stretched much or often, it becomes perfectly elongated. Glass, again, is 
 perfectly elastic, for it will retain no permanent bend; but, unless in very 
 thin plates indeed, or in fine threads, it will not bend far without breaking. 
 
 All hard bodies are elastic, as steel, glass, ivory, &c., and many soft ones, 
 as caoutchouc, silk, a harp string, &c. The aeriform bodies are all per- 
 fectly elastic, as is rudely seen in a bladder filled with air, when squeezed, 
 and allowed to expand again ; and they will change volume to a very great 
 extent. Liquids also are perfectly elastic, but to a small extent. 
 
 A good steel sword may be bent until its ends meet, and yet, when 
 allowed, will return to perfect straightness. 
 
 A rod of bad steel, or of other metal, will be broken in bending, or will 
 retain a bend. 
 
 An ivory ball, let fall on a marble slab, rebounds, owing to the great 
 elasticity of both bodies, nearly to the height from which it fell, and no 
 mark is left on either. If the slab be wet, it is seen that the ivory or mar- 
 
38 CONSTITUTION OF MASSES. 
 
 ble, or both, had yielded considerably at the point of contact, for a circular 
 surface of some extent on the slab is found dried by the blow. The sudden 
 expulsion of air from between the meeting surfaces might contribute to the 
 effect, but the result is very nearly the same when the experiment is made 
 in a vacuum. Billiard balls scarcely lose even their polish by long wear, 
 although the touching parts yield at every stroke. 
 
 A marble chimney-piece, long supported by its ends, is found at last to 
 be bent downwards in the middle ; and the bend is permanent. 
 
 A steel watch-spring, although so much and so constantly bent, resumes 
 its original form when freed at the end of a century ; but occasionally, 
 without evident cause, while in action, it will suddenly give way. 
 
 Elasticity is a property of bodies of great utility to man, as in his time- 
 pieces, carriage-springs, gun-locks, &c., &c. 
 
 " Britileness" designates that constitution of a body where, with hardness, 
 and elasticity perfect as far as it goes, the cohesion among the atoms exists 
 within such narrow limits that a very slight change of position or increase 
 of distance among them is sufficient to produce a rupture. A compara- 
 tively slight force, therefore, if sudden, breaks them. It belongs to most 
 very hard bodies. 
 
 Glass scratches an iron hammer, proving that it is harder than iron yet 
 glass is the very type of fragility j yielding to the stroke of soft wood, or, 
 indeed, of almost any thing which can give a blow. 
 
 Steel, when tempered so as to be very hard, becomes brittle also. The 
 steel chisels and tools with which artificers now shape the stones and metals 
 as they formerly did wood, require, of course, to be exceedingly hard ; but 
 they thereby lose in regard to the extent of their elasticity, and hence are 
 frequently broken. Cast iron, which is much harder than malleable or 
 wrought iron, is very brittle, while soft iron and steel are the toughest things 
 in nature. 
 
 " Malleable" or reducible into thin plates or leaves by hammering. This 
 property, in opposition to elasticity and brittleness, belongs to bodies 
 whose atoms cohere equally in whatever relative situations they happen 
 to be, and therefore yield to force, and shift about among each other, with- 
 out fracture or change of property, almost like the atoms of a fluid. 
 
 Gold is remarkably malleable, for it may be reduced to leaves of the thin- 
 ness of 360,000 to the inch, or of 1,800 to a sheet of common paper. For 
 gold-beaters the metal is first formed into rods, these are ^afterwards rolled or 
 battened into ribbons ; the ribbon is cut into portions, which are extended, 
 by hammering, to great breadth and thinness, and which being again divi- 
 ded into portions, are hammered and extended to the thinness described. 
 
 Silver, copper and tin may also be hammered until very thin. Most other 
 metals crack or are torn before the operation is carried far ; and some, on 
 being struck, are broken at once, almost like glass. 
 
 tl Ductile" or susceptible of being drawn into wire. One might expect 
 malleability and ductility to belong to the same substances and in the 
 same degrees but they do not. In ductile substances, as in malleable, 
 the atoms seem to have no more fixed relation of position than in a liquid, 
 but yet they cohere very strongly. 
 
 One end of a rod of iron, or other ductile metal, being reduced in size so 
 
DENSITY. 39 
 
 as to pass through an opening in a plate of steel, is seized by strong nippers 
 on the other side of the plate, and the whole rod is drawn through. It is 
 thus reduced, of course, to the size of the opening, and is lengthened in a 
 like proportion. By repeating the operation through smaller holes succes- 
 sively, a wire may at last be obtained to the size of a hair. 
 
 Dr. Wollaston's ingenuity produced platinum wire finer than spider's 
 thread. He filled a space in the axis of a silver wire with small platinum 
 wire. He then drew or reduced the compound piece to the smallest wire 
 possible, and on dissolving the silver from the outside, he exposed to view 
 the delicate filament of platinum. 
 
 The order in which metals may be ranged according to their ductility is, 
 platinum, silver, iron, copper, gold, &c. 
 
 Melted glass has great ductility. The workers draw or spin it into threads 
 by merely attaching a point, pulled out from the mass, to the circumference 
 of a turning-wheel. A uniform thread then continues to be drawn out and 
 wound upon the wheel, at a rate of 1,000 yards or more per hour. This 
 glass thread, when lying together in quantities, resembles beautiful white 
 hair, and when cut in bunches, it serves as an ornament to the female head, 
 waving in the air like the delicate plume of a bird of paradise. 
 
 "Pliant." In bodies distinguished by this title, the cohesion is not des- 
 troyed by considerable change of direction among the particles, but there 
 is little elasticity, and unlike what happens in a ductile mass, the same 
 atoms always remain together. 
 
 Of all pliant things, the chief are animal and vegetable fibres and mem- 
 branes as silk, bladder, lint, hemp, &c., &c. 
 
 " Tenacity" means the force of cohesion among the atoms of any mass. 
 It belongs more or less to all solids, and even to liquids. 
 
 This property varies much in different substances. Iron and its modifica- 
 tion called steel possess it in the most remarkable degree. 
 
 The following table shows the comparative tenacity, or strength to resist 
 pulling of certain metals and woods. Supposing similar wires or rods of 
 each to be used, and of such a size that the surface of a broken end or cross- 
 section would be the one-thousandth of a square inch, the weights supported 
 would be nearly as follows : 
 
 METALS. 
 
 Cast Steel . . .134 Ibs. 
 Best wrought iron . 70 
 Cast Iron . ....... . 19 
 
 Copper .",.,- .,- : .1-.^ 19 
 Platinum . . -^ , 16 
 Silver . , ; . ^ 11 
 
 Gold . . ;;:; " 9 
 
 Tin . :,./.,. 5 
 
 Lead . : V.^>: 2 
 
 WOODS. 
 
 Teak . . ',13 
 
 oak . .-;' .:.' 12 
 
 Beech . . . 12* 
 
 Ash . . :$ . 14 
 
 Deal S. 11 
 
40 CONSTITUTION OF MASSES. 
 
 Iron compared in this way, is five or six times stronger than oak. 
 
 Steel wire will support about 39,000 feet, that is, 7? miles of its own 
 length. 
 
 Certain animal substances have great tenacity; as the silk-worm's thread, 
 which is our strongest connecting or sewing material, and has such flexibility 
 united with its strength the ligaments and tendons of the animal body, pos- 
 sessing at once such admirable strength, elasticity and pliancy : these when 
 dried, and otherwise prepared, constituted the tough bow-strings of our re- 
 mote forefathers the hair or wool of animals twisted into threads, and 
 worked into strong and beautiful textures of the loom strips of animal in- 
 testines prepared and twisted, forming the cords of harp and violin, and in 
 strength and uniformity rivaling the steel wires of keyed instruments. 
 
 The gradual discovery of substances possessed of strong tenacity and which 
 man could yet easily mould to his purposes, has been of great importance to 
 his progress in the arts of life. The place of the hempen cordage of Euro- 
 pean navies is still held in China by twisted canes and strips of bamboo ; 
 and even the hempen cable of Europe, so great an improvement on former 
 usage is now rapidly giving way to the more complete and commodious secu- 
 rity of the iron chain of which the material to our remote ancestors existed 
 only as a useless stone or earth. And what a magnificent spectacle is it, at 
 the present day, to behold chains of tough iron stretched high across a chan- 
 nel of the ocean, as at the Menai Strait, between Anglesea and England, 
 and supporting there an admirable bridge-road of safety along which crowded 
 processions may pour, regardless of the deep below, or of the storm ; while 
 under it, ships with full sails spread pursue their course, uninolesting and 
 unmolested ! 
 
APPENDIX 
 
 TO PART I. SECTION I 
 
 BY THE AMERICAN EDITOR. 
 
 Ir the reader has studied the preceding section with attention he is prepared 
 to understand the following propositions. 
 
 Prop. 1 Matter is endowed with properties. 
 
 Prop. 2. The properties of matter are distinguishable into two classes, 
 first, those which are general or belong to all kinds of matter, and second, 
 those which tare peculiar or belong only to particular kinds of matter. 
 
 Prop. 3. The general properties of matter are, indestructibility (p. 22;) 
 extension or the property of occupying a portion of space (jp. 24; ) divisi- 
 bility (p. 23 ;) impenetrability (p. 24; ) and inertia, (p. 42. ) 
 
 Prop. 4. Every particle of matter, and also all masses, have a mutual 
 attraction for one another, or endeavor to get near each other ; and this 
 attraction is inversely as the squares of the distances. 
 
 Attractions may be primarily distributed into two classes : one consisting 
 of those which exist between the molecules or constituent parts of bodies, 
 and the other between the bodies themselves. The former are called mole- 
 cular or atomic attractions, the latter gravitation (p. 26 : ) of the former 
 there are several varieties, 1st, cohesion (p. 27;) when this variety of 
 molecular attraction is exhibited by liquids pervading the interstices of porous 
 bodies, ascending in crevices or in the pores of small tubes, it is called ca- 
 pilliary attraction (p. 28. ) The other varieties of molecular attractions 
 are affinity or chemical attraction (p. 28, ) and electric and magnetic attrac- 
 tion, (p. 30. ) 
 
 Prop. 5. Attraction of gravitation, or that force by which all the masses 
 of matter tend towards each other, is exerted at all distances. 
 
 Prop. 6. Attraction of cohesion acts only within certain limits, and where 
 its sphere of attraction ends, a repulsive force begins. 
 
 Prop. 7. Repulsion, except when dependent on electricity or magnetism, 
 is owing to the presence of heat, which latter pervades all matter. 
 
 Prop. 8. The particles of matter are more or less close, according to the 
 quantity of heat among them ; but they are never in actual contact (p. 30- 
 31,) and hence porosity is usually considered as one of the properties of 
 matter. 
 
 Prop. 9. The peculiar properties of matter are density (p. 35, ) hardness 
 (p. 36, ) elasticity (p. 37, ) brittleness (p. 38, ) malleability (p. 38, ) duc- 
 tility (p. 38, ) pliability (p. 39, ) tenacity, (p. 39, ) &c. 
 
42 MOTIONS AND FORCES. 
 
 SECTION II. THE MOTION OR PHENOMENA OF THE 
 UNIVERSE.* 
 
 ANALYSIS OF THE SECTION. 
 
 The bodies or masses composing the universe may be at rest or in motion, 
 and to change any present state, force proportioned to the quantity of 
 the body and to the degree of change, is equally required, whether to give 
 motion, to take it away, or to bend it : a truth expressed by saying that 
 matter has INERTIA, or figuratively, a stubbornness. Uniform straight 
 motion, then, is as naturally permanent as rest. And the motion in any 
 body, measured by its velocity, quantity of matter and direction, is the 
 measure of the amount and direction of any single force or of any com- 
 bination of forces, which has produced it, as also of the force or momen- 
 tum which the body can exhibit again when opposed or made to act itself 
 as a cause of some new motion. 
 
 The great forces of nature, referred to by the two words ATTRACTION and 
 REPULSION, acting upon INERT matter, produce the equable, accelerated, 
 retarded and bent motions which constitute the great phenomena of the 
 universe. Tides, currents, winds, falling bodies, &c., exemplify attrac- 
 tion. Explosion, steam collision, &c., exemplify repulsion. And as in 
 every case of attraction or repulsion two masses at least must be con- 
 cerned, there is no motion or action in the universe, without an equal and 
 opposite motion or re-action. 
 
 Motion" 
 
 Is the term applied to the phenomenon of the changing of place among 
 bodies. 
 
 Were there no motion in the universe it would be dead. It would be 
 without the rising or setting sun, or river flow, or moving winds, or sound, 
 or light, or animal existence. 
 
 To understand the nature and laws of the motions or changes which are 
 going on around him, is to man of the greatest importance, as it enables him 
 to adapt his actions to what is coming in futurity, and often to interfere so 
 as to control futurity for his special purposes. 
 
 Motion, in any particular case, is described by referring to certain objects 
 to mark place, and to some other motion chosen as the standard of velocity. 
 A man sitting on the deck of a sailing ship, has common motion with 
 the ship : if walking on the deck, he has relative motion to the ship : but 
 if he be walking towards the stern, just as fast as the ship advances, he 
 is at rest relatively to the bottom or shore. A ship sailing against the tide, 
 just as fast as the tide runs, is as much at rest relatively both to the earth 
 and water as if she were at anchor. Absolute motion is that which is rela- 
 tive to the whole universe, or rather to the space in which the universe ex- 
 ists. We have no means of ascertaining such : for although we know how 
 fast our globe whirls upon its axis and wheels round the sun, we have no 
 measure of the motion of the sun himself revolving possibly round some 
 
 * The reader should here re-peruse the title and Analysis at page 22. 
 
MOTION. 43 
 
 more distant centre, but almost certainly having a progress in space, and 
 carrying all the planets along with him. 
 
 Motion is called rapid, as that of lightning slow, as that of the sun-dial 
 shadow ; both terms have reference to the ordinary intermediate velocities 
 observed upon earth. It is called straight or rectilineal, in the apparent 
 path of a failing body bent, or curvilinear, in the track of a body thrown 
 obliquely accelerated, in a stone falling to the earth retarded, in a stone 
 thrown upwards while rising to the point where it stops before again 
 descending. 
 
 " Owing to the INERTIA of bodies, force is equally required to impart motion 
 and to take it away." (Read again the last Analysis.) 
 
 If a man put his hand to the crank of a heavy fly-wheel or grindstone, to 
 turn it, he experiences a certain resistance, which, however, gradually yields 
 to his effort, and he leaves the wheel whirling with velocity proportioned to 
 the effort. If he then puts out his hand again to stop the wheel, he experi- 
 ences an opposite but similar resistance, which however, as before, gradually 
 yields, and he brings the wheel to rest. In the second case the effort re- 
 quired of him is less than the first, by reason of the friction of the turning 
 axle, and the resistance of the air in which the wheel moves, obstructions 
 which, when he was giving motion, opposed him, but when taking it away 
 assisted him. That these obstructions caused the whole difference in such a 
 case, and that they are the great reasons why all ordinary motions on earth 
 seem to tend of themselves to cease, will be shown in subsequent pages. It 
 is the resistance overcome in moving the wheel or in stopping it, and occa- 
 sioning an expenditure of force proportioned to the mass and to the degree 
 of change of state, which is called the INERTIA of the mass, or the vis iner- 
 tias, and sometimes, to help the conception of the student, the stubbornness, 
 sluggishness, or inactivity; but none of these words can originally suggest 
 to the mind all that is intended to be conveyed. 
 
 An exact measure of the amount of inertia is contained in the familiar fact 
 that a body let fall near the surface of the earth, falls rather more than 16 
 feet in the first second of time, the well-known weight of the body, or 
 force of terrestrial attraction acting upon it for one second, being just suffi- 
 cient to overcome its inertia to the extent stated. Were the inertia of matter 
 only half of what it is, a body near the earth would fall 32 feet in the second, 
 instead of 16, as it equally would, if, with present inertia, the attraction of 
 the earth were doubled. And were there no inertia, it would fall or pass 
 through any height, however great, in one instant. As the amount of inertia 
 thus determines the amount of other force required to give motion to a 
 mass, so does it determine the amount of force required to destroy motion 
 in a mass. A heavy cannon-ball, if wanting inertia, might be dispatched 
 with the speed of lightning by the slightest force, but then the stiffness of a 
 stalk of corn would suffice to arrest it ; and while the ball, with the inertia 
 now existing, takes the force of pounds of gunpowder to give it its usual 
 motion, it may not be stopped, even by the cohesion of a block of granite, 
 which accordingly it shivers to pieces. The numerous examples now to 
 follow will prove the immense importance of inertia in the general opera- 
 tions of nature. 
 
 When the sails of a ship are first spread to receive the force or impulse of 
 the wind, the vessel does not acquire her full speed at once, but slowly, as 
 the continuing force gradually overcomes the inertia of her mass. When the 
 
44 MOTIONS AND FORCES. 
 
 sails are afterwards taken in, she does not loose her motion at once, but 
 slowly again, as the continued resisting force of the water destroys it. 
 
 Horses must make a greater effort at first to put a carriage in motion than 
 to maintain the motion afterwards. And a strong effort is required to stop 
 a moving carriage. When a carriage, of which the body hangs from springs, 
 is first moved, the body appears to fall back, and a person within seems to 
 be suddenly forced against the back cushion. When the carriage is stopped 
 again the body swings forward, and if the stoppage be very sudden, a care- 
 less passenger may unwittingly pop his head through a front glass. These 
 particulars prove the inertia, first of rest, and secondly of motion. 
 
 A man standing carelessly at the stern of a boat, when the boat begins to 
 move, falls into the water behind ; because his feet are pulled forward while 
 the inertia of his body keeps it where it was, and therefore behind its sup- 
 port. The stopping of a boat, again, illustrates the opposite inertia of motion, 
 by the man's falling forward. 
 
 An awkward rider on horsback may be left behind, when his horse starts 
 forward suddenly : or may be thrown off on one side by the horse starting 
 to the other. A horse at speed, stopping suddenly, often sends his cavalier 
 over his ears as was mortifyingly experienced by a coxcomb, who, on an old 
 cavalry horse, chose to canter along a foot-path, to the annoyance of the 
 company, and whose horse on hearing the word halt loudly addressed to it 
 by a waggish officer of the regiment, who happened to be there and to recog- 
 nize it, suddenly stood and got rid of its load. The mind or will of the beau 
 had sinned against the law of propriety, but his body very perfectly obeyed 
 the laws of inertia and gravity, by shooting forward in a parabolic curve to 
 the earth. 
 
 A young man not yet accustomed to the whip, drove his phaeton against 
 a heavy coach on the road, and then to his father foolishly excused his awk- 
 wardness, in a way which led to the prosecution of the coachman for furious 
 driving. At the trial, the youth and the servant both deposed that the 
 shock of the coach was such as to throw them over their horses' heads, and 
 thus lost the case, by unconsciously proving, that the faulty velocity was 
 their own. 
 
 A man jumping from a carriage at speed is in great danger of falling for- 
 ward, when his feet reach the ground ; for his body has as much forward 
 velocity as if he bad been running with the speed of the carriage and unless 
 he advance his feet like a running man, to support his advancing body, he 
 must as certainly be dashed to the ground, as a runner whose feet are sud- 
 denly arrested. A man racing who receives a signal to stop, and a man 
 jumping from a flying vehicle, must check their motion^ nearly in the same 
 way. 
 
 A person wishing to leap over'a ditch or chasm, first makes a run, that 
 the motion thereby acquired may help him over. A standing leap falls much 
 short of a running one. 
 
 An African traveller saw himself pursued by a tiger, from which he could 
 not escape by running ; but perceiving that the animal was watching an 
 opportunity to seize him by its usual spring or leap, he artfully led it to 
 where the plain terminated in a precipice hidden by brush-wood, and he had 
 just time to transfer his hat and cloak to a bush, and to retreat a few paces 
 when the tiger sprung upon the bush, and by the moral inertia of its body, 
 was carried over the precipice and destroyed. 
 
 From a glass of water suddenly pushed forward on the table, the water is 
 spilt or left behind \ but if the glass be already in motion, as when carried 
 
MOTION. 45 
 
 by a person Balking, and if it then be suddenly stopped by coming against 
 an impediment, the water is thrown or spilt forward. 
 
 A servant carrying a tray of glasses or china in the dark, and coming sud- 
 denly against an obstacle, hears all his freight slipping forward and crashing 
 at his feet : and a too hurried departure with such a load causes equal des- 
 truction, on the opposite side. 
 
 The actions of beating a coat or carpet with a cane, to expel the dust ; of 
 shaking the snow from one's shoes, by kicking against a door-post ; of clean- 
 ing a dusty book by knocking it against a table, or shutting it violently all 
 illustrate the same principle. 
 
 If a guinea be laid on a card which is already balanced on the point of the 
 finger, a small fillip or blow to the edge of the card will cause it to dart off, 
 but the guinea, owing to its inertia, will remain resting on the finger, its 
 inertia being greater than the friction on it of the card passing from under- 
 neath it. 
 
 When we desire a person, with suspected disease of the brain, to shake 
 his head and tell whether and where he feels pain, we are doing nearly as 
 if we touched the naked brain with the finger to find the tender part ; for 
 the inertia of the brain, when the skull is moved, causes a momentary pres- 
 sure between it and the skull, almost equivalent, for the purpose desired, to 
 such a touch. 
 
 This kind of pressure is sufficient to break and destroy tender wares as 
 glass or eggs in packages which are too suddenly moved or stopped. 
 
 A weight suspended by a spring on ship-board is seen vibrating up and 
 down as the ship pitches with the waves. It seems to fall as the ship rises, 
 and to rise as the ship falls : but the motion is really in the ship, and the 
 comparative rest is in the weight. A heavy weight so supported, and con- 
 nected with a pump-rod, would work the pump. 
 
 Like the weight last mentioned, the mercury ofa common barometer on 
 ship-board is seen rising and falling in the tube; and until the important 
 improvement was lately made, of narrowing one part of the tube to prevent 
 this, the mercurial Barometer was useless at sea. The explanation is, that 
 the tube rises and falls with the ship, from being connected with it ; but the 
 mercury, which plays freely in the tube, and is supported by the atmospheric 
 pressure, tends, by its inertia, to remain at rest, and thus makes the motion 
 of the ship apparent. 
 
 What happens to the mercury in the barometer-tube on ship-board, indi- 
 cates what happens to the blood in the vessels of animals under similar cir- 
 cumstances. In any long vein below the heart, when the body falls, the 
 blood, by its inertia and the supporting action of the vessels, does not fall so 
 fast, and therefore really rises in the vein : and as there are valves in the 
 veins preventing return, the circulation is thus quickened without any mus- 
 cular exhaustion on the part of the individual. This helps to explain the 
 effect of the movement of carriages, of vessels at sea, of swings, &c., and of 
 passive exercise generally, on the circulation, and leaves it less a mystery 
 why these means are often so useful in certain states of weak health. 
 
 If a cannon ball were to break to pieces in its flight, its parts would still 
 advance with the previous velocity. And thus, in the deadly contrivance of 
 the Shrapnell-shell, which is in a case containing hundreds of musket bul- 
 lets, when these are scattered at the desired distance from the devoted body 
 of men, they retain the forward velocity of the shell, and spread death 
 around like the near discharge of a whole battalion of musketry. 
 
 On the awful occasion of a ship in rapid motion being suddenly arrested 
 
46 MOTIONS AND FORCES. 
 
 by a sunken rock, all things on board, men, guns, and furniture, start from 
 their places and dash forwards ; while the onward inertia or moral obstinacy 
 of the hinder parts of the ship, suffices to crush her bow against the rock. 
 
 " Motion as naturally permanent as rest." 
 
 From the instances now given, it is seen that a body at rest would never 
 move if force were not applied, and that a body put in motion retains mo- 
 tion, at least for a time, after the force has ceased; but there is a feeling 
 from common experience, that motion is an unnatural or forced state of 
 bodies, and that all moving things, if left to themselves, would gradually 
 come to rest. It is recollected that a stone projected comes to rest, or a 
 wheel left moving, or a bowl rolled on the green, or the waves heaving 
 after "a storm and in a word, that there is no perpetual motion on the earth. 
 
 On more attentive consideration, however, it may be perceived that there 
 are prodigious differences in the duration of motions, and that the differences 
 are always exactly proportioned to evident causes of retardation, and chiefly 
 to friction and the resistance of the air. 
 
 Friction is the resistance which bodies experience when rubbing or sliding 
 upon each other; and however much it may be diminished by art, it can in 
 no case be annihilated. Air-resistance, again, to motions going on in air, is 
 of the same nature as water-resistance to motions going on in water, only 
 less in degree : and as advancing science has shown the true nature of our 
 atmosphere, the amount of this resistance is perfectly ascertained. 
 
 A smooth ball rolled on the grass soon stops if rolled on a green cloth 
 over a smooth plank it goes longer on the bare plank, longer still on a 
 smooth and level sheet of ice, it hardly suffers retardation from friction, and, 
 if the air be moving with it, will reach a distant shore. 
 
 Two little wind-mill wheels set in motion together with equal velocity, but 
 of which one has the flat sides of the vanes turned to their course, and the 
 other the edges, if moving in the air, will stop at very different times, but if 
 tried in a vessel from which the air has been removed, they will both go 
 much longer, and will then stop exactly together. 
 
 As it is to facilitate the motion of fishes in the water, that they are of 
 sharp form before and behind ; so it is to facilitate the motion of birds in 
 the air that they have somewhat of a similar form. 
 
 A large spinning-top, with a fine hard point, set in motion in a vacuum, 
 and on a hard, smooth surface, will continue turning for hours. 
 
 A pendulum moving in a vacuum has only to overcome slight friction at 
 its point of suspension, and, therefore, if once put in motion, will vibrate for 
 a day or more. 
 
 But it is in the celestial spaces that we see motions completely freed from 
 the obstacles of air and friction and there they seem eternal. 
 
 Had the human eye, unassisted, been able to descry the four beautiful 
 moons of Jupiter, wheeling around him for these thousands of years, with 
 such unabated regularity, and which now form, to the telescope of the astro- 
 nomer, a perfect and magnificent time-piece in the sky, or had science long 
 proved that the velocity imparted to our globe, when first launched into its 
 present orbit, still wheels it along as swiftly as in the days of the first man, 
 this error or prejudice, that motion is always tending to rest, would never 
 have arisen. 
 
 Indeed, had these or other such truths, been long familiar to the common 
 
MOTION UNIFORM. 47 
 
 mind, the opposite prejudice might as well have obtained, that motion is the 
 natural state, and rest a forced or unknown state. We know of nothing 
 which is absolutely at rest. The earth is whirling round its axis and round 
 the sun ; the sun is moving round its axis and round the centre of gravity 
 of the solar system, and possibly, round some more remote centre in the 
 great universe, carrying all its planets and comets about his path. 
 
 If there were any natural tendency in moving bodies to stop, a thing float- 
 ing in a trough of water, on board a sailing ship, should always be found at 
 the end of the trough nearest the stern ; and in all the seas and lakes of the 
 earth, the floating things should be accumulated on the western shores, 
 because the surface of the earth is always turning towards the east. We 
 know that neither of these suppositions is truth. A man on board a mov- 
 ing ship can throw any body just as far towards the bow as towards the 
 stern ; although in the two cases the velocity, as regards the earth is so 
 different. 
 
 Ignorance of the law of moral inertia led a story-telling sailor to assert, as 
 a proof of the speed of his favourite ship, that when a man one day fell from 
 the mast-head, the ship had passed from under him before he reached the 
 deck : the fact in such a case, being, that he must have fallen on the same 
 part of the deck, whether the ship were in motion or at rest, because his 
 body had just the motion or rest which belonged to the ship. 
 
 Another equally sapient man, reflecting that the earth turned round once 
 in twenty-four hours, proposed rising in a balloon, and waiting aloft, until 
 the country which he desired to reach should be passing under him. 
 
 "Motion naturally uniform" (See the Analysis.) 
 
 It is only repeating that a body can neither acquire motion nor lose motion 
 without a cause, to say that free motion must be uniform. 
 
 The perfect uniformity of undisturbed motion is proved by every fact 
 observed in the universe. If any continued motion, as of a planet, for in- 
 stance, be found at one time to have certain relative velocity to some other 
 continued motion, the same relation is found always to hold : or deviations 
 from perfect uniformity are exactly proportioned to the disturbing causes. 
 Thus we can foretell the exact time of an eclipse, a thousand years before 
 its occurrence. 
 
 Had motion not been in its nature uniform, a man could have formed no 
 rational conjecture or anticipation as to future events; for it is by assuming, 
 for instance, that the earth will continue to turn uniformly on its axis, that 
 he speaks of to-morrow and of next week, &c., and that he makes all his 
 arrangements for future emergencies : and were the coming day, or season, 
 or year, to arrive sooner or later than such anticipation, it would throw such 
 confusion in all his affairs, that the world would soon be desolate. 
 
 To calculate futurities, then, or to speak of past events, is merely to take 
 some great uniform motion as a standard with which to compare all others ; 
 and then to say of the remote event, that it coincided or will coincide with 
 some described state of the standard motion. The most obvious and best 
 standards are the whirling of the earth about its axis, and its great revolution 
 round the sun. The first is rendered very sensible to man by his alternately 
 seeing and not seeing the sun, and it is called a day ; the second is marked 
 by the succession of the seasons, and it is called a year. The earth turns 
 upon its axes nearly 365 times while it is performing one circuit round the 
 sun, and thus divides the year into so many smaller parts, and the day is 
 
48 MOTIONS AND FORCES. 
 
 divided into smaller parts, by the progress of the earth's whirling being so 
 distinctly marked, in the constantly varying direction of the sun, as viewed 
 from any given spot on the face of the earth. When advancing civilization 
 made it of importance for men to be able to ascertain with precision the 
 very instant of the earth's revolution, connected with any event, various con- 
 trivances were introduced for the purpose ; as, sun-dials, where the shadow 
 travels progressively round the divided circle ; the uniform flux of water 
 through a prepared opening the flux of sand in the common hour-glass, &c. 
 But the great triumphs of modern ingenuity are those astronomical clocks 
 and watches, in which the counted equal vibrations of the pendulum, or 
 balance-wheel, have detected periodical inequalities even in the motion of 
 the earth itself, and have directed attention to unsuspected disturbing causes, 
 important to be known. 
 
 It is the natural uniformity of undisturbed motion which causes any num- 
 ber of bodies moving together, as the furniture of a sailing ship, to appear 
 among themselves as if at rest, no one tending to pass before, or to fall 
 behind, or to move to one side or another. For the same reason a person 
 who is moving with such bodies is absolutely insensible of his uniform pro- 
 gression, and knows it only by reasoning from such facts as the changing 
 appearance of other objects around which do not share the motion, the rush- 
 ing of the waves or wind, &c. When a ship is becalmed at sea, she may, 
 as numberless sad accidents have proved, be carried by rapid currents in any 
 direction, without one of the crew suspecting that she has motion at all ; and 
 if the suspicion do arise, the truth can be come at only by such means as the 
 sounding line, where the bottom can be reached, or careful observation of the 
 heavenly bodies where it cannot. A man in the hold of a ship in a river or 
 tides-way cannot say whether the rushing of water, which he hears from 
 without, be a rapid tide passing the ship at anchor, or the effect of the ship's 
 advance in the river. A man in a balloon going 80 miles an hour, knows 
 not in what direction he is moving, nor indeed that he is moving at all, but 
 by observing the objects below. 
 
 This explains why men are not sensible of the motion of the earth itself, 
 which they know, however, to be turning around its axis once in twenty -four 
 hours, and therefore to have its surface near the equator moving with a speed 
 of more than 1,000 feet per second ; and as in the case of a ship or balloon, 
 there will be no difference of sensation whether the speed were of one mile 
 per hour or of 10 or 100, so in the case of the earth, there would be none 
 whether it turned as now, once in twenty-four hours ; or, like the planet 
 Jupiter, once in ten. A hunter among the hills, who during the heat of 
 noon, rests and contemplates around him a sublime scfene of solitude and 
 silence, may little think that if, amidst that apparent repose of nature, he 
 were for a moment lifted up from the earth and held at rest above its sur- 
 face, he would see its face of hill and dale sweeping past beneath him at 
 the prodigious rate of 1,000 miles an hour, on account solely of the whirling 
 of the earth. 
 
 The fact that a cannon-ball can be shot just as far upon the surface of the 
 earth, eastward, in the direction of the earth's motion, as westward, against 
 it, illustrates the truth, that whatever common motion objects may have, it 
 does not interfere with the effect of a force producing any new relative 
 motion among them. All the motions seen on earth are really only slight 
 differences among the common motions : as in a fleet of sailing ships, the 
 apparent changes of place among them are in reality only slight alterations 
 of speed or direction, in their individual courses. 
 
MOTION STRAIGHT. 49 
 
 A man continuing to throw upwards a ball or orange, or several of them 
 at once, and to catch and return them alternately, uses no difference of art 
 as regards them, whether he be standing on the earth and whirling with it, 
 or on a sailing ship's deck, or in a moving carriage, or on a galloping horse's 
 back. He and the oranges have always the same forward common motion. 
 And when a man, standing on a galloping horse, leaps through a hoop held 
 across his course, he does not leap forward for this would throw him over 
 the horse's ears but merely jumps up and allows his moral inertia to carry 
 him through. 
 
 The reason why a lofty spire or obelisk stands more securely on the earth, 
 than even a short pillar stands on the bottom of a moving wagon, is, not that 
 the earth is more at rest than the wagon, but that its motion is uniform. 
 Were the present rotation of our globe to be arrested but for a moment, 
 imperial London, with its thousand spires and turrets, would, by the moral 
 inertia, be swept from its valley towards the eastern ocean, just as loose 
 snow is swept away by a gust of wind. 
 
 " Force is required to bend motion" 
 
 If a body moving freely cannot vary its velocity without a cause, neither 
 can it vary its course without a cause ; and free motion, therefore, is 
 straight as well as uniform. 
 
 A ball shot directly up or down gives men their simplest idea of straight 
 motion. 
 
 A bullet or arrow, projected horizontally, is gradually drawn downwards 
 by the attraction of the earth, but it deviates neither to the right nor to 
 the left. 
 
 William Tell, trusting to the natural straightness of motion, obeyed the 
 tyrant's order, and shot an apple placed on his child's head. 
 
 And the right eye of Philip of Macedon is said to have been destroyed 
 by an arrow which* brought a label on it, telling its destination. 
 
 Riflemen shooting at a target, hit the very spot they choose to aim at. 
 
 A stone in a sling, the moment it is set at liberty, darts off as straightly as 
 an arrow from the bow-string or a bullet from a gun-barrel, and it is only 
 because the point of its circle, from which it should depart, cannot in prac- 
 tice be accurately determined, that the same sure aim cannot be taken with it. 
 
 A body moving in a circle, then, or curve, is constrained to do what is 
 contrary to its inertia. A person on first approaching this subject, might 
 suppose that a body, which for a time has been constrained to move in a 
 circle, should naturally continue to do so when set at liberty. But on 
 reflecting that a circle is as if made up of an infinite number of little straight 
 lines, and that the body moving in it has its motion bent at' every step of the 
 progress, the reason is seen why constant force becomes necessary to keep 
 it there, and force just equal to the inertia with which 
 the body tends, at every point of the circle, rather to Fig. 2. 
 
 pursue the straight line, called a tangent, of which that 
 point, as seen in Fig. 2, is the commencement, than the 
 circle itself. The force required to keep the body in the 
 bent course, is called centripetal or centre-seeking force ; 
 while the inertia of the body tending outwards, that is, 
 to move in a straight line rather then in a curve, is called 
 the centrifugal or centre-flying force ; and the term cen- 
 tral forces is applied to both. 
 
50 MOTIONS AND FORCES. 
 
 A sling-cord is always tight while the cord is whirling :,and its tension is 
 of course the measure both of the centripetal and centrifugal force. A means, 
 then, of measuring the tension of a sling-cord would experimentally demon- 
 strate the amount of centrifugal force ; and such a means we possess in the 
 contrivance called the " whirling table/' upon which is a leading sling, or any 
 mass with a string attached to it, may be placed to revolve, at any desired 
 distance from the centre, and with any desired velocity, while the string 
 passing over a pulley at the centre, is made to lift weights proportioned to 
 the outward dragging of the revolving mass. By this apparatus it is found, 
 as would be expected, that centrifugal force in other words the force with 
 which the inertia of moving matter resists the bending of its course from 
 straight to circular, is proportioned, first, to the quantity of matter moved- 
 every separate particle having its own inertia ; second, to the size of the 
 circle or orbit described in the same time a body moving in a circle of 
 double diameter for instance, having to be forced inwards from the tangent, 
 at every departure, twice as far in a given time ; third, that with a double 
 revolution in the same time, the centrifugal force is not double but quadruple 
 (a corresponding proportion existing for other velocities,) because, not only 
 are there twice as many bindings or angular departures from the tangent for 
 the two circles as for one, requiring, as may be said, twice as many tugs or 
 impulses of the centripetal force, but every impulse must be made with 
 double energy, for it has to drive the mass inwards through the required dis- 
 tance in half the time ; and twice as many impulses, every one. being twice 
 as strong, make a quadruple amount of force on the whole ; fourthly and 
 lastly, it is found, agreeing with the relation between inertia and terrestrial 
 gravity described at page 43, that a body revolving, for instance, in a circle 
 of four feet diameter, that it may have centrifugal force just equal to its 
 weight, required to complete its revolution in one second and a half of time. 
 This and similar facts will be more particularly considered when we come to 
 treat of the motions of the planets round the sun. This analysis of central 
 forces will suffice to excite in the student a due interest touching the kindred 
 phenomena now to be described. 
 
 Bodies laid on a whirling horizontal wheel, are readily thrown off. 
 
 In a corn-mill, the grain, after being admitted between the stones through 
 an opening in the centre of the upper stone, is then kept turning round 
 between them, and is, by its centrifugal force, always tending and travelling 
 outwards until it escapes as flour from the circumference. 
 
 A man, if he lie down on a turning millstone with his head near the edge, 
 falls asleep, or dies of apoplexy, from the new pressure of blood on the 
 brain. 
 
 A wet mop, or bottle-brush, made to turn quickly on its handle as an axis, 
 throws the water off in all directions, and soon dries itself. 
 
 Sheep, in wet weather, thus discharge the water from their fleeces, by a 
 semi-rotatory shake of the skin. Water-dogs, on coming to land, dry them- 
 selves by the same action. 
 
 A tumbler of water placed in a sling, may be made to vibrate like a pen- 
 dulum with gradually increasing oscillation, and at last to describe the whole 
 circle, and continue revolving about the hand, without spilling a drop : the 
 water, by its inertia of straightness, or centrifugal force, tending more away 
 from the centre of motion towards the bottom of the tumbler, even when 
 that is uppermost, than towards the earth by gravity. 
 
 As solid bodies laid on a whirling table are thrown off, so water in a ves- 
 sel caused to spin round in any way, as on the centre of a horizontal wheel, 
 
CENTRIFUGAL FORCE. 51 
 
 instead of lying at the bottom, is raised up all round, against the sides of 
 the vessel. 
 
 "Water, poured obliquely into a funnel, runs round the interior of it, and 
 often leaves an open passage of air all the way down through it, as if there 
 were merely a lining of water to the funnel. The centrifugal force of the 
 turning water is a chief reason of this phenomenon : another reason will be 
 considered farther on, under the head of atmospheric pressure. 
 
 Great whirlpools at sea, and smaller ones, or eddies in rivers, occur when- 
 ever a current is obliged suddenly to bend, as in rounding a point of land or 
 a rock, or in meeting and mingling with a contrary current. The water, by 
 tending to continue its straight motion, falls in behind the obstruction, re- 
 luctantly as it were, and leaves there a pit surrounded by a liquid revolving 
 ridge. Charybdis, in the Mediterranean, and the great whirlpool off the 
 Norwegian coast, are noted examples. 
 
 It is owing to the centrifugal force in any bending part of a stream of 
 water, that is to say, the tendency away from the centre of the curvature, 
 that when a bend has once commenced, it increases, and is soon followed by 
 others, until that completeflserpentine winding is produced, which charac- 
 terizes most rivers in their course across extended plains. The water being 
 thrown by any cause to the left side, for instance, wears that into a curve or 
 elbow, and, by its centrifugal force, acts constantly on the outside of the bend, 
 until rock or higher laud resists the gradual progress; from this limit being 
 thrown back again, it wears a similar bend to the right hand, and after that, 
 another to the left, and so on. 
 
 Carriages are often overturned in quickly rounding corners. The inertia 
 carries the body of the vehicle in the former direction, while the wheels are 
 suddenly pulled round by the horses into a new one. A loaded stage-coach 
 running south, and turning suddenly to the east or west, strews its passen- 
 gers on the south side of the road. Where a sharp turning in a carriage-road 
 is unavoidable, the road towards the outside of the bend should always be 
 made higher than at the inside, to prevent such accidents. 
 
 A man or a horse turning a corner at speed, leans much inward, or to- 
 wards the corner, to counteract the centrifugal force, that would throw him 
 away from it. 
 
 In skating with great velocity, this leaning inwards at the turnings be- 
 comes very remarkable, and gives occasion to the fine variety of attitudes 
 displayed by the expert ; and if a skater, in running, finds his body inclined 
 to one side and in danger of falling, he merely makes his skate describe a 
 slight curve towards that side, when the tendency of his body to move 
 fttraightly, or its centrifugal force, refusing to follow in the curve, allows the 
 foot to push itself again under the body, and to restore the perpendicularity. 
 Skating becomes to the intelligent man an intellectual as well as a sensitive 
 or bodily treat, from its exemplifying so pleasingly the law of motion. 
 
 The last example explains, also, why a hoop rolled along the ground goes 
 so long without falling : if it incline to one side, threatening to fall, by that 
 very circumstance, the part touching the ground is made to bend its course 
 to that side, and as in the case of the skater who turns his foot, the sup- 
 porting base is again forced directly under the mass of the body. 
 
 A coin dropped on the table or floor often exhibits the same phenomenon. 
 It is said to run and hide itself in the corner. Just before falling, if not 
 obstructed, it describes several turns of a decreasing spiral, the minute ex- 
 amination of which is a pleasing mathematical exercise. 
 
 The reason also why a spinning top stands, will*;be understood here. 
 
52 MOTIONS AND FORCES. 
 
 While the top is quite upright, the extremity of its peg, being directly under 
 its centre, supports it steadily, and although turning so rapidly, and with 
 much friction, has no tendency to move from the place : but if the top in- 
 cline at all, the edge or side of the peg, instead of its very point, is in con- 
 tact with the floor, and the peg then becoming as a turning little roller, 
 advances quickly, and describes a curve somewhat as a skater's foot does, 
 until it come directly under the body of the top as before. It thus appears 
 that the very fact of the top inclining, causes the point to shift its place, 
 and to continue moving until it comes again directly under the centre of the 
 top. It is remarkable that even in philosophical treatises of authority the 
 standing of a top is still vaguely attributed to centrifugal force. And some 
 persons believe that a top spinning in a weighing scale, would be found 
 lighter than when at rest; and others most erroneously hold that the centri- 
 fugal force of the whirling, which of course acts directly away from the 
 axis, and quite equally in all directions, yet becomes, when the top inclines, 
 greater upwards than downwards, so as to counteract the gravity of the 
 top. The way in which centrifugal force really helps to maintain the 
 spinning of atop is, that when the body inclines.or begins to fall in one direc- 
 tion, its motion in that direction continues until the point describing its 
 curve, like the foot of a skater, has forced itself under the body, again. 
 
 By reason of centrifugal force also, it is easier to do feats of horsemanship 
 in a small ring as at our theatres, than if the animal were running on a 
 straight road. We see the man and the horse always inclining inwards to 
 counteract centrifugal force, and if the rider tend to fall inwards, he -has 
 merely to quicken the pace ; if to fall outwards, he has to slacken it, and all 
 is right again. 
 
 If a pair of common fire-tongs, suspended by a cord from the top, be 
 made to turn by the twisting or untwisting of the cord, the legs will separate 
 from each other with force dependent on the speed of rotation, and will 
 again collapse when the turning ceases. Mr. Watt adapted this fact most 
 ingeniously to the regulation of the speed of his steam-engine. His steam- 
 gouernor may in truth be described as a pair of tongs with heavy balls at 
 the ends, to make their opening more energetic, attached to some turning 
 part of the machine. If the engine move with more than the assigned 
 speed the balls open or fly asunder beyond their middle station, and by a 
 simple contrivance are then made to act on a valve which contracts the steam 
 tube ; on the contrary, with too slow a motion, they collapse and open the 
 valve. 
 
 A half- formed vessel of soft clay, placed in the centre of the potter's 
 table, which is made to whirl and is called his wheel, opens out or 
 widens merely by the force of its sides and thus assists the worker in giving 
 its form. 
 
 A ball of soft clay, with a spindle fixed through its centre, if made to turn 
 quickly, soon ceases to be a perfect ball. It bulges out in the middle, where 
 the centrifugal force is great, and becomes flattened towards the ends, or 
 where the spindle issues. 
 
 This change of form is exactly what has happened to the ball of our 
 earth. It has bulged out seventeen miles at the equator, in consequence of 
 its daily rotation, and is flattened at the poles in a corresponding degree. 
 A mass of lead that weighs one thousand pounds at our pole, weighs about 
 five pounds less at the equator, by reason of the centrifugal force. 
 
 In the planets Jupiter and Saturn, of which the rotation is much quicker 
 than of our earth, the middle or equator bulges out still more even so as 
 to offend an eye which expects a perfect sphere. 
 
QUANTITY OF MOTION. 53 
 
 If the rotation of our earth were seventeen times faster than it is, the 
 bodies or matter at the equator would have centrifugal force equal to their 
 gravity, and a little more velocity would cause them to fly off altogether, or 
 to rise and form a ring around the earth like that which surrounds Saturn. 
 Saturn's double ring seems to have been formed in this way, and is now 
 supported chjefly by the centrifugal force of the parts. Were it to crumble 
 to pieces, the pieces might still revolve, as so many little satellites. His 
 true satellites are only more distant masses sustained in the same manner. 
 And our earth and the other primary planets have the same relation to the 
 sun that these satellites have to Saturn all being sustained by an admira- 
 ble balance between centrifugal force and gravity. 
 
 " The quantity of Motion in a body measured by the velocity and quantity 
 
 of matter. 
 
 If a single atom of matter were moving at the rate of one foot per second 
 it would have a definite quantity of motion expressed by these words ; and 
 if it were moving ten feet per second, it would have ten times the quantity. 
 Again, in a mass consisting of many atoms, the quantity of motion would be 
 still as much greater as there were more atoms in it than one. 
 
 By experiment it is found, that if a ball of soft clay of one pound, sus- 
 pended by a cord as a pendulum, be allowed to fall with a velocity of ten feet 
 per second, against u ball of nine pounds suspended in the same way, but at 
 rest, the two, after contact, will start together at the rate of one foot per 
 second, the original quantity of motion being then diffused through ten 
 times the quantity of matter, and therefore exhibiting only one tenth of the 
 velocity. 
 
 A cannon ball of a thousand ounces, moving one foot per second, has thus 
 the same quantity of motion in it as a musket-ball of one ounce, leaving the 
 gun-barrel with a velocity of a thousand feet in the second. 
 
 " The quantity of motion in a body is the measure of the force which pro- 
 duced it." 
 
 The experiment of the balls of clay mentioned above furnishes one instance 
 of this truth. Again, a body falling for ten seconds, acquires ten times as 
 much velocity as by falling for one second ; its motion thus measuring the 
 force of gravity which has been exerted upon it. 
 
 When a large body or mass of many atoms falls, it of course has as much 
 more motion than a smaller body, as there are more atoms in it than in the 
 smaller ; but as gravity acts equally on every atom, the force causing 
 either body to fall is still exactly indicated by the quantity of motion in it. 
 
 A large body or mass of many atoms falls where there is no impediment, 
 with the same velocity as a smaller body or a single atom ; for gravity pulls 
 equally at each atom, and must overcome its inertia equally, whether it be 
 alone or with others. 
 
 This remark contradicts the popular opinion, that a large and heavy body 
 should fall to the earth much faster than a small and light one ; an opinion 
 which has arisen from our constantly seeing such contrasts, as the rapid fall 
 of a gold coin, and the slow descent of a feather. The true cause of the 
 contrast is, that the atoms of the feather are much spread out, so as to be 
 more resisted by the air than those of the gold. If the two be let fall toge- 
 ther in a vessel from which the air has been extracted as in the common 
 air-pump experiment, they arrive at the bottom in exactly the same time; 
 
54 MOTIONS AND FORCES. 
 
 and even in the air, if the coin be hammered out into gold leaf, it will fall 
 still more slowly than the feather. One brick dropped from a height, be- 
 cause its motion is not much affected by the air, reaches the earth very 
 nearly as soon as ten bricks let fall near it, whether they be connected or 
 separate as a single horse may reach the goal as soon as ten horses gallop- 
 ing abreast. 
 
 A man's force will move a small skiff quickly, a loaded barge very slowly, 
 and a large ship in a degree scarcely to be perceived. In each case, however, 
 the quantity of motion may be the same, and a true measure of the force 
 which produced it. 
 
 A ball of one pound weight, impelled by a given force, moves twice as 
 fast as a ball of two Bounds impelled with the same; yet, although the 
 velocities are different, the quantities of motion, as ascertained by the rule 
 already given, are equal, and indicate an equality of producing force. 
 
 " The quantity of motion in a body is the measure also of the force or mo- 
 mentum which it can exhibit again" (See the Analysis, 42. ) 
 
 Bodies, owing to their inertia, may be regarded as passive- reservoirs of 
 force or motion, always ready to return as much as they have received. Mo- 
 mentum is the name given to the motion in a body; with reference to the 
 production by it of new motions or the overcoming of resistances, and is but 
 another term for the quantity of motion 
 
 A cannon ball, according to the quantity of motion in it, may have only 
 the force or momentum that will bruise a plank, or it may have enough to 
 penetrate a tree, or even to shoot its rapid way through a block of the hardest 
 stone. 
 
 A block of wood floating against a man's leg with moderate velocity, 
 would be little felt; but a loaded barge, coming at the same rate and press- 
 ing it against the quay, might break the bones; a large ship, again, although 
 moving no faster, would crush his body against any fixed obstacle ; and an 
 island of ice, opposed in its approach to another, even by a first-rate man-of- 
 war, would destroy it, as meeting barges destroy a floating egg-shell. 
 
 A hail-stone falling, strikes rudely ; a stone rolled from a height, as of old, 
 by the besieged against besiegers, may carry death with it to many ; an 
 avalanche, breaking from its hold on a mountain steep, may sweep away a 
 village. 
 
 To meeting bodies, the shock is the same, whether the motion be shared be- 
 tween them or be all in one. 
 
 If a running man come against a man who is standing, both receive a cer- 
 tain shock. If both be running at the same rate in opposite directions, the 
 shock is doubled. In some such cases, as where swift skaters have met, the 
 shock has proved fatal. 
 
 The meeting fists of boxers not unfrequently dislocate or break bones. 
 
 A man's skull is fractured as certainly by its being dashed against a tree 
 or beam, while he is on a galloping horse, as by a blow of a similar beam 
 coming upon him with the velocity of the horse. 
 
 When two ships in opposite courses meet at sea, although each may be 
 sailing at a moderate rate, the destruction is often as complete to both as if 
 with a double velocity they had struck on a rock. Many melancholy in- 
 stances of this kind are on record. In the darkness of night a large ship has 
 met one smaller and weaker, and in the lapse of a few seconds, have followed 
 
DIRECTION OF FORCES. 
 
 55 
 
 the shock of the encounter, the scream of the surprised victims, and the hor- 
 ribly silence when the waves had again closed over them and their vessel for 
 ever. In November, 1825, on the coast of Scotland, the Comet steamboat 
 was thus destroyed, and carried to the bottom with her about seventy pas- 
 sengers, into whose ears the drowning water rushed before the sound of ar- 
 rested music and joy had died away, 
 
 " Direction of the force or forces producing motion." 
 
 When only one force acts on a body, the body obeys in the exact direction 
 of the force. 
 
 A ball floating in water, or lying on smooth ice, is driven exactly south 
 by a wind blowing to the south. A bullet issues from the mouth of a cannon, 
 in the direction of the axis of the cannon which is, as the force impels it. 
 
 "When two or more forces, not in the same direction, act upon a body at the 
 same time, as it cannot move two ways at once, it holds a middle course 
 between the directions. This course is called the resulting direction, viz., 
 resulting from the composition of the forces. 
 
 A ball or ship moving south by a direct wind, may, at the same time, be 
 carried east, just as fast, by a tide or current moving east ; every instant, 
 therefore, it will go a little south and a little east, and really will describe 
 a middle line pointing south- east. 
 
 These particulars may be well represented on paper, as by fig. 3 : where 
 b is the original place of the ball or ship, e the east, s the 
 south, and b a the middle line pointing to the south-east, 
 and showing the true ceurse of the vessel. This figure is 
 called the parallelogram of forces, and is an important help 
 to the understanding of many facts in natural philosophy. 
 The minute investigation of the subject belongs to the 
 science of measures, or technical mathematics ; but the 
 general truths are quite intelligble to common sense, or 
 the mathematics of common experience. 
 
 When two forces act upon a body, like the wind and 
 tide in the last example, the result is the same, whether they act together or 
 one after the other. For instance, if the wind drive a vessel one mile south, 
 as from b to s, fig. 3, and immediately afterwards the tide drive it one mile 
 east, as s to a, the vessel will be in the same place at last, viz., at a, as if she 
 had been driven at once south-east, in the line b a, by the simultaneous 
 action of the two. Therefore, by drawing the lines b s and b e to represent 
 the force and direction of the two causes of motion, and by then adding one 
 of them, or an equivalent, to the end of the other as s a to b s, or e a to 
 b e, the square or parallelogram is sketched, of which the middle line or 
 diagonal, as it is called, shows the resultant of the forces, and the true 
 course of the body obeying them. 
 
 What is thus true of the effect of continued forces like wind and tide is 
 
 Fig. 3. 
 
 Fig. 4. 
 
 Fig. 5. 
 
 Fig. 6. 
 
 Fig. 7. 
 
56 " MOTION SAND FORCES. 
 
 true also of momentary impulses, like the blows of clubs simultaneously 
 striking a ball, or of two billiard-balls striking a third. 
 
 When the forces exactly cross each other, and are equal, as in the case of 
 the ship above supposed, the figure becomes a square, at fljg. 3 ; but if one 
 of the forces be greater than the other, the figure becomes oblong, as at fig. 
 4 ; if the forces cross obliquely, the fig. becomes as at fig. 5 ; and if they 
 cross in an opposing direction, it will be as at fig. 6. In all the cases, 
 however, the diagonal still shows the result. It is evident that the same line 
 may be the diagonal of many figures, as seen in b a at fig. 7 } and therefore, 
 that very different degrees and directions of combined forces may produce 
 the same result. 
 
 Forces crossing each other so obliquely as to be represented by lines drawn 
 in almost opposite directions, would form a parallelogram having scarcely 
 any breath, that is to say, the diagonal would approach to nothing; showing 
 thus, that opposing forces neutralize or destroy each other. In fig. 6, by 
 reason of this crossing, the resultant is less than either of the constituents. 
 And for the same reason, when forces cross so acutely as to advance nearly 
 parallel to each other, the resultant is longer than either, as seen in fig. 5. 
 Forces directly opposed, or entirely agreeing in direction, give as their re- 
 sultant their difference or their sum. 
 
 Forces crossing each other directly, or at right angles, as is true of the 
 exactly eastward force b e, and the exactly southward force b s, in figures 3 
 and 4, do not in the slightest degree neutralize or alter each other, for the 
 body, when arrived at a, is just as far east as it would be at e, and as far 
 south as it would be at s. This explains why the progressive motion of the 
 planets in their orbits is not at all affected by the directly crossing centri- 
 petal force of gravity which keeps them at their due ( distances from the sun. 
 In all cases where the two crossing forces are equal, with whatever ob- 
 liquity they cross, the resulting direction must be midway between them. 
 Thus a boat impelled by oars, goes straight, although the direction in which 
 the oar acts is constantly changing ; because the changing obliquity of the 
 force is always the same on both sides. This explains also why a bird fly- 
 ing, or a man swimming, holds a perfectly straight course, although in both 
 cases the direction of the impelling forces is constantly varying. And it 
 explains why a body suspended, as a plummet, or falling to the earth as an 
 apple does from a tree, is always in a line towards the centre of the earth : 
 for, while the part of the earth immediately under the body is pulling it 
 straight down to the centre, the action of parts on any one 
 side of the perpendicular is exactly counterbalanced by the 
 action of corresponding parts on the opposite side ; and the 
 perpendicular is still the diagonal or middle line of every 
 pair of attracting parts. In fig. 8, b a represents the common 
 diagonal. In speaking of the attraction of our earth, there- 
 fore, which really is the united attraction of all the individual 
 atoms, we may always consider it as a single force acting to- 
 wards the centre of the earth. 
 When a body is carried below the surface of the earth, its weight becomes 
 less, because the matter then above it is drawing it up, instead of down, as 
 before. A descent of a few hundred feet makes a sensible difference, and 
 at the centre of the earth, if man could reach it, he would find things to 
 have no weight at all ; and there would be neither up nor down, because 
 bodies would be attracted equally in all directions. 
 
 When more than two forces act on a body, the resulting direction may be 
 
DIRECTION OF FORCES. 
 
 57 
 
 found, first of two, and then of the last resultant with each of the others 
 successively : or the forces may be represented on paper by lines tacked 
 together, of which one denotes the strength and direction of each : the ex- 
 tremity of the last line will mark the place of the body after being acted 
 upon by the combined forces. A sailor, to know the true place of his ship 
 and the course which she has steered, considers, first, the forward progress 
 as found by the log, then the leeway or sideward motion produced by a 
 cross wind, and then the effect of any tide or current in which he may be 
 sailing. 
 
 Resolution of Forces is a phrase pointing to another important use of such 
 parallelograms or figures as have just been described, viz., the enabling us 
 when force or motion is given, to find the forces or motions in any other 
 directions of which it may be the resultant, and those into which it may 
 itself be resolved. 
 
 Thus, if a line b a (in any of the preceding figures 4, 5, 6, &c.) represent 
 a force or motion, and the line b s represent one of two elements composing 
 it, we have but to complete the parallelogram b s a e to obtain the other 
 line, b e representing the only other force or motion which, combined with 
 the first element, can produce the given resultant. If a ship pass from b to 
 a (fig. 5) while sailing through the water eastward, a distance expressed by 
 b e, she must at the same time have been carried by a tide current to the 
 distance and in the direction marked by the line b s. 
 
 Again, if a line be given representing a single 
 force or motion as b a, and if it be desired to know Fig. 9. 
 
 how much there is in this capable of acting in another jy e 
 
 direction as b d ; it is only necessary to draw a line f\ - C 
 
 in the direction, as b d, from the commencement of 
 
 b a, and to cut such line by another drawn directly 
 
 upon it or at right angles to it, as the term is, from 
 
 the other end of b a : the length of b d, so cut off, 
 
 viz., b s, shows the proportion required. 
 
 It is thus that a sailor who knows how far he has 
 sailed in an oblique direction finds out how much 
 he has gone north and east or south and west j in 
 other words, finds out the difference in latitude and longitude between his 
 present place and a former one. In the above figure, b a may represent the 
 course and distance sailed, b s the difference of latitude, and b e the differ- 
 ence of longitude. 
 
 Thus again, if a ball b strike a table a c, 
 with velocity and direction, both represented Fig 10. 
 
 by the line be; and if the ball be supposed 
 afterwards with the same velocity to approach 
 the table in the oblique directign e c, it will 
 then strike with as much less force than before, 
 as the line e a is shorter than e c. For e a is 
 found according to the rule for decomposing a 
 force, given above; and, to common sense, it 
 is obvious, that if the whole velocity of the 
 ball be represented by e c, the rate of approxi- 
 mation towards the table, or merely downward 
 
 . velocity and therefore the downward force is marked by the line e a. The 
 body only falls through the distance e a while moving all the way from e to c. 
 
 O 
 
58 MOTIONS AND FORCES. 
 
 Figure 10, explains the important cases of the force of wind upon ships 
 sails, windmill vanes, &c. ; and the force of water upon float-boards, water- 
 wheels, &c. ; showing that the moving mass exerts force upon a surface, 
 not in proportion to the speed with which it may he passing along or near the 
 surface, but to the rate of perpendicular approximation. It explains also, why 
 the slanting blow of a club or ball is so light, compared with the direct blow. 
 
 " The two great forces of Nature ore Attraction and Repulsion." (Read 
 
 the Analysis.) 
 
 A person, on first approaching this subject, is far from supposing that the 
 beautiful and almost endless variety of phenomena exhibited in the universe 
 around, all are referrable to the two principles, attraction and repulsion^ 
 examined in the first section : but such is the truth. It will first be shown 
 here, how the great classes of accelerated, retarded, and bent motions arise 
 from them. 
 
 Attraction. Until Newton said, that what we call weight of bodies is 
 merely an instance of that universal attraction of matter which diminishes 
 with increasing distance, it was never suspected that weight was less, high 
 up in the air than on the ground ; or on a lofty mountain than on the sea- 
 shore. But this we now know to be the case. However, in studying what 
 goes on in obedience to gravity near the surface of the earth, except in a few 
 very nice cases, gravity may be considered as a uniform power ; for man has 
 neither approached the centre of the earth in mines, nor receded from it in 
 balloons, by more than about a thousandth part of his distance from it; and 
 weight has relation to the distance from the centre, not to the distance from 
 the surface. 
 
 11 Accelerated Motion from Gravity" 
 
 Owing to the inertia of matter, any force continuing to act on a mass 
 which is free to obey it, produces in the mass a quickening or accelerated 
 motion ; for as the motion given in the first instance, continues afterwards 
 without any farther force, merely on account of the inertia, it follows, that 
 as much more motion is added during the second instant, and as much again 
 during the third, and so on. A falling body, therefore, under the influence 
 of attraction, is, as it were, a reservoir, receiving every instant fresh velocity 
 and momentum. 
 
 It is said that Newton's sublime genius read the nature of attraction in 
 the simple incident of an apple falling before him from a lofty branch in 
 his garden. The eye which perceives an apple beginning to fall, can follow 
 it for a time and mark the gradual acceleration of its descent, but soon sees 
 its path only as a shadowy line. 
 
 A boy letting a ball drop from his hand, can catch it again in the first 
 instant, but after a litttle delay his hand pursues it in vain. 
 
 A fragment of rock, detached from thejwow of a hill by the lightning 
 stroke, begins its motion slowly; but once fairly launched, it gathers fresh 
 speed and*momentuni with every instant, and bounds from steep to steep 
 driving every obstacle before it. 
 
 Any liquid falling from a reservoir, forms a descending mass or stream, of 
 which the bulk diminishes from above downwards, in the same proportion as 
 the velocity of the particles increases. This truth is well exemplified in the 
 pouring out of molasses or thick syrup; if the height of the fall be con- 
 siderable, the bulky sluggish mass, wliich first escapes, is reduced, before it 
 reaches the bottom, to a small thread; but the thread is moving proportion- 
 ately faster, and fills the receiving vessel with surprising rapidity. The same 
 
MEASURE OF ATTRACTION. 59 
 
 truth is exhibited on a vast scale in the falls of Niagara; where the broad 
 river is seen first bending over the precipice a deep slow moving mass, then 
 becoming a thinner and a thinner sheet as it descends, until at last, sur- 
 rounded by its foam or mist, it flashes into the deep below, apparently with 
 the velocity of lightning. 
 
 When velocity becomes considerable in any case of falling; it cannot be 
 measured accurately by thereye, but its effects ascertain it. A man leaps 
 from a chair with impunity, from a table with a shock, from a high window 
 with fracture of his bones, and in falling from a balloon his body is literally 
 dashed to pieces. 
 
 The force of gravity or general attraction is such at the surface of this 
 earth, that, in the first second of time, it gives to a body allowed to fall a 
 velocity of 32 feet nearly per second, that is, a velocity which, remaining 
 uniform from the end of the second, would carry it, without farther action 
 of gravity, through 32 feet in the next second Yet the body falls only 16 
 feet in the first second; and the reason is, that the velocity of 32 feet pos- 
 sessed at the end of the second is gradually acquired, the body having only 
 half of it at the half second, and as much less than half at any distance be- 
 fore that time, as it has more than half at the same distance afterwards; and 
 the average, therefore, is only half of the 32, or 16 feet in the whole second. 
 In the next second, it falls, of course, through the whole 32 feet, with 16 
 additional, from the new action of gravity, in all three times as much as in 
 the first second; and in two seconds, therefore, it falls altogether four times 
 as far as in one second. At the end of two seconds the velocity is doubled 
 or is 64 feet per second, so that in the third second the body falls 64, and 
 other new 16, in all five times as much as in the first second ; and in three 
 seconds, therefore, it has descended nine times as far as in one second, &c. 
 Knowing this progress, the velocity acquired by a falling body, and the dis- 
 tance through which it falls, in any given time, are easily calculated ; and 
 the height of a precipice, or the depth of a well, may be ascertained by 
 marking the time required for a body to fall through the space. 
 
 The doctrines of falling bodies are of such importance in the minute ex- 
 amination of many of the phenomena of nature, that much attention has been 
 bestowed upon them. Mr. Atwood's ingenious contrivance by which the 
 motion of falling bodies may be retarded in any desired degree, without the 
 character of the motion being otherwise altered, has enabled experimenters 
 to render evident to the senses all that abstract calculation had anticipated. 
 A pound weight, left quite free, falls towards the ground, sixteen feet in the 
 first second, proving that attraction of one pound is just sufficient to over- 
 come the inertia of one pound at that rate. But if the inertia were doubled, 
 or tripled, or increased in any other degree, the fall of course would be just so 
 much slower. Now Mr. Atwood's machine in effect increases 
 it, by causing falling weights to overcome not only their own Fig. 11. 
 inertia, but also that of other weights; fig. 11. Thus a and b, 
 being weights of two pounds each, balancing each other over 
 the very easily turned pulley c, are moved by a weight of one 
 pound d, hooked to one of them ; and gravity in pulling this 
 down, with force of one pound, has to overcome, not the inertia 
 of one nound, but of five, for the other two weights must move 
 as fast as the one pound does ; and thus, the velocity being 
 reduced to one-fifth of what is natural to a falling body, the d~* <i 
 
 descent can be minutely observed. The experiments with At- a i ] 1 Jb 
 
 wood's machine may be varied exceedingly, and they are most Fj 
 interesting. 
 
60 
 
 MOTIONS AND FORCES. 
 
 11 Retarded Motion" from gravity. 
 
 What has been said of the changing velocity of a falling body, from gra- 
 vity, is exactly true, in a reversed way, respecting a rising body exposed to 
 the same influence. 
 
 A bullet shot directly upwards, every instant loses a part of its velocity, 
 until at last it comes to rest in the sky, where a soaring eagle might see 
 the messenger of death motionless and harmless for a moment by his side : 
 the ball then descends again, and so that, at corresponding points of the 
 ascent and descent, but for the resistance of the air, the velocities would be 
 equal; and, on reaching the ground, it would have acquired exactly the 
 velocity with which it first departed. 
 
 It is explained in a preceding paragraph, that a body falls four times as 
 far in two seconds as in one, although the velocity at the end of two seconds 
 is only doubled. For the same reason, a body shot upwards with double 
 velocity, rises four times as far as if shot with a single velocity; if shot with 
 triple velocity, it rises nine times as far, and so forth. 
 
 In aiming for amusement at bodies thrown up into the air, it is easy to 
 hit them near their point of turning, and more difficult always as they are 
 nearer to the ground, whether rising or falling. 
 
 An upward jet of water is small below, where it issues from the pipe with 
 great velocity, but it becomes more bulky as the water loses velocity in 
 ascending, and at the top it often spreads a little like a palm tree, and any 
 light round solid will continue supported and playing upon its summit. 
 
 The rise of a pendulum from the bottom of its arc, is an exact copy, re- 
 versed, of its previous descent to that point. 
 
 " The Pendulum" 
 
 exemplifies well both accelerated and retarded motion. The name is appli- 
 cable to any body so suspended, that it may swing freely backwards and 
 forwards. When such a body is made of certain form and length, although 
 so simple, it is one of the most admirable contrivances of man's ingenuity. 
 
 Galileo having observed the hanging chandeliers of lofty ceilings to con- 
 tinue vibrating long and with singular uniformity, after any accidental cause 
 of disturbance, was led to investigate the laws of the phenomenon ; and out 
 of what, in some shape or other, had been before men's eyes, but uselessly, 
 from the beginning of the world, his powerful genius extracted the most 
 important results. Independently of the light which the theory of the pen- 
 dulum has thrown on various branches of physics, the instrument itself, with 
 a few wheels attached, to record its vibrations, has' now become the perfect 
 time-keeper, regulating many of the affairs of men. 
 
 A common pendulum consists of a ball, fig. 12, as a, suspended from a rod 
 
 from a fixed point as b, and made to swing 
 backwards and forwards, or to vibrite un- 
 der this point. Being raised to c, and then 
 set at liberty, it falls back to a with an ac- 
 celerating motion, like a ball rolling down 
 a slope, and when arrived there, it has just 
 acquired momentum enough to carry it to 
 d, at an elevation on the other side; from 
 this it falls back again, again to rise ; and 
 would so go on for ever, but for the impedi- 
 ments of air and friction. The pendulum 
 is strictly an object of mathematical study; 
 
 Fig. 12. 
 
PENDULUM. 61 
 
 but we shall give a general idea of its important characteristics in common 
 language. 
 
 1. The times of the vibrations of a pendulum are very neary equal, 
 whether it be moving much or little, that is to say, whether the arc described 
 by it be large or small. This remarkable property is what makes a time- 
 keeper. The reason that a large vibration is performed in the same time as 
 a small one, in other words, that the pendulum always moves faster in pro- 
 portion as its journey is longer is, that in proportion as the arc described 
 is more extended, the steeper are its beginning and ending, and the more 
 rapidly, therefore, the pendulum falls down at first, sweeps along the inter- 
 mediate space, and stops at last. * It is evident, for instance, that the portion 
 c e of the arc (fig. 13) is much more steep than the equal portion e c. 
 A pendulum made to vibrate in the curve called a cycloid, which, in the 
 central part, very nearly coincides with a circular arc, but towards the ex- 
 tremity rises a little more steeply, has its beats perfectly isochronous, or in 
 equal times, whatever their extent. 
 
 A common clock is merely a pendulum with wheel-work attached to it, to 
 record the number of the vibrations, and with a weight or spring having force 
 enough to counteract the retarding effects of friction and the resistance of the 
 air. The wheels show how many swings or beats of the pendulum have 
 taken place, because at every beat, a tooth of the last wheel is allowed to 
 pass. Now if this wheel has sixty teeth, as is common, it will just turn 
 round once for sixty beats of the pendulum, or seconds, and a hand fixed 
 on its axis projecting through the dial-plate, will be the second hand of the 
 clock. The other wheels are so connected with the first, and the numbers 
 of teeth on them so proportioned, that one turns sixty times slower than the 
 first, to fit its axis to carry a minute hand, and another, by moving twelve 
 times slower still, is fitted to carry an hour hand. 
 
 2. The length of a pendulum influences the time of its vibration. Long 
 pendulums virbrate more slowly than short ones, be- 
 cause, in corresponding arcs or paths, the hob or ball Fig. 13. 
 
 of the long pendulum has a greater journey to per- 
 form, without having a steeper line of descent. If a 
 pendulum b a be twice as long as another reaching 
 from b to e, it has twice as much to fall in its descend- 
 ing arc c a, as the other jn its arc d e, while in cor- 
 responding parts of the two paths, the slope or incli- 
 nation is always equal : -the ball of the long pendulum 
 may be considered as having rolled twice as far down 
 a given slope as the ball of the short pendulum. Now 
 as a body falls four times as far, either directly or on 
 any uniform slope, in two seconds, as in one, a pendu- 
 lum must be four times as long, to beat once in two 
 seconds, as to beat every second. A pendulum of a Jittle more than 39 
 inches beat seconds ; one of four times the length is required to beat double 
 seconds, and one of one-fourth the length to beat half seconds. As a pen- 
 dulum to answer its purpose must be of invariable length, one which beats 
 seconds constitutes an easily found standard of measure. 
 
 Because the smallest change in a length of a pendulum alters the rate of 
 going of the clock, it is important to be able to counteract the dilatation or 
 contraction of pendulums caused by the changing heat of the seasons ; and 
 for this purpose various ingenious means have been contrived. One of the 
 best of these is the gridiron pendulum, as it is called, from consisting of various 
 
62 
 
 MOTIONS AND FORCE 
 
 rods of metal. It renders the different dilatability by heat of two 
 Fig. 14. metals composing it, the cause of unchanged length in the whole, 
 o The adjoining sketch may show that if the central rod of brass 
 
 represented by the strong line from b to c, dilate alone just as 
 much as the two rods of steel, represented by the weaker lines on 
 either side of the other, dilate together (the expansion of brass by 
 heat is about double that of steel,) it will exactly counteract the 
 lengthening of these, and will keep the ball d always at the same 
 distance from the point of suspension a. Some astronomical 
 clocks in the present day are so perfect that they do not err one 
 beat of the pendulum in a year. Common clocks are regulated 
 by a screw which lifts or lets down the ball of the pendulum, and 
 so changes the effective length, that is, the distance between the 
 point of suspension and what is called the centre of oscillation, 
 treated of in the next chapter. 
 3. The force of gravity, of course, is what determines how long the pen- 
 dulum shall be in falling to the bottom of its arc, and how long in rising, for 
 the ball of the pendulum, as already stated, may be considered as a body 
 descending by its weight on a slope; a change in the force of gravity, there- 
 fore, would at once alter the rate of all the clocks on earth. At the equator 
 of our earth, where the gravity of bodies is counteracted in a small degree 
 by the centrifugal force arising from the earth's motion (as explained at page 
 53,) a pendulum vibrates more slowly than elsewhere, and must therefore be 
 made shorter to answer the same purpose. Corresponding results take place 
 when a pendulum is carried to a mountain top, and therefore farther away 
 from the centre of the earth, which is the centre of attraction or when car- 
 ried to the bottom of a mine, where it is attracted by the matter above it, as 
 well as by the matter beneath. 
 
 The popular prejudice refuted at page 53, that a large or heavy body 
 should fall to the earth, even in 'a vacuum, more quickly than a small or 
 light body attaches itself also to the case of a heavy and a light pendulum. 
 Now there is'no difference for pendulums of the same kngth, whatever their 
 weight or material, but what depends on the resistance of the air. It is a 
 very remarkable fact thus proved, that in all substances the gravity and 
 inertia perfectly agree. 
 
 There is a small pendulum called a metronome, used 
 Fig. 15. by musicians for marking time ; which, although very 
 
 short, may still be made to beat whole seconds, or even 
 longer intervals. The reason of its slow motion is, that 
 its rod is prolonged beyond its axis of support, at a, up- 
 wards, to b } and has a ball upon the top at b } as well as 
 on the bottom at c ; which upper ball prevents the under 
 one from moving so fast as it otherwise would, just as a 
 small weight attached to one end of a weighing-beam, 
 prevents a greater weight attached to the other end from 
 falling so fast as it would if there were no counterpoise. 
 The rate of motion changes with any change in the dis- 
 tance of the ball b from the centre of motion a ; and to 
 allow of such change, the ball is made to slide. 
 
 A pocket-watch differs from a clock in having a vibrating wheel instead 
 of a vibrating pendulum ; and as, in a clock, gravity is always pulling the 
 pendulum down to the bottom of its arc, which is its natural place of rest, 
 but does not fix it there, because the momentum acquired during its fall 
 
PENDULUM. 63 
 
 from one side is just sufficient to carry it up to an equal height on the other 
 so in a watch, a spring, generally spiral, surrounding the axis of the ba- 
 lance-wheel is always forcing this towards a middle position of rest, but does 
 not fix it there, because the momentum acquired during its approach from 
 either side to the middle position, carries it .just as far past on the other side, 
 and the spring has to begin its work again. The balance-wheel at each 
 vibration allows one tooth of the adjoining wheel to pass, as the pendulum 
 does in a clock, and the record of the beats is preserved by the wheels which 
 follow, as already explained for the clock. A main-spring is used to keep 
 up the motion of a watch, instead of the weight used in a clock ; and as a 
 spring acts equally, whatever be its position, a watch keeps time although 
 carried in the pocket or in a moving ship. 
 
 As the rate of a clock is influenced by the length of its pendulum, so is 
 the rate of a watch by the size or diameter of its balance-wheel ; and heat, 
 which retards the motion of a common clock by lengthening the pendulum, 
 retards the motion of a common watch by dilating the balance-wheel. Inge- 
 nuity, however, has found a remedy for the latter case as for the former, 
 viz., the contrivance called the expansion balance-ivheel. Of this the cir- 
 cumference, instead of being a continuous ring, is made up of two half-rings, 
 each attached by one end only, to a cross bar, and which half rings being of 
 brass on the outside and of steel within, bend or curl inwards by heat as 
 a sheet of damp paper bends when held to the fire and thus diminish the 
 size of the wheel at their loose extremities, so as just to counter balance its 
 increase by the expansion of the cross bar. 
 
 As the motion of a pendulum has relation to the force of gravity, so has 
 the motion of a balance-wheel to the stiffness of the balance-spring ; and the 
 regulator of a watch is merely a pin which bears against the balance-spring, 
 and by sliding backwards or forwards, so as to shorten or lengthen the part 
 of the spring left free to act, changes the degree of its stiffness. A change 
 produced by the variation of temperature is compensated for by the expan- 
 sion-wheel described above. 
 
 It would be exceeding the limit marked out for this general work, to speak 
 more particularly here of those admirable watches which have been produced 
 within the last thirty years under the name of chronometers, for the pupose 
 of ascertaining the longitude at sea; but the author may perhaps be excused 
 for mentioning a moment of surprise and delight which he experienced,* on 
 first seeing their singular perfection actually proved. After months spent 
 in a passage from South America to Asia, his pocket chronometer, with 
 others on board, announced one morning that a certain point of land was 
 then bearing east from the ship at a distance of fifty miles ; and in an hour 
 afterwards, when a mist had cleared away, the looker-out on the mast gave 
 the joyous call of a Land a-head !" verifying the report of the chronometers 
 almost to a mile after a voyage of thousands. It is natural, at such a 
 moment, with the dangers and uncertainties of ancient navigation before the 
 mind, to exult in contemplating what man has now achieved. Had the rate 
 of the wonderful little instrument in all that time been changed even a little, 
 its announcement would have been worse than useless, but in the night 
 and in the day, in storm and in calm, in heat and in cold, while the persons 
 around it were experiencing every vicissitude of mental and bodily condition, 
 its steady beat went on, keeping exact account of the rolling of the earth 
 and of the stars ; and in the midst of the trackless waves it was always ready 
 to tell its magic tale of the very spot of the globe over which it had arrived. 
 The mode of using a chronometer for so valuable a purpose will be explained 
 in the section on ^astronomy. 
 
64: MOTIONS AND FORCES. 
 
 Bent or curvilinear motion from attraction. This takes place whenever 
 attraction is acting across the path of an existing free motion. The flying 
 cannon-ball or stone, drawn down by gravity, is an example, for the pro- 
 jectile force ceases with the first impulse, but the bending force is acting 
 every instant and by every instant producing a new effect, causes a cur- 
 vilinear path. 
 
 An oblique jet of water is to the eye a permanent exhibition of the curve 
 described by a body thus projected. The particles of the liquid move in the 
 line which they would describe if projected singly, and the continued suc- 
 cession of them marks the line of situations through which each passes in its 
 course to the earth. 
 
 A cannon or musket-ball, shot quite horizontally over a level plain, will 
 touch the ground or plain just as soon as another ball dropped at the same 
 instant directly from the cannon's mouth ; for the forward or projectile motion 
 does not, in such case at all interfere with the action of gravity. This re- 
 sult, which most persons, before consideration, would be disposed to doubt, 
 makes strikingly sensible the extraordinary speed of the cannon-ball; viz., 
 that it has already moved, perhaps, six hundred feet forward, during the 
 half second that a ball dropped from the hand of a standing person requires 
 to reach the earth only four feet beneath. This fact also explains why, for 
 a long range, the gun must be pointed more or less upwards. 
 
 A dozen marbles swept horizontally from off a table by a stick, all reach 
 the floor at the same instant, how different soever the distance to which 
 they may respectively be driven. 
 
 The particular study of the subject projectiles is very important to mili- 
 tary engineers ; and we know how successfully they have pursued it by the 
 precision with which they now direct their shot and shells to objects at very 
 great distances. 
 
 A cannon-ball shot horizontally from the top 
 
 Fig. 16. of a lofty mountain, would go three or four 
 
 miles. (The mountain is here represented on 
 an enlarged scale, as standing on the globe b, c, 
 d, at a.) If there were no atmosphere to resist 
 its motion, or if the mountain top were above 
 the surface of the atmosphere, the same original 
 velocity would carry it thirty or forty miles be- 
 fore it fell, as to b : with more force still, it 
 would reach to c, and with still more to d. And 
 if it could be dispatched with about ten times 
 the velocity of a common cannon-shot, it would 
 not have approached nearer to the earth than at 
 first, even when it had again reached round e 
 
 or to a ; and its velocity being undiminished, it would perform a second 
 similar tour, and then a third, and so forth : it would, in fact, have become 
 a little satellite, or planetary body, revolving round the earth. In the suc- 
 cessive ranges represented in the figure, it is seen that the centrifugal force 
 of the ball, or its tendency to move in a straight line becomes more and 
 more nearly a counterbalance to gravity, and at last is exactly equal to it. 
 If the force given to the ball were more than sufficient to bring it round 
 again to the level of a, it would for a time fly off, or increase its distance 
 from the earth, acquiring somewhat the eccentric motion of a comet. There 
 may really be such revolving masses above our atmosphere, although invisible 
 to us, owing to their srnallness. It has been supposed by some, that the 
 
PROJECTILES.. 65 
 
 meteoric stones, which fall to the earth every now and then, came from such 
 bodies, or are the entire masses, having become entangled in our atmosphere, 
 so as to lose their forward velocity. The four little planets discovered lately 
 beyond the orbit of Mars, are not larger than a six-thousandth part of our 
 earth. 
 
 REPULSION, produces accelerated, retarded and lent motions, like attrac- 
 tion, but it acts only at minute distances, while attraction draws from the 
 sun, or from the very limits of the universe ; repulsion acts, for instance, be- 
 tween the adjoining atoms of an elastic fluid. Yet repulsion plays a part in 
 the economy of nature, not at all inferior to its sister attraction. We have 
 already seen, when considering the constitution of masses, in section first, 
 that repulsion prevents or modifies the contact of the atoms of all bodies ; 
 that with increase of temperature, it causes these atoms to separate, and of 
 a solid forms a liquid, or even an air; that it operates around all masses as 
 if it were a film or covering, preventing their mutual cohesion, &c., &c. 
 
 Accelerated motion from repulsion is seen when the atoms of gunpowder 
 explode and propel the bullet from the bottom of a piece to the muzzle with 
 such rapidly increasing velocity. The strength of this repulsion of gun- 
 powder is so much greater than the strength of gravity or common attraction, 
 that its action on a bullet, during the passage along a barrel of five or six 
 feet in length, may not be overcome by gravity, during an ascent of a mile 
 or more. 
 
 A visible retarded motion from repulsion is exemplified by a moving body 
 coming against a spring or a bladder full of air, or against the piston-handle 
 of an air-syringe, so as to compress the air beneath it. 
 
 Any elastic body striking against another body and recoiling, exhibits in 
 conjunction the phenomena of retardation, acceleration, and often also of 
 bending, chiefly from repulsion ; for instance : 
 
 An ivory ball, driven forcibly "against a marble slab, does not stop at the 
 instant that apparent contact takes place, but still advances and compresses 
 that part of the substance which is against the marble, as is proved by the 
 facts mentioned at page 37. While this compression of the ivory is going 
 on, the resistance made by the increasing repulsion of the particles gradually 
 retards, and ultimately destroys the forward motion of the ball ; and at the 
 instant of its final arrest, the parts in contact, both of the ball and of the mar- 
 ble, being in their greatest degree of compression, act on the ball, and repel 
 it again with gradually accelerating motion, until it leaves the marble with 
 the same velocity which it had on approaching. The retardation and accel- 
 eration take place here within so small a space, and in so short a time, that 
 they are not apparent to sense, but the mind perceives the nature of the phe- 
 nomenon as distinctly as if the ball had rolled against the end of a long steel 
 spring. If the ball strike the marble obliquely, as from a to c, in a path form- 
 ing the angle acd, with a perpendicular line, it does not rebound in the same 
 line by which it approached, but just as obliquely to- 
 wards the other side, viz., from c to 6; and it then exhibits 
 a bent motion from repulsion. This case illustrates also 
 the " resolution of motions," for the oblique descent a c 
 being composed of a direct downward motion from a to 
 the table, and a horizontal or forward motion from a to- 
 wards the perpendicular, the table destroys the down- 
 ward motion, and converts it into an opposite directly 
 upward motion, but it does not affect the forward mo- 
 tion, which immediately combines again with the up- 
 
 5 
 
66 MOT I.ONS AND FORCES. 
 
 ward, and carries the ball as far beyond the perpendicular at b as it was 
 distant from it at a. The important law in physics, of which this case is 
 an example, is usually expressed '' The angles of incidence and of reflec- 
 tion are equal." It applies to all reflected bodies, as balls, waves, sound, 
 light, &c. 
 
 If the ivory ball and marble, in the above case, were supposed to be both 
 perfectly hard, and without elasticity, still the repulsion which surrounds all 
 bodies, as a thin covering, preventing their cohesion, (see page 32,) would 
 act exactly as the real elasticity of the ivory, and would cause a retarded 
 motion until perfect rest came, and then an accelerated motion back again, 
 until the ball recovered its primitive velocity. 
 
 Collision between hard bodies always exhibits more or less of the truth 
 now described ; when it occurs between soft bodies, as lumps of lead or of 
 moist clay, the approaching parts mutually displace each other, and there is 
 no recoil. 
 
 When a straight steel plate, of which the end is fixed in a block, is bent 
 as by a ball rolling against it, the particles on the side which becomes con- 
 cave are made to approximate, and there is a resistance or repulsion gradu- 
 ally increasing among them ; the particles on the convex side, again, are 
 drawn a little more from each other, and are therefore exerting attraction to 
 return : the recoil of the spring is thus owing to both forces trying to replace 
 the particles in their former relative situations. 
 
 " Tides, Winds, &c., exemplify ATTRACTION." (Read the Analysis, 
 
 page 42.) 
 
 Until we reflect attentively on this subject, we are far from perceiving that 
 all the phenomena of nature are only instances of attraction and repulsion, 
 acting under a variety of circumstances. 
 
 ATTRACTION. Tides are raised by the attraction of the moon and sun, 
 and fall again by the general attraction of the earth; producing in many of 
 the shallower parts of the ocean very rapid horizontal currents. They do a 
 great deal of work for man. They carry his ships along the coasts, and up 
 and down the rivers; they turn water-wheels for him; they fill his docks 
 and canals at convenient times ; they rise to receive his ships, launched from 
 elevated building-yards, &c. What a busy scene is a great sea-port river, 
 during the rising and falling of the tide with the. thousands of people 
 along its bank, borrowing assistance in their various occupations ! 
 
 Winds are produced chiefly by the fluid atmosphere seeking its level, in 
 obedience to the attraction of the earth, after the action of disturbing causes, 
 such as the heat of the sun, &c. They help man in the important business 
 of navigation ; they turn his windmills, &c. 
 
 The currents of rivers are water constantly descending on slopes, that is, 
 regaining its level, in obedience to the earth's attraction. Water-mills and 
 inland navigation are among the advantages which they afford to man. 
 
 All fatting and pressing bodies exhibit attraction in its simplest form. 
 
 REPULSION is instanced in explosion, steam, the action of springs, &c. 
 
 Explosion of gunpowder is repulsion among the particles when assuming 
 the form of air. 
 
 Steam, by the repulsion among its particles, moves the piston of the 
 steam-engine. In our days it performs half the labour of society. 
 
 Accidental explosion of fire-damp, or hydrogen, in mines, and the tre- 
 
PRODUCTION OF GREAT VELOCITIES. 67 
 
 mendous evolutions of elastic fluid in volcanoes and earthquakes, are other 
 instances of the same class. 
 
 Elasticity, as seen in springs, collision, &c., belongs chiefly to repulsion ; 
 as seen in India-rubber, and other substances resuming their usual length 
 after extension, it belongs chiefly to attraction. 
 
 A spring is often, as it were, a reservior of force, kept ready charged for 
 a purpose ; as when a gunlock is cocked, a watch wqund up, &c. 
 
 It will be remarked, with respect to many of the phenomena now and here- 
 after to be mentioned, that it is not the original Attraction or Repulsion 
 which man uses as his servant, but the momentum gradually accumulated 
 in masses by the exertion of such attraction or repulsion ; in other words, 
 the inertia is used as a great working power or force. 
 
 Electrical, galvanic, magnetical, and optical, phenomena, are also in great 
 part peculiar attractions and repulsions, as will be seen in the chapters devoted 
 to the explanation of them. And even the actions of animals, so infinitely 
 varied, are all results of a shortening of the fleshy threads called muscular 
 fibres, which is produced by the mutual attraction of their component parti- 
 cles ; just as the varied motion of a telegraph, or of a ship's yard, are pro- 
 duced by the shortening of certain ropes of connection. 
 
 Howeyer closely allied the last-mentioned particular attractions and repul- 
 sions may be to the general attraction and repulsion formerly treated of, it 
 is found convenient to consider them apart. 
 
 In the remarkable phenomena of nature and art, all the motions being caused, 
 as now shown, by Attraction and Repulsion, these forces do not operate by 
 a single impulse, but through a repetition of impulses, or a continued action, 
 of which the effect is gradually accumulated in the inertia of matter. Thus 
 all great velocities and momenta are the terminations of an accelerated 
 motion. ^ 
 
 Meteoric stones falling from great heights, bury themselves deep in the 
 earth by the force of their gradually acquired velocity. 
 
 When the wood-cutters among the Alps launch an enormous tree from high 
 on the mountain side, along the smooth wooden trough or channel prepared 
 for it and in fewer minutes than it traverses miles, it is seen plunging into 
 the lake below; it acquires its frightful velocity, not at once, but through 
 the action of gravity continued during the whole of its descent. 
 
 The shock or blow of the ram of a pile-engine, is not the effect of momen- 
 tary attraction between it and the earth, but of that attraction accumulating 
 motal inertia or power, during the descent of the ram through a space of 
 twenty or thirty feet. 
 
 A common hammer, in its instantaneous shock, has the condensed effect 
 of the arm and of gravity, as accumulated through its whole previous course ; 
 and when a powerful blow is intended, the hammer, or hatchet, or club, or 
 fist in boxing, is lifted high, or carried far back, that there may be time and 
 space for imparting greater power. 
 
 The inferior animals, by many of their actions, illustrate the same truth, 
 and prove their experimental or instinctive acquaintance with it. 
 
 Sea-birds carry shell-fish up into the air, and drop them on smooth stones 
 to break them, and to obtain the food. It is related in Grecian story, that a 
 bird once mistook the venerable bald head of a sage meditating on the sea- 
 
68 MOTIONS AND FORCES. 
 
 shore for a smooth stone, and by the same act killed an oyster and the 
 philosopher. 
 
 There are some long-necked birds, that fight and kill their prey by a blow 
 of their beak. They draw back the head, bending the neck like a swan or 
 serpent, and then dart it forward, with a continued effort, until the strong 
 wedge-like beak reaches its destination, almost with the velocity of a pistol 
 bullet. One snake in darting its fangs at another passing swiftly across its 
 coil, has been known to miss its aim and inflict a mortal wound on its own 
 flesh. 
 
 Bulls, rams and goats, in fighting, alternately recede and run at each 
 other, that the shock may be great when their foreheads meet. 
 
 A horse in kicking, from the great length of his leg, and the consequent 
 space through which he can be adding velocity to his foot, dries at last 
 against the object almost like a cannon shot. 
 
 A bow-string propelling an arrow, follows it through a considerable space, 
 and so gives the great velocity at last produced. 
 
 A sling gives to the hand the power of adding velocity to the stone through 
 a long path ; for the hand moves in a small circle while the stone moves in 
 a larger, and the hand being kept always in somewhat advance of the stone, 
 pulls at it without intermission, until the moment of discharge. 
 
 The battering-rams of the ancients allowed those about them to* accumu- 
 late in them the efforts of many hands, and of a considerable duration of 
 action, so as to give at last one great and sudden shock. 
 
 Even the gentle action of the human breath, exerted for a time on a pea 
 or small hard ball of clay while passing through a long smooth tube, gives 
 a velocity which will inflict a sharp and painful stroke on a distant animal. 
 In Borneo and other of the Eastern Islands, poisoned arrows are thrown in 
 this way with great force and precision. 
 
 The action of gunpowder or bullets, although appearing so sudden, is still 
 not an instantaneous, but a gradual, and therefore accelerating action ; and 
 accordingly we find the effect to depend on the length of the piece along 
 which the force pursues the ball. A small fast sailing vessel with a single 
 long gun, has often compelled a very superior vessel, whose guns were 
 shorter, to yield. 
 
 For the same reason that all great velocities require continued action or re- 
 peated impulse to produce them, so do they also destroy them ; the in- 
 ertia of motion and of rest being equal. v 
 
 A vast mass of rock suspended like a pendulum, and allowed to sweep 
 down its curve to a considerable elevation, would arrive at the bottom like a 
 battering-ram, with force sufficient to shake a thick wall or rampart to its 
 foundation. The continued action of gravity would have given this force, 
 and if, instead of the solid resistance supposed, and which would scarcely be 
 sufficient to take the whole momentum away, the mass were merely allowed 
 to continue its course as a pendulum, and to ascend on the other side, the 
 continued action of gravity then opposing its motion, would bring it to pow- 
 erless rest again, by the time when it had reached an elevation equal to that 
 from which it fell. 
 
 Soft air expanding gives gradually the death-carrying velocity to the can- 
 non-ball ; and soft air, or cotton, or wood, resisting in a close strong tube, 
 if the bullet could be directed exactly into it would again gradually anni- 
 hilate the motion. Were the attempt made, however, to stop the ball sud- 
 
PRODUCTION OF GREAT VELOCITIES. 69 
 
 denly, by a block of the hardest granite, the block would instantly be riven 
 by its force. 
 
 Bales of cotton or thick masses of cork, attached round a ship, will receive 
 cannon-balls, and bring them to rest, without themselves suffering much, 
 while the naked firmer side of the ship would be penetrated. The cotton or 
 cork offers an increasing resistance through a considerable space, while the 
 oak opposes its hard front at once, and must instantly suffice or be destroyed. 
 A hard body, that it may at once destroy such a motion as we are supposing, 
 must be able to oppose as much force in perhaps the space of one-hundredth 
 of an inch, that is, in the extent to which its* elasticity .will let it yield with- 
 out breaking, as the moving cause gave, through a much greater space ( a 
 plate of steel will thus oppose a pistol-bullet; ) and when it cannot do this, 
 it must be broken or penetrated by the moving body. It is to be remarked, 
 however, that the continued opposition of a thick mass of wood, stone, or 
 earth, to an entered bullet, brings it to rest at least as an elastic unbroken 
 opposition would. Gunners have ascertained the exact depth in each sub- 
 stance to which a ball will penetrate ; and they call buildings bomb-proof 
 or ball-proof, which have a thickness or depth exceeding that. 
 
 A hempen or silken rope, supporting the scale of a weighing beam, would 
 resist a greater weight falling into the scale than would be resisted by an 
 iron chain which were even stronger than the rope for the purpose of bearing 
 a quiescent weight; because the hemp or silk would yield by its. elasticity, 
 and continue its resistance through a considerable space and time, and thus 
 would at last gradually overcome the momentum ; while the iron, by scarcely 
 yielding at all, would require to be strong enough to stop the mass suddenly 
 or would break. 
 
 Yet for the same reason that iron is weakest in such a case as the last, it 
 is stronger than hemp or rope when used as a cable for a ship, to withstand 
 the sudden force of waves. 
 
 This will be understood on considering, that the chain by its weight hangs 
 as *a curve or inverted arc in the water, while the rope, being nearly of the 
 weight of water, is supported in it almost as a straight line from the anchor 
 to the ship; therefore, when a great wave dashes against the ship, the bent 
 chain will yield until it be drawn nearly straight, by which great extent of 
 yielding, and consequent length of resistance, it will withstand a great 
 shock ; whereas, the straight rope, as it can yield only .by the elasticity of 
 its material, and comparatively, therefore, a little way, will resist much 
 less. 
 
 A heavy ship moving quickly with the tide or wind, could not be stopped 
 instantly by a short rope or chain of any magnitude : if the attempt were 
 made to destroy at once so vast a momentum, something would certainly 
 give way ; but a rope of very moderate size, kept tight between the shore 
 and the ship, and from time to time allowed to slip a little round a wooden 
 block, when the tightness threatened its breaking, would accomplish the end 
 very soon and easily. 
 
 The following are farther proofs that forces are to be measured as much by 
 the time or space through which they act, as by their difference of inten- 
 sity or momentary power. 
 
 A door standing open, and which would yield readily on its hinges to the 
 gentle push of a finger, is not moved by a cannon-ball piercing through it. 
 Now the ball really overcomes the whole force of cohesion among the atoms 
 
70 MOTIONS AND FORCES. 
 
 of tough wood : but that force is allowed to act or resist for so short a time, 
 owing to the rapid passage of the ball, that it is not sufficient to affect the 
 inertia of the door, in a degree to produce a sensible motion. The cohesion 
 of the circle in the door, cut out by the ball, would have borne a weight of 
 more than a hundred pounds laid quietly upon it, but supposing the bullet 
 to fly twelve hundred feet in a second, and the door to be one inch thick, 
 the cohesion being allowed to act for only the 14,400th part of a second, its 
 influence is not perceived. The following are other examples of the same 
 kind. 
 
 A leaden bullet pressed slowly against a pane of glass, breaks it irregu- 
 larly, where the strength happens to be least ; but the same bullet shot at it 
 from a pistol, makes only a small round hole. It has been amusingly said 
 of such a case, that the particles struck and carried away have not time to 
 warn their neighbours of what is happening. 
 
 A cannon-ball, having very great velocity, passes through a ship's side, 
 and leaves but a little mark ; while one with less speed splinters and breaks 
 the wood to a considerable distance around. A near shot thus often injures 
 a ship less than one from a greater distance. 
 
 A sheet of paper standing edgeways on a table, is not driven down by a 
 pistol-ball fired through it. 
 
 The truth at present under consideration explains, with respect to gun-shot 
 wounds, why the man often remains ignorant for a time of his misfortune, 
 and why a rapid bullet only kills the parts which it touches, while a spent 
 ball may bruise and injure all around. In many cases of injury popularly 
 attributed to the wind of a ball, the ball itself has really reached the part. 
 
 A man lying down and receiving the blow of a great hammer on his chest 
 would be killed by it ; but if a heavy anvil be first laid upon the chest, and 
 the blow then received upon the anvil, the man bears it with impunity. 
 Here the quantity of motion in the hammer being diffused through the great 
 mass of the anvil, produces but a trifling velocity, which the elasticity of the 
 chest, in its slow yielding, easily overcomes. 
 
 A circular plate of soft iron, made to turn with extreme rapidity, will cut 
 through the hardest steel file, almost as a knife cuts through a carrot. In 
 cases where a soft powder suffices to polish a hard body, it acts partly like 
 this plate, by the motion or velocity given to the wearing particles. 
 
 " There is no motion or action in the universe, without a concomitant and 
 opposite action of equal amount." (See the Analysis.) 
 
 This truth has otherwise been expressed " action and reaction are equal 
 and contrary." It is evident, that if no action or movement takes place on 
 earth but in consequence of either Attraction or Repulsion and this has 
 now been shown there must always be two objects or masses concerned, 
 and each must be attracted or repelled just as much as the other, although 
 one will have less velocity than the other, as it may be itself greater, or 
 fixed to another mass. 
 
 If a man in one boat pull at a rope attached to another, the two boats will 
 approach. If they be of equal size and load, they will both move at the 
 same rate, in whichever of the boats the man may be ; and if there be a 
 difference in the sizes, and resistances, there will be a corresponding differ- 
 ence in the velocities, the smaller boat moving the fastest. 
 
 A magnet and a piece of iron attract each other equally, whatever dispro- 
 portion there is between the masses. If either be balanced in a scale, and 
 
ACTION AND REACTION EQUAL. 71 
 
 the other be then brought within a certain distance beneath it, the very same 
 counterpoise will be required to prevent their approach, whichever be in the 
 scale. If the two were hanging near each other as pendulums, they would 
 approach and meet; but the little one would perform more of the journey 
 in proportion to its littleness. 
 
 A man in a boat pulling a rope attached to a large ship, seems only to 
 move the boat : but he really moves the ship a little, for supposing the re- 
 sistance of the ship to be just a thousand times greater than that of the 
 boat, a thousand men in a thousand boats, pulling simultaneously in the 
 same manner would make the ship meet them half way. 
 
 A pound of lead and the earth attract each other with equal force, but 
 that force makes the lead approach sixteen feet in a second towards the earth, 
 while the contrary motion of the earth is of course as much less than this as 
 the earth is weightier than one pound, and is therefore unnoticed. Speak- 
 ing strictly, it is true, that even a feather falling lifts the earth towards it, 
 and that a man jumping kicks the earth away. 
 
 A spring unbending between two equal bodies, throws them off with equal 
 velocity ; if between bodies of different magnitudes, the velocity of the smaller 
 body is greater in proportion to its smallness. 
 
 On firing a cannon, the gun recoils with even more motion or momentum 
 in it than the ball has, for it suffers the reaction of the expelled gunpowder 
 as well as of the ball ; but the momentum in the gun being diffused through 
 a greater mass, the velocity is small, and easily checked. 
 
 The recoil of a light fowling-piece will hurt the shoulder, if the piece be 
 not held close to it. 
 
 A ship in chase, by firing her bow guns, retards her motion ; by firing 
 from her stern she quickens it. 
 
 A ship firing a broadside, heels or inclines to the opposite side. 
 
 A vessel, of water suspended by a cord hangs perpendicularly ; but if a 
 hole be opened on one side, so as to allow the water to jet out there, the 
 vessel will be pushed to the other side by the reaction of the jet, and will so 
 remain while it flows. If the hole be oblique, the vessel will constantly turn 
 round. 
 
 A vessel of water placed upon a floating piece of plank, and allowed to 
 throw out a jet, as in the last case, moves the plank in the opposite direction. 
 
 A steamboat may be driven by making the engine pump or squirt water 
 from the stern, instead of making it, as usual, move paddle-wheels. There 
 is a loss of power, however, in this mode of applying it, as will be explained 
 under the head of " Hydraulics." 
 
 A man floating in a small boat, and blowing strongly with a bellows to- 
 wards the stern, pushes himself onwards with the same force with which the 
 &ir issues from the bellows-pipe. 
 
 A sky-rocket ascends, because, after it is lighted, the lower part is always 
 producing a larger quantity of aeriform fluid, which, in expanding, presses 
 not only on the air below, but also on the rocket above, and thus lifts it. 
 The ascent is aided also by the recoil of the rocket from the part of its sub- 
 stance, which is constantly bursting downwards. 
 
 He was a foolish man who thought he had found the means of command- 
 ing always a fair wind for his pleasure-boat, by erecting an immense bellows 
 in the stern. The bellows and sails acted against each other, and there was 
 no motion : indeed, in a perfect calm, there would be a little backward mo- 
 tion, because the sail would not catch all the wind from the bellows. 
 
72 MOTIONS AND FORCES. 
 
 A man supported on a floating plank, by walking towards one end of it 
 gives it a motion in the direction opposite. 
 
 A man using an oar, or a steam-engine turning paddle-wheels, advances 
 exactly with the force that drives the water astern. 
 
 A swimmer pressing the water downwards and backwads with his hands, 
 is sent forwards and upwards with the same force, by the reaction of the 
 water. 
 
 And a bird flying, is upheld with exactly the force with which it strikes 
 the air in the opposite direction. 
 
 A man pushing against the ground with a stick} may be considered as 
 compressing a spring between the earth and the end of his stick, which 
 spring is therefore pushing him up as much as he pushes down ; and if, at 
 the time, he were balanced in the scale of a weighing beam, he would find 
 that he weighed just as much less as he was pressing with his stick. 
 
 Thus an invalid, on a spring plank or chair, who, by a trifling downward 
 pressure of his hand on a staff or on a table, causes his body to rise and fall 
 through a great range, and thus obtains the advantange of almost passive 
 exercise, is really lifting himself while he presses downward. 
 
 When a boy cries on knocking his head against a table or pane of glass, 
 he is commonly told, and truly, that he has given as hard a blow as he has 
 received ; although his philosophy probably, looking chiefly to results, 
 blames the table for his head hurt, and his head for the glass broken. 
 
 The difference of momentum acquired in a fall of one foot or of several, 
 is well known : the corresponding intensities of reaction are unpleasantly 
 experienced by a man who sits down in an easy chair, or who, in sitting 
 down where he supposed a chair to be, unexpectedly reaches the floor. 
 
 What motion the wind has given to a ship it has itself lost, that is to say, 
 the ship has reacted on the moving air : as is seen when one vessel is be- 
 calmed under the lee of another. 
 
 When one billiard-ball strikes directly another ball of equal size, it stops, 
 and the second ball proceeds with the whole velocity which the first had 
 the action which imparts the new motion being equal to the reaction which 
 destroys the old. Although the transference of motion, in such a case, seems 
 to be instantaneous, the change is really progressive, and as follows. The 
 approaching ball, at a certain point of time, has just given half of its motion 
 to the other equal ball, and if both were of soft clay, they would then proceed 
 together with half the original velocity ; but, as they are elastic, the touching 
 parts at the moment supposed are compressed like a spring between the balls, 
 and by then expanding, and exerting force equally both ways, they double 
 the velocity of the foremost ball, and destroy altogether the motion of that 
 behind. 
 
 If a billiard ball be propelled against the nearest one of the row of balls 
 equal to itself, it comes to rest as in the last case described, while the farthest 
 ball of the row darts off with its velocity, the intermediate balls having 
 each received and transmitted the motion in a twinkling, without appearing 
 themselves to move 
 
 As farther illustrative of the truths, that action and reaction are equal and 
 contrary, and that in every case of hard bodies striking each other, they may 
 be regarded as compressing a very small strong spring between them, we 
 may mention, that when any elastic body, as a billiard-ball, strikes another 
 body larger than itself, and rebounds, it gives to that other, not only all the 
 motion which it originally possessed, this being done at the moment when 
 it comes to rest, but an additional quantity, equal to that with which it recoils 
 
ACTION AND REACTION EQUAL. 73 
 
 owing to the equal action in both directions of the repulsion or spring 
 which causes the recoil. When the difference of size between the bodies is 
 very great, the returning velocity of the smaller is nearly as great as its 
 advancing motion was, and thus it gives a momentum to the body struck 
 nearly double of what it originally itself possessed. This phenomena con- 
 stitutes the paradoxical case of an effect being greater than its cause, and has 
 led persons, imperfectly acquainted with the subject, to seek from the prin- 
 ciple, a perpetuum mobile. A hamifler on rebounding from an anvil has 
 given a blow nearly double the force which it had itself, for the anvil felt its 
 full original force while stopping it, and then, equally with itself, was affected 
 by the repulsion which caused its return. 
 
 Many other interesting facts might be adduced as examples of equal action 
 and reaction, but these will suffice. 
 
 This second section of the work has now explained the nature of INERTIA 
 in matter, and has shown that the infinitely varied phenomena of motion, 
 which the universe exhibits, are only attraction and repulsion, acting on 
 inertia of atoms separate or conjoined, under diversified circumstances. 
 And such is the sublime simplicity of the whole scheme of nature. 
 
74 APPENDIX. 
 
 APPENDIX 
 
 TO PAKT I. SECTION II. 
 
 "V 
 
 BY THE AMERICAN EDITOR. 
 
 THE attentive perusal of the preceding section will prepare the reader to 
 understand the following propositions. 
 
 Definitions. 
 
 Prop. 1. When a body is successively changing its place, it is said to be 
 in motion, p. 42. 
 
 The idea of motion involves those of space, time, velocity, direction, the 
 quantity of matter and momentum. 
 
 Prop. 2. The space described is the distance passed over by a body 
 during its motion ; and is measured by the number of units of length, as a 
 foot, a yard, a mile, &c., contained in this distance. 
 
 Prop. 3. The time consists of a certain number of units of time adopted 
 as its measure, as a second, a minute, &c. ; which have elapsed during the 
 motion of a body. 
 
 Prop. 4. The velocity of a body is the rate at which it moves, or the 
 number of these assumed units of space that it passes over during the as- 
 sumed unit of time. 
 
 All the above measures may be represented graphically by lines that are 
 proportioned to them, p. 65. 
 
 Prop. 5. The direction of a body may be straight or curved ; when 
 straight or rectilinear, it is the angle which its path makes with any straight 
 line in the same plane, adopted as an axis ; when the path of a body is a 
 curve, its direction at any point is the angle which the tangent to the curve 
 at the point makes with the fixed axis. 
 
 Prop. 6. The momentum of a body is its quantity of motion, both the 
 mass and velocity being taken into consideration, and its proper measure is 
 the product of the mass into the velocity, pp. 58, 54. 
 
 Prop. 7. A body is said to have a uniform motion when its velocity 
 remains constant, that is, when it describes equal spaces in equal successive 
 intervals of time, p. 47. 
 
 Prop. 8. Every motion that is not uniform is said to be varied, and is 
 called accelerated or retarded as the velocity increases or decreases. 
 
 Prop. 9. When the velocity constantly increases or decreases in the 
 direct ratio of the time that the body has been moved, the motion is said to 
 be uniformly accelerated or retarded, pp. 43, 58, 59, 60. 
 
 . Prop. 10* Whatever is capable of producing or destroying the motion 
 of a body is called force. 
 
APPENDIX. 75 
 
 
 
 Prop. 11. A force that produces its effect instantaneously, and then 
 ceases to act, is called an impulsive force. 
 
 Prop. 12. A force that acts continually and equally is termed a con- 
 stant force. 
 
 Prop. 13. When the constant force acts in lines directed towards a 
 single point or centre, it is called centripetal, and the path of the body its 
 orbit, p. 59. 
 
 Prop. 14. That part of the impulsive force which tends to make a body 
 move directly from the centre, is termed the centrifugal force, p. 49. 
 
 Prop. 15. A force that is capable of destroying motion without being 
 able, under any circumstances, to produce motion, is termed a passive force. 
 
 Prop. 16. The state of rest produced by the action of opposite forces is 
 termed equilibrium. 
 
 Prop. 17. When a body is struck, its particles yield to the impulse, and 
 the form of the body is changed. When the body possesses the inherent 
 power when thus changed, of restoring its form, it is said to be elastic; 
 when it has not this power, it is called non-elastic, p. 37. 
 
 Prop. 18. A body oscillating below a point to which it has in any way 
 attached, is termed a pendulum, p. 60. 
 
 Laws of Motion. 
 
 Prop. 19. 1st. If a body be at rest it will continue at rest, and if in 
 motion, it will continue to advance uniformly in a right line unless com- 
 pelled to change its state by some external force, pp. 47, 49. 
 
 Prop. 20. 2d. The motion of a body is in the direction of the force that 
 produces it, and is proportional to that force, pp. 53, 55. 
 
 Prop. 21. 3d. Action and reaction are always equal and opposed to each 
 other; or when a body communicates motion to another, it loses of its own 
 momentum as much as it gives to the other body, pp. 70, 72. 
 
 Of Impulsive force and Rectilinear motion. 
 
 Prop. 22. The effect of an impulsive force is to produce uniform rec- 
 tilinear motion, p. 49. 
 
 For during the moment of its action on any body, it must set it in motion 
 with a certain velocity j and by the first law of motion, the body must con- 
 tinue to advance in a straight line with that velocity. 
 
 Prop. 23. In rectilinear motion the space is as the velocity multiplied 
 into the time. 
 
 For if a body move with the velocity of three feet per second, it is evi- 
 dent, that it will move over 6 feet in two seconds, i. e. X 32 ; and 9 feet in 
 seconds, i. e. X 33, and 12 feet in four seconds, &c. &c. 
 
 Prop. 24. The time is as the space divided by the velocity. 
 
 For if.a body passes over 12 feet for instance, when its velocity is 3 feet 
 per second, it is evident, that in order to find the number of seconds, which 
 the body has employed in passing over 12 feet of space, we need only 
 divide 12 by 3, (i. e., the space by the velocity) and the quotient 4 is the 
 time sought. 
 
 Prop. 25. The velocity is as the space divided by the time. 
 
 For if a body move over 12 feet in 4 seconds, its velocity is evidently 
 3 feet per second or 12-^-4. 
 
 The velocities of two bodies may be compared, in the same manner : the 
 velocities of two bodies A and B, for instance, of which A moves over 54 
 
76 APPENDIX. 
 
 
 
 feet in 9 seconds, and B, 96 feet in 6 seconds ; their velocities will be as 
 6 (54-9) to 16 (96-f6.*) 
 
 Of a constant force and uniformly accelerated motion. 
 
 Prop. 26. The effect of a constant force acting upon a body, is to pro- 
 duce in it a uniformly accelerated motion, p. 58. 
 
 For since the effect of force is to produce velocity, a constant force must, 
 in successive instants of time, afford continual and equal additions to the 
 velocity of the body it has set in motion ; that is, the velocity will increase 
 in the direct ratio that the body has been moving, which is the definition of 
 uniformly accelerated ti^otion. 
 
 Prop. 27. In uniformly accelerated motion the space described is as the 
 square of the time, pp. 58, 59. 
 
 Thus it is found by experiment, that if a body move with a gradually and 
 constantly increasing velocity that would carry it through a mile in one 
 minute, that at the end of this time it has acquired such a velocity as would 
 carry it through two miles the next minute, if the force that communicated 
 its motion ceased to act at the end of the first minute ; but if the force con- 
 tinues to act, it acquires a velocity that would carry it over an additional 
 mile, so that it will pass over three miles the second minute, or four miles 
 in two minutes. At the end of the second minute it has acquired a velocity 
 that will carry it over double the space in the third minute, that it moved 
 over in the first two minutes, or a velocity of 8 miles in 2 minutes, or 4 
 miles a minute. But the force still continuing to act, it will move a mile 
 farther or five miles in the third minute. Hence, if a body acted upon by a 
 continued force move a mile the first minute, it will move 3 miles the second, 
 5 the 3d, 7 the 4th, 9 the 5th, &c. 
 
 Thus the spaces described in successive equal parts of time, by uniformly 
 accelerated motion, are always as the odd numbers 1, 3, 5, 7, 9, &c., and 
 consequently the whole spaces are as the squares of the times or of the last- 
 acquired velocities. For the continued addition of the odd numbers yields 
 the squares of all numbers from unity upwards. Thus 1 is the first odd 
 number and the square of 1 is 1 ; 3 is the second odd number, and this 
 added to one makes 4, the square of 2 ; 5 is the third odd number and this 
 added to 4 makes 9, the square of 3 ; and so on for ever. Since, therefore, 
 the times and velocities proceed evenly and constantly as 1, 2, 3, 4, &c., but 
 the spaces described in equal times are as 1, 3, 5, 7, &c., it is evident that 
 the space described, 
 
 In 1 minute will be l=squareofl 
 
 In 2 " " 1+3=4= " 2 
 
 In 3 " " - - 1+3+5=9= " 3 
 
 In 4 - 1+3+5+7=16= " 4 
 
 * For the benefit of those who are acquainted with algebra, we subjoin >he follow- 
 ing equation, which expresses all the circumstances of uniform motion. 
 Let t the time of motion, 
 
 * = the space described in the time j?, 
 v = the velocity; 
 
 Then, s = v t from which we obtain 
 
 * 
 
 v = 
 
 t 
 
 s 
 
 and t = 
 
APPENDIX. 77 
 
 Of Gravity. 
 
 Prop. 28. The force which causes bodies to fall to the earth is of the 
 kind named constant, and is called gravity, p. 58. 
 
 Prop. 29. The direction of gravity is in lines perpendicular to the earth's 
 surface. 
 
 Prop. 30. The force of gravity is directly proportional to the mass of 
 the body. 
 
 For however small the parts into which we divide a body, we find them all 
 affected by gravity, since this force must act upon all the particles of a body. 
 
 Hence, in an unresisting medium, all bodies setting out from a state of 
 rest, fall through the same space in the same time, because the force of 
 gravity acting upon them increases in proportion to the mass to be moved. 
 
 Prop. 31. The force of gravity decreases, as the square of the distance 
 from the attracting body increases. 
 
 This is proved by astronomical observations. 
 
 Motion produced by joint forces. 
 
 Prop. 32. When a body is acted upon at the same moment by a plurality 
 of forces, each of these forces produces its full effect : and the place of the 
 body at the end of any given time is the same as it would have been if the 
 forces had acted in succession each during that time, pp. 55, 56, 57. 
 
 Thus let A B represent the direction of a 
 force that would move a body, A the distance 
 from A to B in a certain interval of time, (a 
 second for example,) and A 0, the direction of 
 a force that would propel the same body from 
 A to C in the same interval of time. Suppose 
 the first force acted alone, it would move the 
 body from A to B in one second ; if the force A C then acted at B, by drawing 
 B R equal and parallel to A C, B R will represent the direction and velocity 
 of the force A C, and R the position in which the body would be in at the 
 end of the second interval of time. Unite A and R and the line A R will 
 represent the course of the body A if acted upon at the same moment by 
 the two forces A' B and A C, and R the position of the body at the end of 
 the first interval of time. 
 
 In the same manner the action of anynum- Fig. 19. 
 
 ber of forces may be represented. Thus let 
 A B, A C, A D, A E, represent the separate 
 effects of four different forces acting in the 
 same plane, capable of moving a body the 
 distances A B, A C, A D, A E, in a given 
 interval of time. Draw B c, c d, d R, equal 
 and parallell to A C, A D, A E, respectively, 
 and join A R, A B, c d R, will represent the 
 path of the body if these forces had acted 
 successively each during one interval of time, 
 and A R the path of the body if they all act 
 together, and R the position of the body at the end of the first interval of time. 
 
 Prop. 33. The line A R in figures given to illustrate the preceding 
 propositions represents the direction and measure of a single force equivalent 
 to all the others in each figure ; and hence the process by which it is deter- 
 mined called the composition of forces, pp. 55, 56, 57. 
 
78 APPEXDIX. 
 
 Prop. 34. Any force may be decomposed into any number of other forces, 
 that shall be equivalent to it, by the reverse of the foregoing operation. 
 This process is called the Resolution of forces, p. 57. 
 
 Thus the force A R, fig. 18, may be separated into two forces A B, A C, 
 and the force A R, fig 19, into four forces, A B, A C, A D, and A E. 
 
 Prop. 35. When the forces act in the same right line, we have only, in 
 order to ascertain the spaces described by their combined action, to add or 
 substract the spaces which would be described by their separate action, ac- 
 cording as these forces act, in the same or opposite direction. 
 
 Equilibrium. 
 
 Prop. 36. A body acted upon by a plurality of forces, in opposite direc- 
 tions, will remain at rest, or in equilibria; when these forces were supposed 
 to act in succession each during the same interval of time, the body would 
 arrive at its point of departure. 
 
 The simplest and most evident case of equilibrium is that in which a 
 body is acted upon by two equal and opposite forces. 
 
 On the joint action of an impulsive and a constant force. 
 A. When these forces act in the same manner. 
 
 Prop. 37. When the forces act in the sam'e direction, the place of the 
 body at the end of any given time, may be determined, as in the problem 
 of the composition of forces, by supposing, first, that the impulsive force acts 
 during that time, and then that the action of the constant force commences 
 and acts alone during the same time : the spaces added altogether will give 
 the space passed over by the joint action of these forces during the assumed 
 time. 
 
 Prop. 38. When the forces act in opposite directions, the place of the 
 body may be ascertained by a similar process ; in this case, however, the 
 spaces are to be substracted one from the other, pp. 58, 59. 
 
 When a constant force is acting in a direction contrary to that of a moving 
 body set in motion by an impulsive force, the retardation that the former 
 produces may be determined by comparing the motion wifh that of a body 
 moved by the same force. 
 
 The degrees by which an ascending body loses its motion, are the same 
 as those by which it is again accelerated at the same points, when it has 
 acquired its greatest height and again descends, for the velocity at the corres- 
 ponding parts of the ascent and descent are equal. Thus we may calculate 
 to what height a body will rise when projected upwards by an impulsive 
 force, gunpowder, for instance, and retarded by the force of gravity. Since 
 the force of gravitation produces or destroys a velocity of 32 feet in every 
 second, a velocity of 320 feet will be destroyed in 10 seconds; and accord- 
 ing to what has been premised, a body will fall in 10 seconds through a hun- 
 dred times 16 feet or 1600 feet, which is therefore the height to which a 
 velocity of 320 feet in a second will carry a ball projected, without resistance 
 from other cause than gravity, 'in a vertical direction, p. 60. 
 
 B. When these forces act in different directions. 
 * When the successive directions of the constant forces are parallel. 
 Prop. 39. If the constant force be that of gravity, the successive direc- 
 
APPENDIX. 
 
 79 
 
 Fig. 20. 
 
 tions of which are assumed to be parallel, the investigation of the effects 
 produced constitutes the doctrine of projectiles; a projectile being a body 
 thrown in any direction by an impulsive force and at the same time acted 
 upon by the force of gravity, pp. 59, 60. 
 
 Prop. 40. The place of a projectile at' the end of any given time may 
 be determined as in the problem of the composition of forces, by supposing 
 first that the impulsive force alone has acted during that time, and then that 
 the action of gravity commences, and acts alone during the same time. 
 
 Thus let A H represent a hori- 
 zontal plane, and A B the initial 
 direction and velocity of a body 
 projected from the point A in 
 the same plane. If the impul- 
 sive force alone acted on the 
 body it would describe the path 
 ABB' B" B'" &c. with uniform 
 velocity. But as the force of 
 gravity acts from the moment of 
 projection, the body will be 
 drawn downwards from the line 
 A B'" so as to be found after the 
 successive intervals of time, at 
 the points g g' g", &c., and as 
 the force of gravity produces a 
 velocity which increases as the 
 
 squares of the distances^ if the distances A B, B B', B' B", B" B'" be 
 equal, B g, B' g', B" g", B"' g"', &c., will be as the squares of these dis- 
 tances 7 and the path of the projectile through the points g g' g" g'" will 
 be a curve, and this curve mathematicians have called aparabola. 
 
 ** When the successive directions of the constant force tend to a common 
 
 centre. 
 
 Prop. 41. This case constitutes the doctrine of central forces, see prop. 
 13, p. 75. 
 
 Prop. 42. The place of the body at the end of any given time may be 
 determined here also by the problem of the resolution of forces. 
 
 Thus, suppose A represent a body impelled 
 towards H with such a force, as, by itself, would 
 enable it to run over the equal spaces A B, B F, 
 F G-, &c., in equal portions of time : suppose like- 
 wise that it is acted upon the same time by con- 
 stant force, which would enable it to pass over the 
 unequal spaces A I, I K, K L, &c., in the same 
 equal portions of time. It is evident, that the 
 joint action of both these forces would compel the 
 body A to pass over the curvilinear path A N 
 P, &o. Through B draw the line B C, (viz., in 
 the centre of attraction); through I draw I N par- 
 allel to A B ; and at the end of the first portion of 
 time the body will be found at N, whence it would 
 proceed in the straight direction N R (by the first 
 law of motion), if the constant force then ceased 
 
 Fig. 21. 
 
80 APPENDIX. 
 
 to act. But as this force continues to act, the body at the end of the second 
 portion of time will be found in ; for the like reason, at the end of the 
 third portion of time, it will be found in P, and so on. The course then A 
 N P is not straight, but consists of the lines A N, N 0, P, forming certain 
 angles with each other. Now it will not be difficult to conceive that, because 
 the attractive force acts not by intervals but constantly and unremittedly, 
 the real path of the body must be a polygonal course, consisting of an infi- 
 nite number of sides; or more justly speaking, a continuate curved line, which 
 passes through the points A, N, 0, P, &c., as is shown by the dotted line. 
 
 Prop. 43. Should the action of the centripetal force cease at any instant, 
 the body would proceed straight forward, p. 49. 
 
 The portion of the impulsive force by which this is affected is called the 
 centrifugal, prop. 14. 
 
 Prop. 44. Whilst the distance from the centre remains unchanged, as 
 when the body moves in a circular orbit, the centripetal and centrifugal 
 forces are equal. 
 
 Laws of Central forces. 
 
 Prop. 45. When bodies revolve in equal circles, their centrifugal forces 
 are proportional to the squares of their velocities. 
 
 Prop. 46. When two bodies revolve with equal velocities at different 
 distances, the centrifugal forces are inversely as the distances. 
 
 Consequently (prop 45,46) the centrifugal forces are in all cases directly 
 as the squares of the velocities, and inversely as the distances. 
 
 Prop. 47. When two bodies revolve in equal times at different distances, 
 their centripetal forces are simply as their distances. 
 
 In general, the centripetal forces are as the distances directly and as the 
 squares or the times of revolution inversely. 
 
 Prop. 48. WheR the forces vary inversely as the squares of the distances, 
 as in the case of gravitation, the squares of the times of revolution are pro- 
 portional to the cubes of the distances. 
 
 Thus, if the distance of one body be four times as great as that of another, 
 the cube of 4 being 64, which is the square of 8, the times of its revolution 
 will be 8 times as great as that of the first body. 
 
 Prop. 49. Where the orbit deviates more or less from a circular form, a 
 right line joining the revolving body and its centre of attraction, always de- 
 scribes equal areas in equal times, and the velocity of the body is therefore 
 always inversely as the perpendicular drawn from the centre to the tangent ; 
 and the velocity at any point less than three-eighths, greater than that neces- 
 sary to make the body describe a circle. 
 
 Prop. 50. To propel a body in an elliptical orbit, the force directed to 
 its focus must be inversely as the square of the distance. 
 
 This is proved by astronomical observations, but we have no other proof of it. 
 
 The motion of the planets round the sun in the solar system is governed 
 by the laws of central forces, the centripetal force in this case being that of 
 gravity. 
 
 On the joint effect of active and inactive forces. 
 A. When they have opposite directions. 
 
 Prop. 51. The effect of passive forces is to restrain and modify the action 
 of other forces so as to confine the motion of a body to a particular course or 
 path, and the direction of the passive force affecting a body at any moment 
 
APPENDIX 
 
 81 
 
 s the line perpendicular to that part of this path at which the body is found 
 at this moment. If the direction of the active force be also perpendicular 
 to this path, the body must evidently remain at rest, since no part of this 
 force can be resolved into the direction of the path in which alone the body 
 can move. 
 
 B. When they have different directions. 
 
 General rule. 
 
 Prop. 52. Resolve the active force into two, one perpendicular, and the 
 other a tangent to the path of the body, the effect of the former force will be 
 entirely destroyed (prop. 51,) and the body will advance by the latter alone, 
 
 , * On the motion of a body impelled obliquely against a plane. 
 
 Prop. 53. Let M N represent the plane, Fig. 22 
 
 and A B the direction and velocity from the 
 impulsive force, resolve A B into the forces 
 A C perpendicular to the plane, and C B in 
 its direction, then by the general rule (prop. 
 52) the body will move along the plane witlra 
 velocity of which C B is the measure. 
 
 K On the motion of a body impelled obliquely against a curved surface. 
 
 Prop. 54. Let M N represent the curve and Fig. 23. 
 
 A B the direction and velocity from the impul- 
 sive force. Resolve A B into two forces, C B 
 perpendicular to the curve at B, and B D (equal 
 to A C) a tangent to the curve at the same point. 
 Then B D will represent the velocity at the point 
 B. 
 
 Prop. 55. If the curve be interrupted at any 
 point, or change the direction of its concavity, the body will advance with 
 its last velocity in a tangent to the curve at that point. 
 
 *** On the descent of a body along an inclined plane. 
 
 Prop. 56. Let M N represent an in- 
 clined plane and A B (perpendicular to the 
 horizontal base H NJ the force of gravity 
 as measured by the distance which would 
 cause a body to descend in the first second 
 of time. Resolve A B into two, A C, per- 
 pendicular to the plane, and C B in its 
 direction, then the body will be urged 
 down the plane by the constant force 
 measured by C B. 
 
 Laws of the descent of bodies down inclined planes. 
 
 Prop. 57. 1st. The motion of a body drawn down an inclined plane is 
 uniformly accelerated. 
 
 Prop. 58. 2d. The velocity acquired is proportional to the perpendicular 
 
 6 
 
82 APPENDIX.. 
 
 descent, so that a body falling from M to H has the same velocity at H as 
 one descending the whole length of the plane at N. 
 
 Prop. 59. 3d. The times of descent down planes of the same heights 
 are as their lengths. 
 
 Prdp. 60. 4th. The times of descent down all planes which are cords 
 drawn to the lowest point of the same circle, are equal. 
 
 Thus, if the balls A, B, C, be placed at different 
 Fig- 25 points of the circle and suffered to descend at the 
 
 same instant along as many planes which meet at 
 the lowest point of the circle, they will arrive there 
 at the same time. 
 
 Or it may be enunciated in the following terms : 
 the times of descent down all the cords drawn from 
 the same point or circumference of a circle will be 
 the same. 
 
 This will be made evident by supposing the above 
 figure inverted, D being made the upper point and the 
 balls allowed to fall from that point to A, B, and C.* 
 
 **** On the descent of a body down a vertical curved line. 
 
 Prop. 61. The times of descent down the cords of different circles are 
 to each other as the square roots of their diameters. 
 
 Prop. 62. If a body fall from a state of rest down a curve, the velocity 
 acquired is equal to that which it would have by falling through the same 
 perpendicular height. 
 
 For if the curve be considered as made up of an infinite number of con- 
 tiguous planes, it is evident that the angle of inclination of any two of these 
 adjacent planes is infinitely small, or nothing, and consequently there is no 
 velocity lost by a change of direction in passing from one to the other. 
 Therefore, as the effect of gravity is not impeded, the truth of the proposi- 
 tion becomes evident. 
 
 Prop. 63. If a body be projected up a curve, the perpendicular height to 
 which it will rise is equal to that through which it roust fall to acquire the 
 velocity of projection. For the body in its ascent will be retarded in the 
 same degree that it was accelerated in its descent* 
 
 Thus, let B A B' be a curve in which the lowest 
 Fig. 26. point is A, and the parts A B, A B' are similar ; 
 
 a body in falling down B A will acquire a velocity 
 that will carry it to B', and since the velocities in 
 all equal altitudes in the ascent and descent are 
 equal, the times of ascent and descent are equal. 
 
 The foregoing proposition is equally true whether 
 the body actually move over a solid surface or be 
 retained in its path by a string which is in every part perpendicular to it. 
 
 Of the simple Pendulum. 
 
 Prop. 64. The simple pendulum is conceived to be a mere material point 
 suspended by an imponderable and inextensible thread, p. 60. 
 
 Prop. 65. If the simple pendulum vibrates through very small arcs, 
 these may, without sensible error, be conceived to coincide with their chords, 
 and we may derive from this consideration the following theorems : 
 
 1st. As the times of descent of the body down different chords of the same 
 
APPENDIX. 83 
 
 verticle circle are equal (prop. 60.) the vibrations of the same pendulum, 
 although performed through unequal arcs, will be very nearly equal, 
 p. 61. 
 
 2d. The times of vibrations of different pendulums will be to each other 
 as the square roots of the lengths of these pendulums, or, which is the same 
 thing, their lengths are proportioned to the squares of the times of vibration, 
 p. 61. 
 
 The times of descent down the chords of different circles are the same as 
 would be occupied in descending vertically through their diameters, and are 
 consequently proportional to the square roots of these diameters. 
 
 Of the impact of bodies. 
 
 Prop. 66. When a body in motion strikes directly another body, it always 
 communicates motion to the second body, and loses part of its own, and from 
 the third law of motion it is evident that the momentum gained by the 
 second body is exactly equal to that lost by the first. 
 
 Prop. 67. When one non-elastic body strikes against another, the two 
 bodies will move on together since there is no force to separate them ; and as 
 one of the bodies gains all the momentum which the other loses, the momen- 
 tum after impact will be equal to the sum of the momentum before impact. 
 
 Prop. 68. When an elastic body strikes against another, the second is 
 impelled forward with double the momentum which it would have received 
 under the same circumstances if non-elastic. 
 
 For at the moment of impact the form of the body struck is changed by a 
 force equivalent to the momentum which it receives from the striking body, 
 and if this body be perfectly elastic, its form will be restored to it by a force 
 exactly equal to that by which it was changed, and this force (which we 
 have just seen to be equal to the original impulse,) will be exerted in driving 
 the body forward. The body thus receives, besides its original impulse, the 
 equal force of the re-bound. 
 
 Prop. 69. The striking body, when elastic, is also acted upon by the 
 rebound, and loses twice as much momentum as it would have lost if 
 non-elastic. 
 
 In this case, as in the former, the sum of the momenta is the same after 
 impact as before it ; but the bodies after impact do not move on together. 
 
 Prop. 70. If an elastic body strike against a firm plane, the angle of 
 reflection will be equal to the angle of incidence, p. 66. 
 
84 MECHANICS. 
 
 PART II. 
 
 PHENOMENA OF SOLIDS. 
 
 THE FOUR FUNDAMENTAL TRUTHS USED TO EXPLAIN THE PECULIARI- 
 TIES OF STATE AND MOTION WHICH DEPEND ON THE SOLID FORM OF 
 BODIES j A DEPARTMENT COMMONLY CALLED MECHANICS. 
 
 ANALYSIS OF THE CHAPTER.* 
 
 A force, which moves part of a solid body, must effect the whole or break 
 off the part. 
 
 If the force be directed towards a certain central point in the mass, it will 
 effect the whole equally, whether simply to support the mass, or to move it 
 or to stop it when in motion. The point, according to circumstance, is 
 called THE CENTRE OF GRAVITY OF INERTIA, OT OF ACTION. 
 
 In solid bodies moving about an axis, as exemplified in a wheel or weigh- 
 ing beam the various parts describe circles or move through spaces which 
 are greater in proportion to their respective distances from the centre of 
 motion. Hence forces differing as to speed, may still, through a solid 
 medium, be brought exactly to co-operate or to oppose one anotJier j a slow 
 force counter -balancing or being equivalent to a quicker one, provided 
 tliat it be more intense in proportion as it is slower. The SIMPLE 
 MACHINES, or MECHANICAL, MEDIA Called LEVER, WHEEL AND AXLE, 
 PULLEY, INCLINED PLANE, WEDGE, SCREW, &c., are so many arrange- 
 mentsof solid parts, by which forces of different velocities and intensities 
 maybe thus connected or opposed, or may be conveniently substituted one 
 for another . 
 
 By solid connecting parts, also, the direction of any existing motion or force 
 may be changed, as when the straight motion of running water is con- 
 verted into the rotary motion of a water-wheel, &c. Hence arises an end- 
 less variety of COMPLEX MACHINES. 
 
 In all machines, an important circumstance to be considered is the resist- 
 ance among moving parts which arises from FRICTION : and in solid 
 structures generally, the forms and positions of parts have to be adjusted 
 to the STRENGTH OF THE MATERIALS, and to the strains which the parts 
 have to bear. 
 
 " Solid" is the term applied to a mass in which the mutual attraction of 
 the atoms is so strong, that the mass may be moved about as one body, with- 
 out the relative positions of the component parts being thereby disturbed. 
 
 " Force moving part of a solid must effect the whole or break off the part." 
 
 This is a necessary consequence of the description or definition of a solid 
 just given. And it follows that in all cases of breaking, the cohesion of the 
 
 * The reader .should here re-peruse the general table or synopsis at page 19. 
 
CENTRE OF GRAVITY. 85 
 
 atoms at the fractured part must have been less strong than the weight of 
 the remaining mass, or its inertia resisting the degree of change attempted, or 
 the force fixing it to its place, or than some combination of these particulars. 
 
 The sharp blow of a hammer given to 'an ivory ball, causes it to dart off 
 swiftly, but does not injure it, because the -cohesion among the atoms struck 
 is stronger than the opposing inertia of the mass, even under a rapid change ; 
 but the blow of a hammer on a large elephant's tusk indents or breaks the 
 part because the opposing inertia of the larger mass is stronger than the 
 cohesion of the atoms which receive the blow. 
 
 A vessel of pottery- ware may be safely suspended by its handle ; proving 
 that the cohesion which fixes the handle to it is stronger than the weight of 
 the vessel ; but if the attempt be made to lift the vessel quickly, the handle 
 may rise and leave the vessel behind ; because then the weight and inertia 
 are acting together to destroy the cohesion. Thus servants attempting to 
 lift too quickly the loaded stone-ware dishes at a dinner-table, often break 
 off the part by which they take hold. 
 
 Centre of Gravity or Inertia. 
 
 If any uniform beam or rod be supported by its middle, like a weighing 
 beam, the two ends will just balance each other. This is in accordance with 
 the general truth or law of attraction already explained; for as there is just 
 as much similarly situated matter on one side of the support as on the other, 
 there will also be just as much attraction, and therefore no reason why the 
 matter on one side should overpower that on the other. If equal weights be 
 afterwards attached in corresponding situations on the two arms of the beam 
 the balance will not be thereby disturbed; and the operation of adding 
 weights that counterpoise, above and below, and near and far from the centre 
 may be continued, until a bulky mass is built up upon the beam and in- 
 stead of a beam a wheel may be used yet the whole will remain perfectly 
 supported and in equilibrium about the original centre. In the pages now 
 to follow, it will be shown that, in every body or mass, or system of con- 
 nected masses, in the universe, there is a point of this kind about which all 
 the parts balance or have equilibrium, and it is this point which is called 
 the centre of gravity or of inertia. Although in any mass, therefore, every 
 atom has its separate gravity and inertia, and the weight and inertia of the 
 whole are really diffused through the whole, still by supporting this one 
 point, either from above or from below, the whole mass is equally supported ; 
 by lifting it, the whole is lifted ; by stopping it, the whole is brought to 
 rest ; and when it rises or falls, the general mass is really rising or falling. 
 Thus for many purposes, a body, however large, may be considered as com- 
 pressed into or existing only in the single point called its centre of gravity 
 or of inertia. 
 
 This centre in a mass of regular shape and of uniform substance, as a ball 
 or cube of metal, is easily found, because it is the evident centre of the 
 form ; but in bodies that are irregular, either as to density or form, it must 
 be found by rules of calculation hereafter explained. 
 
 To say that the centre of gravity will always take the lowest situation 
 which the support of the body will allow, is only to repeat, that bodies tend 
 by their gravity towards the centre of the earth. In a suspended body, there- 
 fore, as the lowest situation which the centre of gravity can find is, when it 
 is immediately under the point of suspension, all bodies hanging freely must 
 have their centre of gravity directly under that point. A plummet is an 
 interesting example of this; and the truth furnishes, in many cases of irre- 
 gular masses, a very simple practical mode of finding the centre. 
 
86 
 
 MECHANICS. 
 
 Thus if an irregular piece of plank or of pasteboard, represented here by 
 the figure a e b d, be suspended from any point, as a, and 
 the cord of a plummet a g be attached at the same point, the 
 centre of gravity of the board must be somewhere in the 
 direction of the plummet, and a chalk line left on the board 
 where the cord touched it, must pass over the centre of 
 gravity. If the board be then suspended by another point, 
 as d, and another chalk line d e be made in the same man- 
 ner, the place c, where the two lines cross or cut each other, 
 will indicate the centre of gravity ; and the board when sup- 
 ported by a cord attached there, will hang evenly balanced. 
 The following cases further illustrate the truth, that the 
 centre of gravity always seeks the lowest place. They seem 
 at first to be exceptions to the law ; but when more fully 
 considered, are interesting proofs of it. 
 
 A wooden cylinder or roller e d c, placed on a slope 
 or inclined plane a b, will naturally descend, because 
 its centre of gravity is thereby approaching the earth ; 
 but if there be a heavy mass of lead c introduced at one 
 side, which must rise before the roller can descend, the 
 rise of the mass being contrary to gravity, the motion 
 will be arrested. Indeed if the roller were placed on 
 the plane with the lead in the position d, the lead would 
 fall down to the position c, and so would move the roller towards &, exhibit- 
 ing the singular phenomenon of a body rolling up hill by the action of its 
 weight. 
 
 If a billiard-ball be placed upon the small ends of two billiard sticks or cues 
 
 a b and c d, laid on a table with 
 their points c and a in contact, but 
 with the larger ends b and d so far 
 apart that there may be just room 
 for the ball to touch the table be- 
 'tween them, the ball will roll along 
 between the cues, sinking gradu- 
 ally from its high situation near 
 their points, to its lower situation near b. To a careless observer, it 
 would then have the appearance of rolling upwards, because the cues on 
 which it rests are thicker towards the end d and b ; but it would really be 
 descending in obedience to gravity. If a double cone, as represented at/, 
 were substituted for the ball, it would similarly roll from c to e, and with still 
 niore of the fallacious appearance of rolling upwards, because its ends would 
 always be resting on the upper and rising surfaces of the cues. 
 
 The board or stick c d resting on the edge of 
 the table a b would naturally fall if left to itself, ' 
 because more than half of it is beyond the edge 
 of the table ; but strange to say, an additional 
 weight e attached to its projecting part as at b 
 by the cord b e, instead of pulling it down fast- 
 er, shall fix or steady it on the table, provided 
 the weight be pushed inwards a little by a rod 
 d e resting against it and against a niche in the 
 stick at d. It is evident that the stick c d, in 
 falling, must turn round the edge of the table at 
 
 Fig. 30. 
 
CENTRE OF GRAVITY. 
 
 87 
 
 b; but in so doing, after the arrangement now supposed, it must lift the 
 weight e along the path e f- which rise, as the weight is heavier than the 
 stick (that is to say, as the common centre of gravity of the connected objects 
 is near e, ) gravity forbids, and therefore the stick and weight will both remain 
 supported by the table. An umbrella or walking cane, hanging on the edge 
 of a table by a crooked handle, is another instance of the same kind. And 
 the common toy of a little man standing on tiptoe upon the top of a pillar, 
 and supporting two leaden bullets by wires descending from his hands, is 
 another combination of parts which places the centre of gravity of the whole 
 the support, making the combination a kind of pendulum. 
 
 By attending to the centre of gravity of the bodies around us on earth, we 
 are enabled to explain why, from the influence of gravity, some of them are 
 stable 'or firmly fixed, others tottering, others falling. 
 
 If we find that a body, from its form or position, cannot be overturned 
 without its centre of gravity being lifted, knowing now that the general 
 mass is then lifted in the same degree, we see why a weak cause cannot effect 
 the change. The rise of the centre of gravity, or body, in any case of falling 
 over when the centre of gravity is over the middle of the sustaining base, 
 will be proportioned to the breadth of the base of the body, compared with 
 the height of the centre of gravity above the base. This is shown in the 
 annexed figures of which the two particulars of base and height &rv combined 
 
 in a series of proportions. In the figures, the dot c marks the place of the cen- 
 tre of gravity, and the curved line beginning from the dot marks the path 
 of the centre of gravity, when the body is overturned. This curved line is a 
 portion of a circle which has^the edge or extremity of the base (Z>, in fig. A.) 
 as a centre, because the body in turning must rest upon such extremity or 
 corner as the centre of its motion. The farther inwards, therefore, from this 
 extremity that the centre of gravity is, as marked by where a plumb-line as 
 p, hanging from it, crosses the base, the farther, of course, is the centre of 
 gravity from the top of the circle which it has to describe in moving, and the 
 steeper, consequently, will be its commencing path j and as in the case of 
 bodies made to roll up slopes, the steeper the ascent, the greater will be the 
 force necessary to give motion. The line of a plummet hanging from the 
 centre of gravity is^called the line of direction of the centre, or that in 
 which it tends naturally to descend to the earth. 
 
 In fig. A, which has a broad base and little height of the centre of gravity, 
 we see that the centre must rise almost perpendicularly before it can fall 
 over, and the resistance to overturning is therefore nearly equal to the 
 whole weight of the body. Hence the firmness of a pyramid. 
 
 In figures B, C, and D, progressively, the commencing path of the centre 
 is less steep, because the base is narrower, and hence the bodies are so much 
 the less stable. B, may represent an ordinary house, C, a tall narrow house, 
 and D. a lofty chimney. 
 
88 
 
 MECHANICS. 
 
 Fig. E, shows a tottering position, for the centre of gravity being directly 
 over a base which is a mere point, the least inclination places it on a 
 descending slope, and the body- must fall. 
 
 Fig. 32. 
 
 K. 
 
 In F, the position is tottering on one side, and stable on the other. This 
 explains how the least inclination of a standing body virtually narrows, in 
 one direction, its sustaining base. 
 
 In Gr, which represents a ball upon a level plane, the whole mass is sup- 
 ported on a single point, as in E, yet the body has no tendency to move, 
 because, in any other possible position, the centre would still be as far from 
 the sustaining plane. In moving, the centre describes the straight level- 
 line a b. 
 
 In H, the ball is on an inclined plane, and rolls down, the centre of 
 gravity describing the oblique line b a. 
 
 In I, which is an oval body resting on a level plane, when the body is 
 moved to either side, the centre of gravity must rise, as in the case of a pen- 
 dulum. Hence an oval body on a level will rock or vibrate like a pendulum. 
 
 K, is a true pendulum, whose centre of gravity describes the curve here 
 shown, as explained in Section II., at page 60. 
 
 The importance of the subject of the centre of gravity will be farther judged 
 of by the facts which are now to be reviewed. 
 
 A cart loaded with metal or stone may go safely along a road of which one 
 side is higher than the other, as here shown, but were the same cart loaded 
 with wool or hay it would be overturned ; because, although the sustaining 
 base be the same in the two cases, the line of direction falls much within it 
 from the low centre of gravity of the metal at (^but falls very near the wheel 
 at P, or altogether on the outside, from the high centre 
 of the wool at a, and in the latter case the centre has 
 offered to it a descending path. 
 
 This explains why lofty stage coaches or vans are so 
 dangerous, and particularly when heavy luggage is 
 placed on the top, and why lofty gigs and curricles 
 have led to so many fatal accidents. As regards any 
 of these, a defect of smoothness or of level in the road, 
 or even, in a case of quick driving, a slight lateral 
 bend often suffices to produce the catastrophe. The 
 safety-coaches of late times are made with the wheels 
 far apart to give a broad base, and with the luggage 
 receptacles and seats for outside passengers placed low down before and 
 behind the body of the carriage, instead of on the top, as formerly. 
 
 The feet of tripods are generally expanded below to give a broad base. 
 The same is true of our common chairs ; but a thoughtless child often leans 
 so far over the back of the chair, that he causes the line of the general centre 
 of gravity to fall beyond the base, and the chair with its load is overturned. 
 
 Fig. 33. 
 
CENTRE OF GRAVITY. 89 
 
 The small lofty chairs made to raise children to the parents' elbow at the 
 dinner-table, are very dangerous if the feet are not made to spread much. 
 Pillar-and-claw tables, candle-sticks, table-lamps, and many other articles of 
 household furniture, have stability given in the same manner. 
 
 The least inclination of a standing body virtually narrows the supporting base. 
 
 This truth is explained by jig. F. It shows the necessity of building the 
 thin walls and tall chimneys of modern houses perfectly upright. And hence 
 the extreme importance and utility of that simple instrument, the plummet 
 or plumb-line, which, when applied to a body, is a visible indication of the 
 line of its centre of gravity. The mason and many other workmen cannot 
 proceed a step without their guiding plummet. 
 
 The brick walls of ordinary houses are so thin, that, to have standing 
 strength, they require to rest against one another ; and hence they occasion- 
 ally exhibit the kind of stability which belongs to a child's house built of 
 cards. As contrasted with the masses of masonry which remain to us from 
 antiquity, resting on firm-spreading basements, they are examples of what 
 is truly ephemeral, in comparison with that which has partaken of the per- 
 manency of nature's own works, covering regions with mighty ruins. What 
 magnificent illustrations of strength and durability, dependent on propor- 
 tions, are those ancient pyramids and temples, which still give such interest 
 to the banks of the Nile, and to the valleys and plains of Asia ! 
 
 There are many remarkable structures on earth which lean or incline a 
 little } yet so long as the line of their centre of gravity remains within the 
 base, and the parts of the mass have tenacity among themselves sufficient to 
 hold together the structure will stand. The famous tower of Pisa was built 
 intentionally inclining, to frighten and surprise : with a height of one hun- 
 dred and thirty feet, it overhangs its base sixteen feet, and assumes nearly 
 the air of fig. F. in page 88. 
 
 The tall monument near London Bridge inclines so much, that in high 
 winds, from a particular quarter, timid minds have doubted of its stability. 
 And many of the most lofty and beautiful of our cathedral spires or tow- 
 ers, as that of Salisbury, have lost something of their perpendicularity. 
 
 An oval body on a flat level surface, as already explained by fig. I, page 
 88, oscillates somewhat like a pendulum, because, when disturbed from its 
 middle position, its centre of gravity has risen and seeks to return. The 
 same is true of any regular slice or portion of a solid globe, which will con- 
 sequently always come to rest with its plane .face turned directly upwards. 
 
 The rocking-horse of children and the common cradle are exemplifications 
 of the same class. 
 
 But perhaps the most curious instances are those rocks called Loggan or 
 Laggan stones, of which there are several among the picturesque barriers 
 of the British coast. An immense mass, loosened in some convulsion of 
 nature, is found with a slightly rounded base resting on a flatter surface of 
 rock below; and is so nearly balanced, that the, force of a man suffices to 
 move it. Some of these have been objects of much superstitious veneration 
 to their neighbourhood. 
 
 There is an amusing Chinese toy, made in obedience to the same princi- 
 ple. It has the appearance of a little fat, laughing man, sitting on the 
 ground with his feet concealed under him; but where the feet should be, 
 there is only a rounded smooth surface, with heavy lead ballast placed in it, 
 so low, as always, when allowed, to raise the body to the erect or sitting 
 
90 MECHANICS. 
 
 attitude. A child pushes the little fellow down again and again, and would 
 persuade him to be still, but is surprised to see him always up the moment 
 after, shaking about and as lively as ever. 
 
 The vibratory motion of a pendulum, as dependent upon the circumstance 
 of the centre of gravity having been moved from its lowest place, which it 
 again constantly seeks, was so fully considered in the last chapter, that it 
 need not be again dwelt upon here ; but we have to enumerate the follow- 
 ing phenomena as being of the same class : 
 The vibrations of a common swing. 
 The rocking of a balloon when it first ascends. 
 
 The spontaneous shutting of those gates or doors of which the upper 
 hinge overhangs or projects beyond the lower, causing the gate, when in the 
 shut position, to have its lock lower than when in any other. Such a gate 
 always returns of itself, from either side, to the shut position, just as a pen- 
 pulum returns to the lowest part of its arc : the gate, in fact, is but a 
 sloping pendulum. 
 
 Of the same nature also is the rocking or rolling of a ship, in particular 
 states of wind and sea. When the centre of gravity of a ship is too low, 
 owing to all the heavy load being placed near the keel, this pendulum- 
 motion, in rough weather, becomes excessive and dangerous. 
 
 The actions and postures of animals, and particularly of man, illustrate beau- 
 tifully the observations made above with respect to the centre of gravity. 
 
 A body, we have seen, is tottering in proportion as it has great altitude 
 and narrow base but it is the noble prerogative of man to be able to sup- 
 port his towering figure with great firmness, on a very narrrow base, and 
 under constant change of attitude. This faculty is acquired slowly because 
 of the difficulty. A child does well who walks at the end of ten or twelve 
 months j while the young of quadrupeds, which have a broad supporting base, 
 are able to stand and even to move about almost immediately after birth. 
 
 The supporting base of a man is the space occupied by and included be- 
 tween the feet. The advantage of turning out the toes is, that without taking 
 much from the length of the base, it adds considerably to the breadth. 
 
 If there be much art in walking on two perfect feet, there is still more in 
 walking on two slender wooden legs, with rounded extremities : which, 
 however, we often see done, by mutilated soldiers and sailors. 
 
 All the ladies of the empire of China have to acquire nearly the same 
 talent as these victims of war,; for barbarous custom has crippled them, by 
 confining their feet for life in such shoes as fitted them in infancy. 
 
 But surpassing in difficulty any of these instances is the practice, which is 
 general among the inhabitants of the sandy plains, called the Landes, in the 
 south-west of France, of walking on stilts. The Landes afford tolerable 
 pasture for sheep ; but during one portion of the year are half covered with 
 water, and during the remainder are still very unfit walking ground, by 
 reason of their deep loose sand and thick furze. The natives meet the incon- 
 veniences of all seasons by doubling the length of their natural legs, through 
 the addition to them of the stilts mentioned, which they call des echasses. 
 Mounted on these, which are wooden poles, put on and off as regularly as 
 the other parts of dress, they appear to strangers a new and extraordinary 
 race of long-legged beings, marching over the loose sand, or through the water, 
 with steps of eight or ten feet in length, and with the speed of a trotting 
 horse ; their moderate journeys being of thirty or forty miles in a day. While 
 
CENT RE OF GRAVITY. 91 
 
 watching their flocks, they fix themselves in convenient stations by means of a 
 third staff which supports them behind, and then with their rough sheep-skin 
 cloaks and caps, like thatched roofs over them, they appear like little watch- 
 towers, or singular lofty tripods, scattered over the face of the country. 
 
 Still beyond the art of walking on stilts is that which some persons 
 attain of walking and dancing on a single rope or wire ; or even of keeping 
 the centre of gravity above the base, while standing on the moveable support 
 of a galloping horse. A rope-dancer usually carries a long pole in his hand, 
 to balance him ] it is loaded at each end, and when he inclines, he throws 
 it a little towards the side required, that the reaction may restore his 
 perpendicularity. 
 
 Much art of the same sort is shown, in the attitudes and evolutions of the 
 skater ; in the amusements of supporting a stick upright on the end of the 
 finger ; and many other feats of a like kind. 
 
 Attitudes generally depend on the necessity of keeping the centre of 
 gravity of the body over the base under variety of circumstances, as in the 
 straight or upright part of a man who carries a load on his head ; the 
 leaning forward of one who carries it on his back ; the hanging backwards 
 of one who bears it between his arms; the leaning to one side of him who 
 is carrying a weight on the other side ; the habitual carriage of very fat 
 people, whose head and shoulders are thrown back, giving a certain air of 
 self-satisfaction, an air which belongs also to the expectant mother, and 
 even to the dropsical patient, although producing in the latter so sad an 
 incongruity. 
 
 When a man walks or runs, he inclines forward, that the centre of gravity 
 may overhang the base : and he must then be constantly advancing his foot 
 to prevent his falling. He makes his body incline just enough to produce 
 the velocity which he desires. 
 
 A man, in pulling horizontally at a load, is merely causing his body to 
 overhang its base, so that its tendency to fall may become a force or power 
 applicable to the work. 
 
 When a man rises from a chair, he is seen first to bend the body forward, 
 or to draw the feet backward, so as to bring the feet or base under the centre 
 of gravity, and then he lifts the body up. If he lifts too soon, that is, 
 before the body be sufficiently advanced, he falls back again. 
 
 A man standing with his heels close to a perpendicular wall, cannot with- 
 out falling, bend forward sufficiently to pick up any object that lies before 
 him on the ground } because the wall prevents him from throwing part of 
 his body backward, to counterbalance the head and arms which must pro- 
 ject forward. A person little versed in such matters, might agree to give 
 ten guineas for permission to possess himself, if he could, of a purse of twenty, 
 laid on the ground before him : he of course would lose his stake. 
 
 When a man walks at a moderate rate, his centre of gravity comes alter- 
 nately over the right and over the left foot. This is the reason why the 
 body advances in a waving line, and why persons walking arm in arm shake 
 each other, unless they make the movements of their feet to correspond, as 
 soldiers do in marching. 
 
 Sea Sickness is a subject closely related to the present. Man requiring, 
 as now explained, so strictly to maintain his perpendicularity, that is, to keep 
 the centre of gravity always over the supporting part of his body, ascertains 
 the required position in various ways, but chiefly by comparing the perpen- 
 dicularity, or other known position of things about him, with his own posi- 
 
92 MECHANICS. 
 
 tion. Vertigo and sickness are the consequences of depriving him of his 
 standards of comparison, or of disturbing them. 
 
 Hence on shipboard, where the lines of the masts, windows, furniture, &c. 
 are constantly changing, sickness, vertigo and other affections of the same 
 class are common to persons unaccustomed to ships. Many persons experi- 
 ence similar effects in carriages, and in swings j or on looking from a lofty 
 precipice, where known objects being distant, and viewed under a new as- 
 pect, are not so readily recognized ; also in walking on a wall or roof; in 
 looking directly up to a roof, or to the stars in the zenith, because then all 
 standards disappear ; on entering a round room, where there are no perpen- 
 dicular lines of light and shade, as when the walls and roof are covered with 
 a paper which has no regular arrangement of spot ; on turning round, as in 
 waltzing, or if placed on a wheel ; because the eye is not then allowed to 
 rest long enough on any standard, &c. 
 
 People when in the dark, and therefore blind people, always use standards 
 belonging to the sense of touch ; and it is because, on board of a ship, 
 the standards both of sight and touch are lost, that the effect is so very 
 remarkable. 
 
 But sea sickness also partly depends on the irregular pressure of the 
 bowels among themselves and against the containing parts, when the influ- 
 ence of their inertia and weight varies with the rising and falling of the ship. 
 
 From the nature of sea sickness, as discovered in these facts, it is seen 
 why persons unaccustomed to the motion of a ship, often find relief by keep- 
 ing their eyes directed to the fixed shore, where visible ; or by lying down 
 on their backs and shutting their eyes ; or by taking such a dose of exhilar- 
 ating drink as shall diminish their sensibility to all objects of external sense. 
 
 As no condition or form of matter escapes from the great laws of nature, 
 we find the attitudes and general condition of vegetable as well as of animal 
 bodies, characterized by the necessity of having the centre of gravity sup- 
 ported over tho base. With what admiration may we contemplate the pine 
 and other trees in the forests or nature, springing up to heaven as perpen- 
 dicularly as if the plummet had been at work to direct them j and no less 
 on the brows of precipitous hills than in the level plains. On a smaller scale, 
 we see the grasses and corn-stalks of our fields illustrating the same truth. 
 And whenever, in tree or shrub, accident or peculiar nature causes a devia- 
 tion from perpendicularity, additional strength and support are provided. 
 
 Beauty of form or position is often felt to exist in bodies, merely because 
 they possess the shape and support required, that the centre of gravity may 
 be stable. 
 
 In architecture, how displeasing is a wall or pillar that is not quite upright ; 
 or a column with too small a base ; or a very tall narrow house ; or a long 
 slender chimney. On the other hand, how beautiful in a lofty edifice is the 
 suitable succession of columns, from the massive Doric of the basement, 
 supporting the whole superstructure, to the light Corinthian or kindred forms 
 seen above. The Chinese pagoda is a fine example of the union of certain 
 requisites for stability, viz., perpendicularity and expanding base, with the 
 other qualities of perfect symmetry, graceful proportion, and fanciful orna- 
 ment. When seen crowning a rising ground in a wooded island, or spring- 
 ing up from the centre of a rich garden, it forms, perhaps, one of the most 
 beautiful objects which fancy has ever designed. 
 
 Beauty of attitude and grace of carriage in the human individual are in 
 great part referable to the same principle. 
 
CENTRE OP GRAVITY. 93 
 
 The postures of opera dancers might pass as intentional illustrations of the 
 number of ways in which the centre of gravity may be kept above a narrow 
 base, by counteracting one disturbing motion or extension of a limb by some 
 opposite and corresponding motion. The common statute of the god Mercury 
 on tip-toe is a permanent familiar illustration of such a beautifully balanced 
 attitude. 
 
 Grace of carriage includes not only a perfect freedom of motion, but also 
 a firmness of step, or steady bearing of the centre of gravity over the base. 
 It is usually possessed by those who live in the country, taking much and 
 varied exercise, or who make gymnastics a part of their discipline. What 
 a contrast is there between the gait of the active mountaineer, enjoying the 
 consciousness of perfect nature, and that of the mechanic or shop-keeper, 
 whose confinement to the cell of his trade soon produces in his body a shape 
 and air corresponding to it and in the softer sex what a difference is there, 
 between that active and graceful fair one who recalls to us the fabled Diana 
 of old, and that other sedentary being who, having scarcely trodden but on 
 smooth pavements and carpets, under any new circumstances, carries her 
 person as if it were a load quite new and foreign to her. 
 
 The centre of gravity is also the centre of inertia. When a person lifts 
 a uniform rod by its middle, the inertia of both ends being equal, he over- 
 comes it equally, and raises them evenly together. When he lifts by a part 
 nearer to one end, the shorter and lighter portion having less inertia will rise 
 the first, and there will be a turning motion of the rod round the finger as a 
 centre, proportioned to the excess of inertia in the greater side. 
 
 The centre of gravity, or inertia , however, is not necessarily in the centre 
 of the mass; for if a weight of three pounds, a, 
 be affixed to one end of a rod, and a weight of Fig. 34. 
 
 only one pound, b, be affixed to the other, the J> ^ -v 
 
 two will still be balanced, if supported or lifted CD & ( "* J 
 
 by a point of the rod, c, three times nearer to the ^ S 
 
 centre of the large weight than to the centre of 
 
 the small one. This fact is explained under the head of lever, a few pages 
 hence. For the sake of simplicity, in describing such experiments, the 
 weight of the connecting rod itself is neglected. 
 
 The centre of gravity or inertia is also the centre of centrifugal force : 
 for if the balls a and b of the last figure were made to spin round a common 
 centre, as by making the connecting rod rest and turn upon a point or pivot 
 at c; unless the point c were the centre of inertia of the two, the pivot 
 would always be drawn in the direction of that end of the rod at which there 
 was the greatest centrifugal force. It is on this account that in the case of 
 a mill-stone, or great fly-wheel, or of the balance wheel of a watch, the axis 
 must pass through the centre of inertia, to prevent its being more worn on 
 one side than on the other. 
 
 When we say in astronomy, that the earth revolves round the sun, or that 
 the moon revolves round the earth, we do not speak with absolute correct- 
 ness, for in all such cases, both bodies are revolving round the common cen- 
 tre of inertia of the two. In the case of the sun and earth, as the former is 
 about a million times larger than the latter, the common centre of inertia of 
 the two is a million times nearer to its centre than to the centre of the earth, 
 and is therefore within its body or circumference. 
 
 The centre of inertia in a body moving evenly is also its centre of action 
 OY percussion ; because, if such centre come against an obstacle, the whole 
 
94 MECHANICS. 
 
 momentum of the body acts there and is destroyed ; while if any other part 
 than the centre hit, the body loses only part of its momentum, and revolves 
 round the obstacle as a pivot or centre of motion, to pass it on the side to- 
 wards which the greater inertia happened to 'be. 
 
 In a hammer, or a bar of iron used as a hammer, or in a pendulum, the 
 motion is not said to be even, because the velocity of the different parts is 
 different, being greatest far from the hand or centre of motion, and the centre 
 of all the motal inertia is nearer to the fast moving end than to the other. 
 Its exact place, in many cases, is easily ascertained by calculation. In a uni- 
 form rod moving as a pendulum, for instance, it is at a distance of one-third 
 from the lower end. In the pendulum it is called the centre of oscillation. 
 If a man use a bar or rod of iron as a hammer, he must take care to make 
 it strike the object by its centre of action, or his own hand will receive a part 
 of the shock. A very heavy mass thus carelessly used will seriously strain 
 the wrist. In a common hammer, as the chief part of the matter is at the 
 end, the centre of percussion is there too, and no precaution of the kind men- 
 tioned is required. 
 
 If a rod or small log of wood be suspended horizontally by a string tied to 
 its middle, or be floating in water, and if a forward blow be given directly 
 across it near to one end, the other end will be found, in the first instant to 
 have moved a little backward, or in a direction contrary to the blow, as if the 
 rod had been fixed upon an axis. The inertia of the general mass, by re- 
 sisting the motion becomes in effect a 
 
 Fig. 35. fixed axis. This fact is amusingly 
 
 a n i illustrated by laying the ends of a 
 
 long stick on two wine-glasses, and 
 then breaking the stick by a smart 
 downward blow of a poker on its cen- 
 tre. Instead of breaking the glasses 
 also, as by such a blow might be expected, the ends of the stick rise at the 
 instant of the stroke, to turn round certain centres of resistance in the frag- 
 ments, as at a and b , and then fall harmless on the table. 
 
 In this section we have seen what admirable simplicity is given to many 
 of our reasonings and operations, by considering bodies in reference only to 
 their centre of inertia, under one or other of its names. 
 
 u In a solid body moving about an axis, like a wheel or weighing-beam, 
 the different parts have different velocities, according to their respective 
 distances from the axis or centre" ( Read the Analysis. ) 
 
 The truth of this proposition is perceived at pnce on comparing the motion 
 in the rim of a wheel, or near the ends of a weighing-beam, with that in 
 parts nearer the centres. Suppose a d to be a 
 Fig. 36. line drawn across a wheel, or, along a weighing- 
 
 beam, the centre of motion in either case being 
 at c; then the outer circular line or path, c, 
 which a point in a describes when moving, is 
 longer than the "corresponding inner line, b f, 
 which a point at b describes in the same time, as 
 a is farther from the centre than b. This admits 
 of easy mathematical 'demonstration, and is in- 
 deed merely an instance of the truth, that the 
 proportions existing between any parts or lines in 
 one circle, hold with respect to the corresponding 
 parts and lines in all circles. 
 
 T 
 
f 
 
 SIMPLE MACHINES. 95 
 
 " Hence forces with different speed may still be placed in continued connec- 
 tion or opposition ; and they will balance or be equivalent, if the one be as 
 much more intense than the other as it is slower." (Read the Analysis.) 
 
 This is the important truth unon which the whole of mechanics may be 
 said to hinge. It gives to man the simple machines or mechanical powers, 
 as they have been called, the Lever, Wedge, Pulley, &c , which enable him 
 to adapt any species and speed of power which he can command to almost 
 any work which he has to accomplish : and the discovery of it and of means 
 to apply it may be said to have subjected external nature to his control. His 
 works are of a thousand kinds, from the displacing of a rock to the spinning 
 of a delicate thread ; while the natural powers or forces at his command are 
 chiefly wind, waterfalls, fire and animal effort and of which in any particu- 
 lar case, he may have only one kind at his service ; still, being able to con- 
 nect together his power and resistance by solid media, of which different 
 parts move with any desired difference of velocities, he can employ any force 
 for a purpose of almost any kind. 
 
 There is, however, a false and most pernicious prejudice very generally 
 existing with respect to the simple machines, which we must begin by 
 removing, viz., that they increase the quantity of power or force applied to 
 them. For instance, when one pound, as a at the end of a beam or lever, 
 is seen balancing two pounds, as b, at half the distance on the other side of 
 the axis, or four pounds as c, at a quarter of the distance, many persons be- 
 lieve that the lever itself gives or begets 
 
 a force equal to the difference of the Fig. 37. 
 
 weights so balanced. But we shall now <s 
 
 show, that levers and all the other me- 
 chanical powers (as from the erroneous 
 
 idea above mentioned they have been # , . , fl '\ ^ & 
 called,) merely enable us to make such ,JJ 
 
 substitutions, so that of a small weight U j p>, 
 
 descending far, in place of a greater * J... ! 2- :< 
 
 weight descending a little way, or of an .".^ 
 
 inferior force working long, instead of a | \ 
 
 superior force working for a shorter '*"" 
 
 time, and thus often to accomplish 
 ends to which the force possessed would 
 
 be quite unsuited if applied directly. In other words the simple machines 
 enable us to concentrate or divide any kind or quantity of force which we 
 possess, so as to suit it to our various purposes, just as mill-ponds and 
 branching channels enable us to accumulate or divide the force of a stream 
 of water; but they no more increase the quantity of power than a mill-pond 
 increases the quantity of water. When any slender force is caused through 
 a machine, to produce some effect which seems proportioned to an intense 
 force, it has always to act longer, or through more space than the other, 
 just in proportion as it is more slender; as a small stream of water acting 
 for ten minutes, may produce the same effect as a greater gush in one 
 minute. Twenty feet of the action of a small horse near the circumference 
 of a great wheel, may be rendered, by intervening machinery, equivalent to 
 ten feet of the action of a heavy ox or elephant nearer the centre. And one 
 horse in drawing through six hundred feet, or a hundred horses in drawing 
 through six feet, or the piston of a great steam-engine, in raising one from the 
 bottom to the top of its cylinder, &c., may all be made to do the same work. 
 
96 MECHANICS. 
 
 To illustrate this subject farther; we shall suppose a weighing-beam x 
 with a weight of one pound hanging at the end x ; then if a spring issuing 
 from the fixed box at E, with uniform force of one pound, be made to push 
 at the other end of the beam y, it will just balance the weight; and if it be 
 
 in the slightest degree stronger than 
 
 Fi S- 38- the weight, it will push the end of 
 
 fl. the beam y down to B, and will raise 
 
 / 3> 33 tne weight to F. If, instead of the 
 single spring of one pound at the end 
 of the beam, two such springs be 
 
 A 
 
 p T 
 
 T ............ I / 
 
 "" '""' 
 
 .^ 
 ...... -U ........ + applied at half way from the centre 
 
 "Jj to the end so as to press at A, where 
 
 there is just half the extent of mo- 
 tion, or room to act, as at B, exactly 
 
 the same effect will follow. Now because one spring at the end of the beam 
 is seen here doing the same work as two similar springs, or a single spring 
 of double strength at the middle, it might at first appear that there was a 
 saving of power by using the single spring and longer lever ; but let it be 
 observed, that the two middle springs have each issued from their box only 
 one inch, while the single spring at the end has issued two inches : in both 
 cases, therefore, exactly two inches of one-pound spring have been used. 
 
 In the last experiment, pound weights or little buckets of water might be 
 used instead of the springs, and with exactly the same result one pound or 
 pint at the end of the arm producing the same effect as two pounds or pints 
 at the middle of it ; but it would be observed that the single quantity fell two 
 inches, while the double quantity at half distance fell only one inch ; and to 
 replace them after they had done their work, there would evidently be the 
 same labour, whether a person had to lift the single quantity first one inch, 
 and then another, or had to lift, first, one half of the double equipoise an inch, 
 and then the other half as much. Each atom of matter may be considered 
 as held to the earth by its thread of attraction, a.nd if one atom rise or fall 
 ten inches, just as much of the supposed thread of attraction will be drawn 
 out or returned as if ten atoms rise or fall one inch. And so where a weight 
 of one pound is made to do any work, instead of a weight of two pounds, 
 there is no more saving than in giving away two yards of single rope instead 
 of one yard of double rope ; and in like manner for all other differences of 
 intensity. 
 
 If a man were to exert a force of one hundred pounds at A, in the above 
 figure, to lift the weight, a boy at B, with force of fifty pounds, might do 
 the same work ; but the man would only have worked or pressed down 
 through one foot, while the boy would have worked through two; and there- 
 fore, although the boy, with the assistance of the lever, seemed to become as 
 strong as the man, the case would merely be, again, that of the one-pound 
 spring unbending two inches, to produce an effect equal to that of the two- 
 pound spring unbending one inch. The boy would be using two feet of his 
 smaller force, where the man used one foot of his greater force ; and if the 
 work had to be long continued, the boy would have completely exhausted 
 himself, when the man remained yet fresh. 
 
 A case of the lever, exhibited in this diagram, serves well to explain the 
 nature of mechanical powers in general. Suppose A to be a weight of four 
 pounds at the end of a rod or lever A B, (p. 97) made to turn on c as an 
 axis or fulcrum, and having the arm c B four times as long as the arm c A, 
 (but the two arms of the lever being equipoised so as not to conceal the action 
 
SIMPLE MACHINES. 97 
 
 * 
 
 of weights subsequently attached to them ;) then one pound at' the end B, 
 would balance the four pounds at the end of A, and with the slightest addi- 
 tional weight would preponderate. Now let us suppose the arc B b to have 
 been fixed to the long arm of the lever with the four projections or shelves 
 here shown on which balls of one 
 
 pound might rest; then if one of the Fig. 39. 
 
 four balls from the plane d were to roll 
 upon the first shelf, it would just 
 balance A, and, with one grain more, 
 would descend to the plane e, one inch 
 below ; then a second ball of one pound 
 would occupy the second shelf, and 
 would descend in the aume way, to be 
 
 followed by a third, and afterwards by / "" 
 
 a fourth ; and when the whole four had 
 
 fallen from d to e, they would Just have lifted the four pound mass, at the 
 other end of the lever, one inch. So that, although one pound was seen 
 here lifting four pounds, it would only have lifted them one-fourth part as 
 far as it fell itself, and the sum of the phenomena would be, that four pounds 
 by falling one inch at the long end of the lever, had raised four pounds 
 through the same distance of one inch at the short end. No mechanical 
 power or machine generates force more than the lever does in this case. 
 
 It appears, then, from all this, that as the quantity of motion in a body is 
 measured by its velocity and the number of atoms in it conjointly, so the 
 quantity of force exerted in any case, is measured by the intensity of the 
 force conjointly with the space through which it moves. A clear mode, 
 therefore, of comparing forces, is to state the lengths and the intensities 
 for instance, to speak of ten feet of one-pound force, as equal to one foot of 
 ten-pound force, &c. 
 
 A horse pulling with the force of fifty pounds goes generally at the rate of 
 six miles an hour ; the steam-engine piston is generally made to move at the 
 rate of two hundred feet per minute, bearing a pressure of steam 'of about 
 twenty pounds to each square inch of its surface ; a particular mill-stream 
 may have a force of one hundred pounds, with a velocity of a hundred and 
 fifty feet per minute : now it is easy, by simple arithmetic, and the rule of 
 length and intensity above explained, to compare all these and other forces 
 as applicable to any given work. We must warn the reader, however, that 
 there are many important considerations connected with the practical employ- 
 ment of forces, according to their respective nature and that of the resistance 
 to be overcome, which cannot be entered upon in this elementary work. In 
 very many cases there is a great waste or unavoidable loss of force, because 
 the resistance, in yielding, runs away or escapes from the force ; as when a 
 ship runs away from the wind which is driving her, or the floats of a quick 
 moving water-wheel, from the stream which turned it. Horses drawing boats 
 or carriages at the rate of five miles an hour, might exert great force, but to 
 have a speed exceeding twelve miles they might require tl^ir whole effort 
 to move their own bodies. As a general rule, although equul quantities of 
 force balance each other when applied to parts of a lever or wheel altogether 
 or nearly at rest, still when a force is made to act near its axis or fulcrum, 
 to produce considerable velocity in a more distant part of the machinery, 
 much of it is wasted in pressure against the fixed fulcrum. 
 
 What an infinity of vain schemes yet some of them displaying great 
 ingenuity for perpetual motion, and new mechanical engines of power, &c. ; 
 
 7 
 
98 MECHANICS. 
 
 % 
 
 would have* been checked at once, had the great truth been generally under- 
 stood, that no form or combination of machinery ever did or ever can increase, 
 in the slightest degree, the quantity of power applied. Ignorance of this is 
 the hinge on which most of the dreams of mechanical projectors have turned. 
 No year passes, even now, in which many patents are not taken out for such 
 supposed discoveries; and the deluded individuals, after selling perhaps even 
 their household necessaries to obtain the means of securing the expected 
 advantages, often sink into despair, when their attempts, instead of bringing 
 riches and happiness to their families, end in disappointment and ruin. The 
 frequency, and eagerness, and obstinacy, with which even talented individuals, 
 owing to their imperfect knowledge of this part of natural philosophy, have 
 engaged in such undertakings, is a remarkable phenomenon in human nature. 
 Examples of such schemes will be noticed in different parts of this work, 
 where they may serve to illustrate points under consideration. 
 
 " Lever ) wheel and axle y &c" (Read the Analysis, at page 84.) 
 
 These are the simplest of the contrivances which the circumstances of 
 solidity in masses has enabled man to adopt for the purpose of connecting 
 or opposing forces and resistances of different intensities. We proceed to 
 describe them, and to explain some of their, useful applications. 
 
 " Lever." 
 
 A beam or rod of any kind, resting at one end of a prop or axis, which 
 becomes its centre of motion, is a lever ; and it has been so called, probably, 
 because such a contrivance was first employed for lifting weights. 
 
 This figure represents a lever employed to move a block of stone ; a, is 
 the end to which the power or force is applied,/, is the prop or fulcrum, and 
 the mass 6, is the weight or resistance. According to the rule already given 
 
 and explained at page 96, the power may be 
 
 Fig. 40; as much less intense than the resistance as 
 
 it is fajrther from the fulcrum, or moving 
 through a greater space. A man at a, there- 
 fore, twice as far from the prop as the centre 
 of gravity of the stone 5, will be able to lift 
 a stone twice as heavy as himself; but he 
 will lift it only one inch for every two inches 
 
 that he descends : and two men would be required, acting at half the dis- 
 tance, to do the same work. 
 
 There is no limit to the difference, as to intensity, of forces which may be 
 made to balance each other by the lever, except the length and strength of 
 the material of which levers have to be formed. Archimedes said, " Give me 
 a lever long enough, and a prop strong enough, and with my own weight I 
 will lift the world. " But he would have required to move with the velocity 
 of a cannon-ball for millions of years, to alter the position of the earth by 
 a small part of an inch. As stated in a former part of the volume, this feat 
 of Archimedes is, in mathematical truth, performed by every man who leaps 
 from the ground ; he kicks the world away from him when he rises, and 
 attracts it again when he falls back. 
 
 To calculate the effect of a lever, in practice, we must always take into 
 account the weight of the lever itself, and the fact of its bending more or 
 less ; but in expounding the theory of the lever, it is usual to consider, first, 
 
SIMPLE MACHINES. 99 
 
 what would be the result if the lever were a rod without weight and without 
 flexibility. 
 
 The rule for the lever, that the opposing forces, to balance each other, must 
 be more or less intense, exactly as they act nearer to or farther from the 
 centre, holds in all cases, whether the forces be on different sides of the prop 
 or both on the same side, and whether the force nearest to the prop have the 
 office of power or of resistance ; it holds, also, whether the lever be straight 
 or crooked. 
 
 The following are examples of levers with the prop between the forces. 
 
 The handspiJce, represented in page 98, is a lever moving a block of stone* 
 The same form, when made of iron, with the extremity formed into claws, is 
 called croio-bar. Both kinds are used by gunners, in working cannon during 
 battle : they are also used generally for lifting and moving heavy masses 
 through small spaces, as the materials of the mason, the ship-builder, the 
 warehouse-man, &c. A short crow-bar is the instrument of house-breakers, 
 for wrenching open locks or bolts, tearing off hinges, &c. 
 
 The common daw-hammer, for drawing nails, is another example. A 
 boy who cannot exert a direct force of fifty pounds, may yet, by means of 
 this kind of hammer, extract a nail to which half a ton might be quietly sus- 
 pended, because his hand moves through, perhaps, eight inches, to make 
 the nail rise one-quarter of an inch. The claw-hammer also proves, that 
 it is of no consequence whether the lever be straight or- crooked, provided 
 "it produces the required difference of velocity between power and resistance. 
 The part of the hammer resting on the plank is the fulcrum. 
 
 A pincers or forceps consists of two levers, of which the hinge is the 
 common prop or fulcrum. In drawing a nail with steel forceps or nippers, 
 we have a good example of the advantages of using a tool : 1, the nail is 
 seized by the teeth of steel instead of by the soft fingers : 2, instead of the 
 griping force of the extreme fingers only, there is the force of the whole 
 hand conveyed through the handles of the nippers : 3, the force is rendered, 
 perhaps, six times more effective by the lever-length of the handles : and 4, 
 by making the nippers, in drawing the nail, rest on one shoulder as a 
 fulcrum it acquires all the advantages of a lever or claw-hammer for the 
 same purpose. 
 
 Common scissors are also double levers, and those stranger shears with 
 which, under the power of a steam-engine, bars and plates of iron are now 
 cut as readily as paper is cut by the force of the hand. 
 
 The common fire-poker is a lever. It rests on the bar of the grate as its 
 prop, and displaces or breaks the caked coal behind as the resistance. 
 
 The mast of a ship, with sails set upon it, may be regarded as a long 
 lever, having the sails as the power, turning upon the centre of buoyancy of 
 the vessel as the fulcrum, and lifting the ballast or centre of gravity as the 
 resistance. For this reason lofty sails make a ship heel or lean over greatly, 
 and if used in open boats, are dangerous. In some of the islands in the 
 Eastern and Pacific Oceans, for the sake of sailing swiftly, boats are used 
 so extremely narrow and sharp, that to counteract the overturning tendency 
 of their large sails, they have an outrigger or projecting plank to wind- 
 ward, on the extremity of which one or more of the crew may sit as a 
 balance. 
 
 Perhaps no instance of the lever, with the prop between the forces, is 
 more interesting than the weighing -beam ; whether with equal arms, form- 
 ing the common scale-beam ; or with unequal arms, forming the steel-yard. 
 
100 MECHANICS. 
 
 We have seen why quantities of matter attached at equal distances from 
 the prop, must be equal to each other in order to balance. A lever, there- 
 fore, which enables us to place quantities thus exactly in opposition to each 
 other, and which turns easily on its axis, becomes a weighing-beam. Of this 
 
 the annexed figure shows a common form. 
 41 - The axis or pivot at c is sharpened below, 
 
 wedge-like, that the beam may turn easily, 
 and that its centre of motion may be nicely 
 determined ; in a delicate balance for philo- 
 sophical purposes, the axis is almost as sharp 
 as a knife edge, and rests on some hard smooth 
 surface of support, so as to turn with the 
 weight of a small part of a grain. The scales 
 
 also of a weighing-beam are suspended on sharp edges to facilitate motion, 
 and to determine nicely the points of suspension. If the two arms of a beam 
 be not of perfectly equal length, a smaller weight at the end of the longer will 
 balance a greater weight at the end of the shorter. An excess of half an 
 inch in the length of a beam-arm, to which merchandise is attached, where 
 the arm should be eight inches long, would cheat the buyer of exactly one 
 ounce in every pound. This case might be detected instantly, by changing 
 the places of the two things balanced ; for so, the lightest would be at the 
 short arm, and would then appear doubly too light. A beam intended for 
 delicate purposes, and required, therefore, to turn easily, must have its centre 
 of gravity very near the axis on which the beam turns; for if otherwise, th<? 
 beam will be in the predicament of a ship with the ballast too high or too low : 
 in the former case, when once inclined, it would fall over, and not to recover 
 itself: in the latter, it would tend to remain horizontal, and therefore would 
 be less free to move. The proper situation of the centre of gravity is a little 
 below the axis or line of support, that the beam may return with sufficient 
 readiness from any state of inclination, to its horizontal position of rest. 
 
 There is a mode of arriving at very accurate results, even with a weigh- 
 ing-beam which is not itself accurately made, provided it has very free 
 motion, viz., first, very nicely to balance in one scale the substance to be 
 weighed, and then to remove it, and to put weights into the same scale, until 
 a perfect balance is produced. Such weights must be the exact equivalent 
 or weight of the substance, however unlike to each other the arms of the 
 balance may be. A projecting rod, or plank, or branch of a tree, may thus 
 be made to answer the purpose of a weighing-beam, by attaching any sub- 
 stance to its extremity and observing minutely how far such substance 
 bends it, and then trying what weights would bend it as much. 
 
 The steel-yard is a lever with unequal arms, and any weight as 6, on the 
 
 long arm, will balance as much more 
 
 Fig. 42. weight as a on the short arm, as the for- 
 
 mer is supported farther from the fulcrum 
 
 j 2 3 .4 .5 G than the latter. Thus, if the hook at the 
 1 ' ' T * short end be one inch from the centre of 
 (5 support, c, a pound weight 5, on the long 
 
 arm at four inches, will balance four 
 
 \a, pounds a, at the short arm. This sup- 
 
 poses, however, that the steel-yard when 
 bare, hangs horizontally, from having a 
 greater mass of matter in the short arm to counterbalance the long slender 
 
SIMPLE MACHINES. 101 
 
 arm from which the shifting weight hangs. When this is not the case, a 
 corresponding allowance has to be made. 
 
 The Chinese, who are so remarkable for the simplicity to which they have 
 reduced all their common implements, weigh any small objects by a delicate 
 pocket steel-yard. It is a rod of wood or ivory, about six inches long, with 
 a silk cord passing through it at a particular part, to serve as a fulcrum, and 
 with a sliding weight on the long arm, and a small scale attached to the 
 short one. 
 
 The following are examples of levers with both forces on the same side of the 
 prop, and where the more distant force acts as the power. 
 
 A common wheel-barrow is a lever, in using which a man bears as much 
 less than the whole weight of the load as the centre of gravity of the load is 
 nearer to the axle of the wheel than to his hands. 
 
 When two porters carry a load placed midway between them, on a pole, 
 they share it equally, that is to say, each bears a half, for the pole becomes a 
 lever, of which each porter is a fulcrum, as regards the other; but if the load 
 be nearer to one end than to the other, he to whom 
 it is nearest bears proportionably more of its weight. F g 43. 
 
 A load at c is equally borne by a porter at a and a & G ~b 
 
 by one at b ; but a load at d gives thre.e-quarters i i 
 
 of its weight to the man at a and only one-quarter <Ja f* 
 to him at b. LJ LJ 
 
 Two horses drawing a plough, act from the ends 
 
 of a cross bar, of which the middle is usually hooked to the plough. The 
 horses must thus pull equally, to keep the bar directly across. When on 
 heavy land, three horses are yoked, and two of them are made to draw from 
 one end of the bar, it must be attached to the plough by a hook, not at its 
 middle, but half as far from one end of it as from the other. 
 
 The oar of a boat is a lever of this kind, where singularly the purpose of 
 fulcrum is served by the unstable water. 
 
 The common nut-crackers furnish another instance, by the lever-power of 
 which a person can break a shell many times stronger than he could break 
 with the bare fingers. 
 
 The consideration of this kind of lever explains why a finger caught near 
 the hinge of a shutting door is so much injured. The momentum of the 
 door acts by a comparatively long lever, upon a resistance placed very near 
 the fulcrum. Children pinching their fingers near the hinge of a door, or of 
 the fire-tongs, which furnishes a similar case wonder why the bite is so keen. 
 
 The phenomenon of the branch of a tree giving way, when in autumn 
 overloaded with fruit, or in winter with snow, also exhibit the action of this 
 kind of lever. The resistance is the cohesion of the upper side of the branch 
 to the tree, and the fulcrum is the part below which is last broken. 
 
 The following are examples of the lever, where the two forces are on the 
 same side of the pivot, but where that nearest to the pivot acts as the power. 
 In this kind, the power is more intense than the resistance. 
 
 The hand of a man who pushes open a gate while standing near the hinges, 
 moves through much less space than the end of the gate, and hence must act 
 with great force. 
 
 When a man uses the common fire-tongs, the ends move much further 
 
102 MECHANICS. 
 
 than his fingers, and therefore with less strength. No one fears a pinch 
 with the ends of the fire- tongs. 
 
 The most beautiful and remarkable instances of this modification of lever 
 are in the limbs of animals. The object in these was to give to the extremi- 
 ties great range and freedom of motion, 1 without clumsiness of form ; and it 
 has been attained most perfectly by the tendons or ropes which move them, 
 being attached near to the joints, which are the pivots or fulcra of the bone 
 levers. 
 
 In the human arm, the deltoid muscle, which forms the cushion of the 
 shoulder, by contracting its fibres less than an inch, raises the elbow twenty 
 inches, and of course, if it overcome a force of fifty pounds at the elbow, it 
 must itself be acting with a force at least twenty times as intense, or of one 
 thousand pounds. What extraordinary strength of muscle, then, is displayed 
 by a man who lifts another at the end of his extended arm ; yet this feat is 
 frequently accomplished, and even on both sides of the person at once. 
 
 How powerful again must be the wing-muscles of birds, which, by this 
 kind of action, sustain themselves in the sky for many hours together. The 
 great albatros, with wings extended fourteen feet or more, is seen in the 
 stormy solitudes of the Southern Ocean, accompanying ships for whole days, 
 without ever resting on the waves. 
 
 A little contraction of the glutaei muscles of the hips gives to the human 
 step a length of four feet. 
 
 While the erroneous opinion prevailed, that machines increased power, 
 instead of, as they do, merely accommodating forces to purposes, this last 
 kind of lever, where a great force acting through a short distance is made to 
 gain great extent of motion and other benefits, was regretted by many as a 
 most unprofitable, contrivance and was called the losing lever. 
 
 It is almost unnecessary to say, that the same rule of comparative veloci- 
 ties ascertains the relations required between power and resistance, where a 
 combination of levers is used, as where there is only one. If a lever which 
 makes one balance four, be applied to work a second lever which does the 
 same, one pound at the long arm of the first will balance sixteen pounds at 
 the short arm of the second, and would balance sixty-four at the short arm 
 of a third such, &c. 
 
 The general rule for the lever, that a force may be less intense the farther 
 it is from the pivot, supposes always that the force acts at right angles, or 
 directly across the lever; for if there be any obliquity, there is a correspond- 
 ing diminution of effect, as explained under the head of resolution of forces, 
 at page 57. For instance, one pound at b on the end of the long arm of the 
 bent lever b d a, because its weight does not act 
 Fig- 44. directly across b d, has influence only as if it were 
 
 acting directly at the end of a shorter horizontal 
 arm df; and the two-pound weight at a acts only 
 as if it were on a horizontal arm at e; now e being 
 only half as far from the centre as/, two pounds 
 at a, in the position of the lever here shown, 
 would just balance the one pound at b. In every 
 case, the exact influence of weights is known by 
 referring them to places directly above or below 
 them, on a supposed horizontal lever ef. What 
 is called a bent-lever balance, is made on the principle here explained. It has 
 on one side a heavy weight <fs at a, and on the other side a scale attached at 
 b ; and the weight of any thing put into the scale is indicated by the position 
 
SIMPLE MACHINES 
 
 103 
 
 then assumed by the lever, marked by the point at which it cuts an arc of 
 divisions placed behind it. In any common weigh-beam-, the point of sus- 
 pension of the scales being a little below the axis of motion of the beam, 
 there is a degree of the property of the bent-lever balance, and enough to 
 require notice in very nice experiments. 
 
 The Wheel and Axle" 
 
 Fig. 45. 
 
 a, 
 
 is the next to be mentioned of the simple machines. The letter d here marks 
 a wheel, and e an angle affixed to it; and we see that in turning together, the 
 wheel would take up or throw off as much 
 more rope than the axle, as its circumference 
 or diameter were greater than that of the axle. 
 If the proportions were as four to one, one 
 pound at b hanging from the circumference of 
 the wheel, would balance four pounds at a, 
 hanging from the opposite side of the axle. 
 The proportions are equally indicated, and are 
 usually expressed by comparison of the diame- 
 ters of the wheel and the axle. 
 
 This figure represents the same object as the 
 last, viewed endways. It explains why the 
 wheel with its axle has been called a perpetual 
 
 lever ; for the two weights hanging in opposition, on the wheel at a, and on 
 the axle at 5, are always as if they were connected by a 
 horizontal lever at a c 6, of which the arms are respectively 
 the diameters of the wheel and the axle turning on the 
 centre c as the prop; and while a simple lever could only 
 lift through a small space, it is evident that this construc- 
 tion will lift as ^mg as there is rope to be wound up. 
 
 A common crane for raising weights consists of an axle 
 to wind up or receive the rope which carries the weight, 
 and of a large wheel at the circumference of which the 
 power is applied. The power may be animal effort exerted 
 on the rim or outside of the wheel, or the weight of a man 
 or beast, walking within it, and moving it as a squirrel 
 moves the cylinder of his cage. 
 
 The capstan used on board of ships, is merely a large upright axle or 
 
 Fig. 46. 
 
 Fig. 47. 
 
 spindle b, which by turning, pulls the 
 cable or rope a b c ; and it is moved by 
 the men pushing at the capstan-bars d, e, 
 /, &c., which for the time are stuck into 
 holes made for them in the broader part 
 or drum, usually 'appearing above the 
 deck at the top of the spindle. These 
 bars may be considered as the spokes of 
 a large wheel, and the effect produced by 
 a man working at one of them is in pro- 
 portion to his distance from the centre. The capstan is chiefly used on board 
 ships for lifting the anchor, and for doing any other very heavy work ; but it 
 is also applied for certain purposes on shore. 
 
 The common winch (represented as attached to the wheel and axle at the 
 letter c,) with which a grindstone is turned, or a crane worked, or a watch 
 
104 
 
 MECHANICS. 
 
 wound up, is really in principle a wheel : for the hand of the worker 
 describes a circle, and there is no difference in the result whether an entire 
 wheel be turning with the hand or only a single spoke of a wheel. 
 
 That part of a common watch called the fusee is as beautiful an illustra- 
 tion of the principle of the wheel and axle now under consideration, as it is 
 
 a useful and ingenious contri- 
 
 Fig. 48. vance. The spring of a watch, 
 
 r) <i immediately after winding up, 
 
 being more strained, is acting 
 more powerfully than after- 
 wards when slacker, and if 
 there were no means of equal- 
 
 izing its action, it would de- 
 
 [j !] stroy the wished-for uniform- 
 
 ity in the motion of the time- 
 piece. The fusee is this means. It may be considered as a barrel or spindle, 
 gradually diminishing from its large end 6, to its small end a, with the 
 surface cut into a spiral grove to receive the chain, by pulling at which the 
 spring in the box c moves the watch. Now when the watch has been wound 
 up, by a key applied on the axis of the fusee, the fusee is covered with the 
 chain up to the small end a, and the newly bent and strong spring begins 
 to pull by this small end or short lever; and afterwards, exactly as the 
 spring becomes relaxed and weaker, it is pulling at a larger and larger part 
 of the fusee-barrel, and so keeps up an equal effect on the general move- 
 ment. 
 
 A large fusee in place of a common cylindrical axle, is often used with a 
 winch, for drawing water by bucket or rope from very deep wells. When 
 the bucket is near the bottom of the well, and the Iab6rer has to overcome 
 the weight of the long rope, in addition to that of the bucket and water, he 
 does so more easily by beginning to wind the rope on a sm|ill axle, that is 
 to say, on the small end of the fusee ; and in proportion as the length of rope 
 diminishes, he lifts by a larger axle. 
 
 By the double axle a b, very unequal .intensities of force maybe balanced. 
 
 We see that in turning it, a rope unwinding 
 from the small end a is taken up by the large 
 end b, turn for turn, and that the rope below 
 must be shortened at each turn by the differ- 
 ence between the circumference of the ends 
 a and b. If the weight rise half an inch 
 only, while the handle of the winch describes 
 a circle of fifty inches, one pound force at the 
 winch would balance one hundred pounds 
 at d. 
 
 By means of a wheel, which is very large 
 in proportion to its axle, forces of very differ- 
 ent intensities may be balanced, but the machine becomes of inconvenient 
 proportions. It is found preferable, therefore, when such an end is desired, 
 to use a combination of wheels of moderate size. In the adjoining figure, 
 three wheels are seen thus connected. Teeth on the axle rf, of the first 
 wheel c, acting on six times the number of teeth in the circumference of 
 the second wheel g, turn it only once for every six times that c turns; and 
 in the same manner the second wheel, by turning six times, turns the 
 
 Fig. 49. 
 
SIMPLE MACHINES. 
 
 105 
 
 third wheel h once ; the first wheel, there- 
 fore, turns thirty -six times for one turn of 
 
 the last ; and as the diameter of the wheel 
 
 c, to which the power is applied, is three 
 
 times as great as that of the axle/, which 
 
 has the resistance, three times thirty-six, 
 
 or one hundred and eight, is the difference 
 
 of velocity, and therefore of intensity, be- 
 tween weights or forces that will balance 
 
 here. An axle with teeth upon it, as d 
 
 or e, is called a pinion. 
 
 On the principle of combined wheels, 
 
 cranes are made,, by which one man can 
 
 lift many tons. It is even possible to 
 
 make an engine, by means of which a little windmill, of a few inches in 
 ( - diameter, should tear up the strongest oak by the roots ; but of course it 
 would require a very long time for its work. 
 
 The most familiar instances of wheel-work are in our clocks and watches. 
 One turn of the axle. on which the watch-key is fixed, is rendered equiva- 
 lent, by the train of wheels, to about four hundred turns or beats of the 
 balance-wheel ; and thus the exertion during a few seconds, of the hand 
 which winds up, gives motion for twenty-four or thirty hours. By increas- 
 ing the number of wheels, time-pieces are made which go for a year ] if the 
 material would last, they might easily be made to go for a hundred or a 
 thousand years. 
 
 Wheels may be connected by bands as Fig. 61. 
 
 well as by teeth. This is seen in the com- 
 mon spinning-wheel, turning-lathes, grind- 
 stones, &c. &c. A spinning-wheel, as a c, 
 of thirty inches in circumference, turns by 
 its band a pirn or spindle of half an inch, 6, 
 sixty times for every turn of itself. 
 
 " The Inclined Plane" 
 
 is the third means which we shall describe, of balancing, by solid media, 
 forces of different intensities. A force push- 
 ing a weight from c to d } only raises it Fig. 52. 
 through the perpendicular height e d, by 
 acting along the whole length of the plane 
 c d; and if the plane be twice as long as it 
 is high, one pound at b acting over the 
 pulley d would balance two pounds at a, or 
 
 anywhere on the plane : and so of all other 
 
 quantities and proportions, as already ex- 
 plained under the head of " Eesolution offerees," at page .86. 
 
 A horse drawing on a road where there is a rise of one foot in twenty, 
 is really lifting one-twentieth of the load, as well as overcoming the friction 
 and other resistances of the carriage. Hence the importance of making roads 
 as level as possible ; and hence our forefathers often erred in carrying their 
 roads directly over hills, for the sake of straightness considered vertically, 
 where by going round the bases of the hills they would scarcely have had 
 greater distance, and would have avoided all rising and falling. Hence, also, 
 
106 MECHANICS. 
 
 
 
 a road up a very steep hill must be made to wind or zig-zag all the way ; for 
 to reach a given height, the ease of the pull to the horses is greater exactly 
 as the road is made longer. This rule of road-making is exhibited remark- 
 ably in various parts of the world, where hills with almost perpendicular 
 faces have very safe and commodious roads upon them, leading to forts or 
 residences near their summits. An intelligent driver, in ascending a steep 
 hill on which there is a broad road, winds from side to side of the road all 
 the way to save his horses a little. 
 
 The railways of modern times offer a beautiful illustration of this subject. 
 They are made generally quite level, so that the drawing horse or steam- 
 engine has only to overcome the friction of the carriage ; or where heavy 
 loads are passing only in one direction', as from mines, they are made to slope 
 a very little, leaving to the horse or other power only the office of regulating 
 the movement. 
 
 A hogshead of merchandize, which twenty men could not lift directly, is 
 often seen moved into or out of a wagon, by one or two men, who have the 
 assistance of an inclined plane. In some canals, or rather particular situa- 
 tions on canals, the loaded boats are drawn up by machinery or inclined 
 planes, instead of being raised by water in locks, as is the usual mode. 
 
 It is supposed that the ancients (the Egyptians particularly) must have 
 used the inclined plane, to assist in elevating and placing those immense 
 masses of stone, which still remain from their times, specimens of their 
 gigantic architecture. 
 
 Our common stairs are inclined planes in principle ; but being so steep, 
 are cut into horizontal and perpendicular surfaces, called steps, that they 
 may afford a firm footing. 
 
 We may here recall, that a body falling freely, in obedience to gravity, 
 descends about sixteen feet in the first second, and that if made to descend 
 on an inclined plane, it moves just as much less quickly (besides- the loss 
 from the friction and the turning produced) as the length of the plane is 
 greater than the height. On a plane sloping one foot in sixteen of its length, 
 a body would descend only one foot in the first second. 
 
 The descent of a pendulum in its arc is investigated mathematically by 
 the laws of the inclined plane. And the laws of the inclined plane itself 
 are mathematically explained by the principle of the resolution of forces, 
 explained at p. 57. 
 
 " The Wedge" 
 
 is merely an inclined plane forced in between resistances to separate or over- 
 come them, instead of, as in the last case, being stationary while the resistance 
 is moved along its surface. The same rule as to mechanical 
 Fig. 53. advantage has been applied to the wedge as to other simple 
 machines ; the force acting on a wedge being considered as 
 moving through its length c d, while the resistance yields to 
 the extent of its breadth a b. But this rule is far from ex- 
 plaining the extraordinary power of a wedge. During the 
 tremor produced by the blow of the driving-hammer, the 
 wedge insinuates itself, and advances much more quickly 
 than the above rule anticipates. 
 
 The wedge is used for many purposes; as for splitting 
 blocks of stone and wood : for squeezing strongly, as in the 
 oil-press ; for lifting gfeat weights, as when a ship of war, in 
 dock is raised by wedges driven under the keel, c. 
 
SIMPLE MACHINES.. 107 
 
 An engineer in London, who had built a very lofty and heavy chimney, 
 common to all his steam-engines and furnaces, found after a time that, 
 owing to a defect in the foundation, it was beginning .to incline. However, 
 by driving wedges under one side of it, he succeeded in restoring it to 
 perfect perpendicularity. 
 
 Nails, awls, needles, &c., are examples of the wedge; as are also all our 
 cutting instruments, knives, razors, the axe, &c. These latter are often 
 used somewhat in the manner of a saw which is a series of small wedges 
 by pulling them lengthwise at the same time that they are pressed directly 
 forward against the object. They themselves, indeed, when viewed through 
 a microscope, are seen to be but finer saws. It appears that the vibration of 
 the particles produced by the drawing of a saw, enables its edge to insinuate 
 itself more easily. The sharpest razor may be pressed directly against the 
 hand with considerable force, and will not enter, but if then drawn along 
 ever so little, it starts into the flesh. 
 
 " The Screw" 
 
 is another of the simple machines. It may be called a winding wedge, for 
 it has the same relation to a straight wedge that 
 a road winding up a hill or tower has to a straight 
 road of the same length and acclivity. 
 
 A screw may be described as a spindle a d, with 
 a thread wound spirally round it, turning or work- 
 ing in a nut c, which has a corresponding spiral fur- 
 row fitted to receive the thread. The nut is some- 
 times called the female screw. Every turn of the 
 screw carries it forward in a fixed nut, or draws a 
 moveable nut along upon it, by exactly the distance 
 between two turns of its thread : this distance, there- 
 fore, is the space passed through by the resistance, 
 while the force moves in the circumference of the 
 circle described by the handle of the screw, as at /, in the figure. The dis- 
 parity between these lengths or spaces is often as a hundred or more to one ; 
 hence the prodigious effects which a screw enables a small force to produce. 
 
 Screws are much used in presses of all kinds : as in those for squeezing 
 oil and juices from such vegetable bodies as linseed, rapeseed, almonds, 
 apples, grapes, sugar-cane, &c. : they are used also in the cotton press, 
 which reduces a great spongy bale, of which a few, comparatively, would 
 fill a ship, to a compact package, heavy enough to sink in water ; also, in 
 the common printing-press, which has to force the paper strongly against the 
 types : a screw is the great agent in our coining machinery, and in letter- 
 copying machines : it is a screw which draws together the iron jaws of a 
 smith's vice, &c. The screw, although producing so -much friction as to con- 
 sume a considerable part of the force -used in working it, is an exceedingly 
 useful contrivance. 
 
 As a screw can easily be made with a hundred turns of its thread in the 
 space of an inch, at perfectly equal distances from each other, it enables the 
 mathematical instrument maker to mark divisions on his work, with a minute- 
 ness and accuracy quite extraordinary. If we suppose such a screw to be 
 pulling forward a plate of metal, or pulling round the edge of a circle, over 
 which a sharp-pointed steel marker can be let down perpendicularly, always 
 in the same place, the marker, if let down once for every turn of the screw, 
 will make just as many lines on the plate as the screw makes turns; but if 
 
103 
 
 MECHANICS. 
 
 made to mark at every hundredth or a thousandth of a turn of the screw, 
 which it will do with equal accuracy, it may draw a hundred thousand dis- 
 tinct lines in one inch. 
 
 The instruments called micrometers, by which the sizes of the heavenly 
 bodies, and of microscopic objects, are ascertained, are worked by fine 
 screws. 
 
 A perpetual screw is the name given where a screw acts on the teeth of a 
 wheel, so as to produce a continued rotation of the wheel. 
 
 A commoii cork-screw is the thread of a screw without the spindle, and 
 is used, not to connect opposing forces, but merely to enter and fix itself in 
 the cork Complicated cork-screws are now made, which draw the cork by 
 the action of a second screw, or of a toothed rod or rack and pinion. 
 
 The Pulley" 
 
 is another simple machine, by which forces of different intensities may be 
 balanced. A simple pulley consists of a wheel as 
 Fig. 55. a b, which rests with its grooved circumference 
 
 . -ft on the bend of a rope, c a b d, and to the axis of 
 which the weight or resistance is attached, as 
 at e. I 
 
 In such a construction, it is evident that the 
 weight (let it be supposed ten pounds) is equally 
 supported by each end of the rope, and that a man 
 holding up one end, only bears half of the weight, 
 or five pounds ; but to raise the weight one foot, 
 he must draw up two feet of rope ; therefore, 
 with the pulley, he is as if lifting five pounds two 
 feet, where, without the pulley, he would have to 
 lift ten pounds one foot. 
 
 Many wheels may be combined together, and 
 in many ways to form compound pulleys. Where- 
 
 ever there is but one rope running through the whole, as shown here, the 
 relation of power and resistance is known by the number of folds of the 
 rope which support the weight. Here there are four supporting folds, and 
 a power of one hundred pounds would balance a resistance of four hundred. 
 As persons using pulleys generally find it more convenient 
 Fig. 56. to stand upon the ground than to go up and apply their 
 
 force directly to one of the supporting ropes, the last of 
 these is commonly made to pass over a wheel above, and 
 to come down apart from the others, as shown here. This 
 portion not being directly connected with the weight, adds 
 convenience to the pulley, but is not to be counted with 
 the others, in estimating the relation of the power and 
 resistance. 
 
 In fixed pulleys, like those shown at a and c, p. 109, 
 there is no mechanical advantage, for the weight just moves 
 as fast as the power ; yet such pulleys are of great use in 
 changing the direction of forces. A sailor without mov- 
 ing from the deck of his ship, by means of such a pulley, 
 may hoist the sail or the signal-flag to the top of the 
 loftiest mast. And in the building of lofty edifices, where 
 heavy loads of material are to be sent up every few 
 
SIMPLE MACHINES. 109 
 
 minutes, a horse, trotting away with the end of the rope from d, in a level 
 courtyard, causes the charged basket b to ascend to the summit of the 
 building as effectually as if he had the power of climbing, at the same rate, 
 the perpendicular wall. jij g 5^ 
 
 There is a case, however in which a 'fixed 
 pulley may seem a balancer of different intensi- 
 ties of force; viz., where one end of a rope is 
 attached to a man's body, and the other is carried 
 over a pulley above, and brought down again to 
 his hands; for safety this end also should be 
 attached to his body. By using the hands then 
 to pull with force equal to half his weight, he 
 supports himself, and may easily raise himself to 
 the pulley. A man, by a pulley thus employed, { 
 
 may let himself down into a deep well, or from, 
 the brow of a cliff, with assurance of being able 
 easily to return, although no one be near to help 
 him ; and cases have often occurred where, by 
 such means, a fellow-creature's life might have been saved, or other im- 
 portant objects attained. How easily, for instance, might persons either 
 reach or escape from the elevated windows of a house on fire, by such a 
 pulley, which might readily be found and used where ladders could not be 
 obtained ! This kind of pulley furnishes a convenient means of taking a 
 bath from a ship's stern windows, &c. 
 
 The chief use of the pulley is on ship-board. It is there called a block, 
 although, strictly speaking, the block is only the wooden mass which sur- 
 rounds the wheel or wheels of the pulley. It aids so powerfully in over- 
 coming the heavy strains of placing the anchor, hoisting the masts and sails, 
 &c., that, by means of it, a smaller number of sailors are rendered equal to 
 the duties of the ship. Pulleys are also used on shore, instead of cranes 
 and capstans, for lifting weights, and overcoming other resistances. 
 
 Surgeons, in former days, when they trusted rather to force than to the 
 address which better information gives, used pulleys much to help in the 
 reductions of luxations, but often hurtfully, from not understanding the 
 force of the pulley. A good surgeon now rarely needs a pulley, and he who 
 should ignorantly stretch his patient on the rack, would be .well requited by 
 similar treatment. 
 
 The cranks of bell-wires, seen in the corners of our rooms, are bent levers 
 nearly equivalent to fixed pulleys. * 
 
 There is no reason, but old usage, why the appellation of mechanical power 
 should be confined to the six contrivances now explained, for those of 
 which the account is yet to follow equally deserve it ; and, as will be seen 
 under hydrostatics and pneumatics, the most powerful mechanical engines 
 do not belong to solids at all. 
 
 Engine of oblique action, is a title which may include a considerable 
 variety of contrivances for connecting different velocities. 
 
 Suppose c a and c b to represent two strong rods connected together, like 
 a carpenter's folding rule, by a hinge or joint at c. If the distant ends be 
 made to bear against notches in two obstacles, at a and 6, and by force then 
 applied to c, either to push or to pull, the joint' c be straightened or carried 
 towards d, the joint c will move through a much greater space than the sirnul- 
 
110 
 
 MECHANICS. 
 
 Fig. 58. 
 
 Fig. 59. 
 
 taneous increase of distance produced between a and b ; and, in proportion to 
 the disparity, the power applied at c will overcome 
 a more intense resistance at the extremities. The 
 mechanical power of this contrivance increases 
 rapidly, the nearer the jointed rods approach to 
 straightness. 
 
 If we suppose the end a to be steadied by a hinge 
 on frame-work, and .the end b to bear upon that 
 part of a printing-press which carries the paper 
 against the types, we have imagined the simple 
 press called, from its contriver, the Russell-press. A 
 man's force at d, at the moment when the rods are 
 drawn nearly to a straight line, becomes equivalent 
 to a pressure of many tons. 
 For the same reason, that by urging c toward d, in the last figure, the 
 extremities a and b are separated with great force, so by urging c in the con- 
 trary direction, the extremities would be drawn together with corresponding 
 force : and if we suppose a c b to be part of a rope coming through pulleys 
 at a and 6, to one end of which rope, beyond a, great resistance is attached, 
 one man, by pulling at c, may move a weight or resistance many times 
 greater than he could move by his direct power. 
 
 The following is another mode of connecting an oblique and a direct force 
 so as to balance them, although of different inten- 
 sities. If to turn a wheel (represented here by 
 the circle) a weight be suspended from d, it is act- 
 ing directly, for it descends just as fast as the cir- 
 cumference of the wheel moves, and would, there- 
 fore, be impelling with its whole strength : but if 
 it were suspended from the point e, it would then 
 be acting obliquely to the motion of that part of the 
 wheel, and from not descending so fast as if at c?, 
 it would have as much less effect on the wheel 
 than if there, as the line e b is shorter than the line 
 d c. The reason of this will be understood by 
 referring to the subjects of resolution of forces 
 and of bent levers, in former parts of the work. 
 For the same reason, if such a wheel were used 
 in lifting weights, a man turning it could lift as 
 
 much more attached at the point e than at the point d } as the line d c is 
 longer than e b. A man turning -this wheel in the direction from e to a, 
 with a weight hanging at e, would be lifting that weight exactly as if he 
 were rolling it up the inclined plane or curve e a. This figure is useful 
 
 in explaining the varying intensity of the ac- 
 tion of a crank or winch, in different parts of 
 its revolution, and of the combination of levers 
 used in the Stanhope printing-press^ in their 
 different positions : it explains also the degrees 
 of strength and support afforded by oblique 
 stays in buildings and in ships' rigging, and 
 many other kindred matters. 
 
 The arrangement of cross-jointed wires, re- 
 presented here, connects different velocities, and 
 therefore is really a mechanic power. It has 
 
 Fig. 60. 
 
SIMPLE MACHINES. Ill 
 
 been applied to some curious purposes, but to none of much utility. By 
 pressing the ends a and b towards each other, the wires, from being in the 
 position represented in the upper figure, immediately assume the position 
 represented in the lower ; so that the end c darts outward much farther than 
 the ends a and b approximate. 
 
 Different intensities of force are balanced, although not simultaneously, by 
 the following means ; which, therefore, according to the old idea, have 
 some claim to the name of mechanic powers. 
 
 A man may have a purpose to effect, which a forcible downward push 
 would accomplish : but his body being too weak to give that push directly, 
 he may employ a certain time in carrying a weight to such an elevation, 
 above his work, that when let fall its momentum may do what is required. 
 Here the continued effort of the man in lifting the weight, to a height of 
 perhaps thirty feet, may be just sufficient to produce a blow which will 
 cause a stake or pile to sink into the earth one inch ] and the contrivance 
 has therefore balanced forces, of which the relation as to intensity is marked 
 by the spaces thirty feet and an inch. 
 
 So also hammerSj clubs, battering-rams, slings, &c., are machines which 
 enable a continued moderate effort to overcome a great but short resistance. 
 
 The fly-wheel, which by persons ignorant of natural philosophy, has often 
 been accounted a positive power, in common cases merely equalizes the 
 effect of an irregular force. 
 
 In using a winch to turn a mill, for instance, a man does not act with 
 equal force all around the circle : but a heavy wheel fixed on the axis mode- 
 rates acceleration, and receives or absorbs momentum, while his action is 
 above par, and returns it again, giving to the machine, while his action is 
 below par, thus equalizing the movement. And in the common instances 
 of circular motion produced by a crank, as when, by the pressure of the foot 
 on a treadle, we turn a lathe or grind-stone, or spinning-wheel, the force is 
 only applied during a small part of the revolution, or in the form of inter- 
 rupted pushes ; yet the motion goes on steadily, because the turning grind- 
 stone, or wheel, or lathe, becomes a fly and reservoir, equalizing the effect 
 of the force. In a steam-engine which moves machinery by a crank, the 
 upward and downward pushes of the piston are converted, by means of a 
 heavy fly-wheel into a very steady rotatory motion. 
 
 A heavy wheel, however, has sometimes been used as a concentrator of 
 force or a machanic power. By means of a winch, or a weight, or otherwise, 
 motion or momentum being gradually accumulated in the wheel, is then 
 made to expend itself in producing some sudden and proportionally great 
 effect. Thus, a man may lift a very heavy weight by first, in any way 
 giving motion to a fly-wheel, and then suddenly hooking a rope from the 
 weight to the axle of the wheel, which rope being wound upon the axle, 
 lifts the weight. 
 
 A fly-wheel moved in the same manner, and containing the result of a 
 man's action during perhaps one hundred seconds, if made to impel a screw- 
 press, will, with one blow or punch, stamp a perfect medal, or from a rough 
 flat 'plate of silver will form a finished spoon, or other utensil. 
 
 A spring, in the same sense, may become a mechanical power. A person 
 may expend some minutes in bending it, and may then let fly its accumulated 
 energy in an instantaneous blow. A gun-lock shows this phenomenon on a 
 small scale. The slow bending of a bow, which afterwards shoots its arrow 
 with such velocity, is another instance. 
 
112 MECHANICS. 
 
 These, then, are the principal means which the solid state of bodies affords 
 us of balancing forces of different intensities. We s*hall find other such means 
 or mechanic powers belonging to liquids and airs. All of them are of inesti- 
 mable value to man, by enabling him to accommodate the forces which he 
 can command to any kind of work which he has to perform. Thus he makes 
 his millstone turn with the same velocity, whether it be moved by the slow 
 exertion of a horse or bullock, walking in a ring, or by the quicker motion 
 of a river gliding under the wheel, or by the rapid gush of a water-fall, or 
 by the invisible swiftness of the wind. And again, each of these forces he 
 can equally apply to turn the heavy millstone or to twist a cotton thread. 
 
 The wants of men seem first to have led them to use the simple machines, 
 for the purpose of raising great weights, or overcoming great resistances 
 and hence the name long used of mechanic powers, particularly for the 
 Lever, Wheel and Axle, Plane, Wedge, Screw, and Pulley : but the term 
 conveys to the uninformed a false idea of their real nature, and has begotten 
 the common prejudice with respect to them, that they generate force, or 
 have a sort of innate power for saving labour. Now so far is this from 
 being true, that in using them in any case, even more labour or bodily 
 exertion is expended than would suffice to do the work without them. This 
 assertion is intentionally rendered paradoxical to arrest, attention, but its 
 truth will appear from the following considerations. 
 
 One man may be able, with a tackle of pulleys having ten plies of the 
 rope, to raise a weight which it would require ten men to raise at once with- 
 out pulleys. But if the weight is to be raised a yard, the ten men will raise 
 it by pulling at a single rope and walking one yard, while the one man at his 
 tackle must walk until he has shortened all the ten plies of rope of one yard 
 each j that is, he must walk ten yards, or ten times as far as the ten men 
 did. In both cases, therefore, to accomplish the same end, we have just 
 the same quantity of man's* work expended, in the first, performed by ten 
 men in one minute, in the second, by one man in ten minutes ; and if the 
 work were of a nature to continue longer, let us say a whole day for the ten 
 men, it would last ten days for the single man, and there would be ten days 7 
 wages of a man to pay in both cases. There is, therefore, no direct saving of 
 human effort from using pulleys j indeed, there is a loss, because of the 
 great friction which has to be overcome. Now exactly the same is true of 
 all other simple machines, or mechanic powers ; none of them save labour, 
 in a strict sense of the phrase ; they only allow a small force to take its time 
 to produce any requisite magnitude of effect, at the expense of additionality 
 overcoming a certain amount of friction or other such risistance. 
 
 The real advantage of these machines are such as the following : 
 
 That one man's effort, or any small power, which is always at command, 
 by working proportionally longer, will answer the purpose of the sudden 
 effort of many men, even of hundreds or thousands, whom it might be most 
 inconvenient and expensive, or even impossible, to bring together. 
 
 A ship's company of a few individuals easily weighs a heavy anchor by 
 means of the capstan. 
 
 A solitary workman, with his screw or other engine, can press a sheet of 
 paper against types, so as to take off a clear impression ; to do which without 
 the press, the direct push of fifty men would scarcely be sufficient : and 
 these fifty men would be idle and superfluous except just at the instants of 
 pressing, which occur only now and then. In this way the screw may be 
 said to do the work of fifty men, for it is as useful. 
 
 A man with a crow-bar may move a great log of wood to a convenient 
 
SIMPLE MACHINES. 113 
 
 place, where twenty men would have been required to move it without the 
 crow-bar ; and although the single man takes twenty minutes, perhaps, to 
 do what the many men would have done in one minute, as the twenty might 
 not have been wanted again for the rest of the day, the crow-bar may really 
 be as useful as the twenty men. 
 
 It is so important to have correct notions on the subject of the simple 
 machines or mechanical powers, that more space has been here allotted to 
 the explanation of the general principle, than has been usual in such works. 
 After the examination which it has now undergone, however, the author 
 hopes that none of his readers will have difficulty in conceiving clearly, that 
 "whatever, through a machine, is gained in power, is lost in 'speed or in 
 time, and vice versa" or will have difficulty in detecting immediately any 
 common fallacy connected with the subject ; as that of supposing, for 
 instance ; that a lever, or great pendulum, or spring, or heavy fly-wheel, &c., 
 can never exert more force than has passed into it from some source of 
 motion. 
 
 11 By solid connecting parts, also, the direction of any existing motion or 
 force may be changed. Hence the endless variety of COMPLEX MACHINES." 
 (Read the Analysis at p. 84. ) 
 
 It is this power of changing the direction of motion, added to the power 
 of connecting and adjusting various intensities of force and resistance by the 
 simple machines last described, which has enabled man to make complex 
 machines, rivaling in their performances the nicest work of human hands. 
 It would be endless to attempt the enumeration of the modes in which the 
 directions of motions may thus be changed, for it would be to enumerate 
 and describe the whole apparatus of the arts and sciences ; but we shall 
 advert to a few as specimens. 
 
 Straight motion changed into rotatory. The straight motion of wind or 
 water becomes rotatory in wind or water-wheels. The straight downward 
 pressure of the human foot, acting at intervals on a treadle and crank, turns 
 round the grindstone, and common lathe, and spinning-wheel. The alter- 
 nate rising and falling of the piston of a steam-engine is made, by means of 
 a crank, to turn the great fly-wheel and any other wheels which a steam- 
 engine may move. 
 
 Rotatory motion into straight. An axle in turning will wind up a rope, 
 and lift a weight in a straight line. A crank on a turning axle, if connected 
 with a pump rod, will work the piston up and down; or it will work a saw. 
 Pallets or teeth on a turning-wheel act on the handle of a great forge ham- 
 mer, so that every one in passing lifts the hammer and produces a blow. 
 
 We need not multiply instances. By a visit to great manufacturing towns, 
 or, indeed, by simply directing the eyes to what is passing around, in any 
 part of the civilized world, we discover miracles of mechanic art: machines 
 driven by wind, water or steam for grinding corn; machines for sawing 
 wood and giving it various forms ; machines in which rods of metal are 
 seized between great rollers, and are flattened at once into thin plates, as if 
 they were of clay, and these plates again are slit into bars or ribbons 
 spinning machines, which perform their delicate office even more uniformly 
 than human hands, forming thousands of threads at once, in obedience to the 
 impulse of a single ste im-engine; weaving machines, which accomplish their 
 difficult task with the most admirable perfection; paper-making engines, 
 which convert worn-out and apparently useless remnants of our apparel, into 
 
114 MECHANICS. 
 
 the uniform and beautiful texture of paper, a texture which, with the farther 
 assistance of the pen, or types, or engraved plate, becomes a magic conserva- 
 tory of mind, shutting up among its folds the brightest effusions of genius, 
 and ready, at any instant, to disclose them again to the delighted student, 
 nothing changed after revolving centuries ; coining machinery, which form 
 a bar or plate of metal cuts out and stamps thousands of beautiful medals in 
 an hour, and keeps an exact record of its work; cranes, pile engines, 
 turning-lathes, time-pieces, all the implements of agriculture, of mining, 
 of navigation, &c., &c. If Aristotle deemed the title or definition of tool- 
 using animal appropriate to man two thousand years ago, what title should 
 be given now ? 
 
 In many of the complex machines, several of the simple ones are found 
 as elements; and in the same machine maybe comprised many of the means 
 of changing the direction of motion. 
 
 " Friction." (Read the Analysis, p. 84. ) 
 
 In estimating the effects of mechanical contrivances, by the rule of compara- 
 tive velocities of the power and resistance, there is an important correction 
 to be made, on account of the mutual friction of the moving parts. In 
 the steam-engine, where the rubbing parts are numerous, the loss of 
 power, from friction often amounts to one-third of the whole. 
 
 Impediment from friction seems to be owing to two causes; 1st, a degree of 
 cohesive attraction between the touching substances; 2nd, the roughness of 
 these surfaces, even where, to the naked eye, they appear smooth. 
 
 It is supposed to be, because the roughness, or little projections and cavi- 
 ties, in pieces of the same or of homogeneous substances mutually fit each 
 other, as the teeth of similar saws would, so as to allow the bodies, in a 
 degree to enter into each other, that the friction is greater between such than 
 between pieces of different or of heterogeneous substances with dissimilar 
 grain. 
 
 The friction of one piece of iron, wood, brick, stone, &c., on another piece 
 of the same substance, has been measured by using the second piece as an 
 inclined plane, and then gradually lifting one end of it until the upper mass 
 began to slide, the inclination of the plane, just before the sliding com- 
 mences, is called the angle of repose. This angle, different for different sub- 
 stances, is found to be, for metals, generally such as to mark that the force 
 required to overcome the friction between small pieces of them is equal to 
 about a fourth of the weight of the moving piece, and for woods it is about 
 a half. But for large pieces or great pressures, the friction is proportionably 
 much less. 
 
 It is this angle in the substances concerned, which determines the degrees 
 of acclivity which can exist in the sides of hills composed of sand, gravel, 
 earth, &c., in the banks of canals, rivers, &c 
 
 If the thread of a screw winds round the spindle with an angle less than 
 this, the screw can never recoil or slide back from force acting against its 
 point. 
 
 But for friction, men walking on the ground or pavement would always be 
 as if walking on ice; and our rivers, that now flow so calmly, would all be 
 frightful torrents. Friction is, therefore, in these cases of great use to men. 
 
 Friction is useful, also, when it enables men, out of the comparatively 
 short fibres of cotton, flax, or hemp, to form their lengthened webs and 
 cordage, for it is friction alone, consequent upon the interweaving and 
 
SIMPLE MACHINES. 115 
 
 twisting of the fibres and threads, which keeps the material of these fabrics 
 together. 
 
 The following means'are used to dimmish friction between rubbing sur- 
 faces ; and they are used singly or in combination, according to the circum- 
 stances. 
 
 1. Making the rubbing surfaces smooth; but this must be done within 
 certain limits, for great smoothness allows the bodies to approach so near 
 that a degree of cohesion takes place. 
 
 2. Letting the substances which are to rub on each other be of different 
 kinds. Axles are made of steel, for instance, and the parts on which they 
 bear are made of brass ; in small machines as time-keepers, the steel axles 
 often play in agate or diamond. The swiftness of a skater depends much 
 on the great dissimilarity between steel and ice. 
 
 8. Interposing some lubricating substance between the rubbing parts; as 
 oils for the metals, soap, grease, black-lead, &c., for the woods. There is a 
 laughable illustration of this in the holiday sport of soaping a lively pig's 
 tail, and then offering him as the prize of the clever fellow who can catch 
 and hold him fast by his slippery appendix. 
 
 4. Diminishing the extent of the touching surfaces ; as in making the 
 rubbing axis of a wheel very small. 
 
 5. Using wheels as in wheel-carriages, instead of dragging a rubbing load 
 along the ground. Casters on household furniture are miniature wheels. 
 
 6. Using what is called friction-wheels ; which 
 
 still farther diminish the friction even of a smooth Fig. 61. 
 
 axis, by allowing it to rest on their circumferences, 
 which turn with it. Here a represent the end of 
 an axis, resting on the exteriors of two friction- 
 wheels, b and c. 
 
 7. Placing the thing to be moved on rollers or 
 balls as when a log of wood is drawn along the 
 ground upon rounded pieces of wood; or when a 
 
 cannon with a flat circular base to its carriage, turns round by rolling on 
 cannon-balls laid on a hard level bed. In these two cases there is hardly 
 any friction, and the resistance is merely from the obstacles which the rollers 
 or balls may have to pass over. 
 
 Of all rubbing parts the joints of animals, considering the strength, fre- 
 quency and rapidity of their movements, are those which have the least 
 friction. The rubbing surfaces in these are covered, first, with a layer of 
 elastic cartilage, and then with an exceedingly smooth membrane, over which 
 there is constantly poured from the glands around, a fluid called synovia, 
 more emollient and lubricating than any oil. and which is renewed constantly 
 as may be required. We study and admire the perfection of animal joints, 
 without being able very closely to imitate it. 
 
 Wheel-carriages merit notice here, as illustrating many of the circum- 
 stances connected with friction ; and moreover as being among the most 
 common of machines. 
 
 Wheel-carriages have three advantages over the sledges for which they 
 are the substitutes : 
 
 1. The rubbing or friction, instead of being between an iron shoe and the 
 stones and irregularities of the road, is between the axle and its bush, of 
 which the surfaces are smoothed and fitted to each other, and well lubricated. 
 
116 
 
 MECHANICS. 
 
 2. While the carriage moves forward, perhaps fifteen feet, by one revolu- 
 tion of its wheel, the rubbing "part, viz., the axle, passes over only a few 
 inches of the internal surface of its smooth greased bush. 
 
 3. The wheel surmounts any abrupt obstacle on the road by the axle 
 
 describing a gently rising slope 
 
 Fig. 62. or curve, as shown in this 
 
 /?,. figure, where a represents an 
 
 obstacle, and where the curve 
 from c, of which the beginning 
 has the direction shown by the 
 line c e, represents the path 
 of the axle in surmounting it. 
 The wheel is as if rising on an 
 inclined plane, and gives to'the 
 drawing animal the relief which 
 such a plane would bring. This 
 kind of advantage is greater in a large wheel, for evidently the smaller wheel 
 here represented, in having to surmount the same size of obstacles, has to 
 rise in the steeper curve beginning at d, but the difference of advantage, in 
 this respect, is not so great as the difference of size. It is true again, that 
 a small wheel would sink to the bottom of a hole, where a larger one would 
 rest on the edges as a bridge, and would sink less. The fore wheels of car- 
 riages are usually made small, because such construction, by allowing the 
 wheel to go under the body of the carriage, facilitates the turning of the 
 carnage. It is not true, however, according to the popular prejudice, that 
 the large hind wheels of coaches, wagons, &c., help to push on the little 
 wheels before them, as if the carriage were on an inclined plane resting on 
 the wheels ; but there is the accidental advantage, that in ascending a hill, 
 when the horses have to put forth their strength, the load rests chiefly on 
 the hind wheels, and in descending, when an increased resistance is 
 desirable, the load falls chiefly on the fore-wheels. 
 
 From the causes mentioned in the last paragraphs, the difference in per- 
 forming the same journey of a mile, by a sledge and by a wheel carriage, is 
 that while the former has to rub over e"very roughness in the road and to be 
 jolted by every irregularity, the rubbing part of the latter, the axle, glides 
 very slowly over about thirty yards of a smooth oiled surface, in a gently 
 waving line. Thus, by wheels, the resistance is reduced to about the hun- 
 dredth part of what it is for a sledge. 
 
 On hilly roads, in descending, it is common to lock or fix one of the wheels 
 of a carriage, and the horses have then to pull nearly as much as on a level 
 road with the wheel free ; showing the effect of a little increase of friction. 
 The wheel of a carriage, simple as from our extreme familiarity with it, 
 it now appears to us, is a thing of very nice workmanship, and which has 
 exercised much ingenuity. It acquires astonishing strength, indeed that of 
 the arch, from what is called its dished form, seen 
 here in the wheel c, as contrasted with the flat wheel 
 a. In a wheel of this form, the extremity of a spoke 
 cannot be displaced inwards, or towards the car- 
 riage, unless the rim of the wheel be enlarged, or 
 all the other spokes yield at the same time, and it 
 cannot be displaced outwards, or away from the car- 
 riage, unless the rim be diminished, or the other 
 spokes yield in the opposite direction: now the 
 
 Fig. 63. 
 
FRICTION. 117 
 
 rim being strongly bound by a ring, or tire of iron, cannot suffer either 
 increase or diminution, and the strength of all the spokes is thus by it 
 conferred on each individually. In &flai wheel a given degree of displace- 
 ment outwards or inwards of the extremities of a spoke, would less affect the 
 magnitude of the circumference, and therefore the rim of such a wheel, 
 secures much less firmly. A watch-glass and a round piece of egg-shell are 
 stronger than flat pieces of like substances, for the same reason that a dished 
 wheel is stronger than a flat wheel. The dished form of a wheel is farther 
 useful by leaving more room between the wheels for the body of the carriage, 
 and is useful, also, in this, that when the carriage is on an inclined road, 
 and more of the weight consequently falls upon the wheel of the lower side, 
 the inferior spokes of that wheel become nearly perpendicular, and thereby 
 support the increased weight more safely. The strongest form of wheel is 
 the doubly dished, that is, a wheel having half of the spokes passing from 
 within to the rim as from c to d, fig. 63, and the other half similarly from 
 without. This form is adopted in the wheel recently constructed entirely 
 of iron, in which there is a farther peculiarity that the load is supported 
 more by hanging by the upper spokes than by resting on the lower. When 
 wheels, instead of standing upright, like b and d shown, fig. 63, are made 
 to incline outwards, as is common, owing to the ends of the axletrees being 
 bent down a little to give a security against the accident of the wheels falling 
 off, the pull to the horses in deep or sandy roads is much increased ; for an 
 inclining wheel would naturally describe a curved and outward path, as is 
 seen when a hoop or wheelbarrow inclines ; and the horses, therefore, in 
 drawing straight forward, have constantly to overcome the deviating tendency 
 in all inclining wheels. This cause of resistance is still more remarkable 
 when the wheels have broad rims. Such wheels must be conical, that is, of 
 smaller diameter at the outer than at the inner edge, as the end of a cask is 
 smaller than its middle, and then, as the iron hoops or tires which cover the 
 different parts cannot all, by an equal number of turns, truly measure the 
 same length of road, there will be a constant rubbing or grinding forward of 
 the lesser rings, and a grinding backward of the larger, injuring the road, 
 rapidly wearing the iron, and exhausting the strength of the pulling animals. 
 Such wheels rolling free would describe a circular path, as is exemplified 
 when a thimble, or drinking glass, or sugar-loaf, which also are conical, is 
 pushed forward on any plane surface. 
 
 The application of springs to carriages, which is an improvement of com- 
 paratively recent date, not only renders them soft moving vehicles on rough 
 roads, but much lessens the pull to the horses. When there is no spring, 
 the whole load must rise with every rising of the road, and if time be given, 
 must sink with every depression, and the depression costs as much labour 
 as the rising, because the wheel must be drawn up again from the bottom of 
 it ; but in a spring-carriage moving rapidly along, only the parts below the 
 springs are moved in correspondence with the road-surface, while all above, 
 by the inertia of the matter, have a soft and even advance. Hence arises 
 the superiority of those modern carriages, furnished with what are called 
 under-springs, which insulate from the effect of shocks, all the parts except- 
 ing the wheels and axletrees themselves. When only the body of the car- 
 riage is on springs, the horses have still to rattle the heavy frame-work 
 below it over all irregularities, and then the wheels as well as the structure 
 generally require to be of much greater strength and weight to bear the 
 consequent shocks. 
 
118 MECHANICS. 
 
 The subject of wheel carriages is interesting to medical men, from their 
 having often to direct in transporting the sick or wounded. 
 
 It is perhaps difficult to conceive anything more elegant and perfect than 
 the carriages of modern refinement; and, therefore, a man, who sees them 
 gliding swiftly along the prepared levels and slopes of our present landscapes, 
 and thinks of the clumsy vehicles on the bad roads of former times, may 
 readily imagine that absolute perfection is at last attained. Yet we are per- 
 haps now on the eve of a farther change which, for many purposes, will be 
 of greater importance than all that has yet been achieved viz., the general 
 adoption of rail-roads, with new-fashioned carriages to suit them. To all 
 who study such subjects, it is now known, that to draw a loaded wagon up 
 one inconsiderable hill, costs more force than to send it thirty or forty miles 
 along a level rail- way ; and the conclusion is obvious, that although the origi- 
 nal expense of forming the level line might considerably exceed that of mak- 
 ing an ordinary road, still, in situations of great traffic, the difference would 
 soon be paid for by the savings, and when once paid, the savings would be 
 as a profit for ever. To readers conversant with political economy, it 
 would be superfluous to speak here of the advantages of any great facility 
 of intercourse, but to those who are not, the following reflections may be 
 interesting. 
 
 In reviewing the history of the human race, we find that every remarkable 
 increase in civilization has taken place very much in proportion to the facili- 
 ties of intercourse offered in the particular situation. First, therefore, civili- 
 zation, grew along the banks of great rivers, as the Nile, the Euphrates and 
 the Ganges ; or along the shores of inland seas or archipelagoes, as in the 
 Mediterranean and the numerous islands of Greece ; or over fertile and ex- 
 tended plains, as in many parts of India. When the situation thus bound a 
 great number of individuals into one body, the useful new thought or action 
 of any one unusually gifted, and which, in the insulated state, would soon 
 have been forgotten and lost, extended its influence immediately to the whole 
 body, and became the thought or action of all who could benefit'by it, besides 
 that it was recorded for ever, as part of the growing science of art of the com- 
 munity. And in a numerous society, such useful thoughts and acts would 
 naturally be more frequent, because persons feeling that they had the eyes of 
 a multitude upon them, and that the rewards of excellence would be propor- 
 tionally great, would be excited to emulation in all the pursuits that could 
 contribute to the well-being of the society. Men soon learned to estimate 
 aright these and many other advantages of easy intercourse, and after having 
 possessed themselves with avidity of the stations naturally fitted for their 
 purposes, they began to improve the old and to make new stations. They 
 created rivers and shores, and plains of their own, that is, they constructed 
 canals and basins, and roads ; and so connected, artificially, regions which 
 nature seemed to have separated forever. In the British isles, whose fa- 
 voured children have taken so remarkable a lead in showing what prodigies 
 a wise policy may effect, the advantages arising from certain lines of canal 
 and road first executed, soon led to numberless similar enterprises, and within 
 half a century the empire has been thus bound together in all directions : and 
 it seems as if the noble work was now to be crowned by the substitution of 
 level railways for many of the common roads and canals.* Several rail- 
 
 * These observations were first published (the substance had been written long 
 before,) soon after the Darlington rail-road, the first of any note intended for pas- 
 sengers", was opened. The Manchester and Liverpool rail-road has since then 
 admirably verified the anticipations. 
 
STRENGTH OF MATERIAL. 119 
 
 roads of short extent have already been established, and although they and 
 the carriages upon them are far from having the perfection which philosophy 
 says they will admit, the results have been very satisfactory. If we sup- 
 pose the progress to continue, and the price of transporting things and per- 
 sons to be thus reduced to a fourth of the present charge and in many cases 
 it may be less and if we suppose the time of journeying with safety also to 
 be reduced in some considerable degree, of which there can be as little 
 doubt the general adoption of such roads would operate an extraordinary 
 revolution and improvement in the state of society. Without in reality 
 changing the distances of places, it would in effect bring all places nearer to 
 each other, and would give to every spot in the kingdom the conveniences 
 of the whole, of town and country, of sea-coast and of highland district. 
 A man, wherever residing, might consider himself virtually near to any other 
 part, when, at the expense of time and money now expended in travelling a 
 short way, he might travel very far, and he would thus find remarkably 
 extended, the sphere both of his business and of his pleasures. The 
 over-crowded and unhealthy parts of towns would scatter their inhabitants 
 into the country for the man of business could be as conveniently at his 
 post from a distance of several miles, as he is now from an adjoining street. 
 The present heavy charges for bringing distant produce to market being 
 nearly saved, the buyer everywhere would purchase cheaper, and the pro- 
 ducer would be still better remunerated. 
 
 In a word, such a change would be affected, as if by magic the whole of 
 Britain had been compressed into a circle of a few miles in diameter, yet 
 without any part losing aught of its magnitude or beauties. All this may 
 appear visionary ; but it is less so than seventy years ago it would have 
 been to anticipate much of what, in respect to travelling, has really come to 
 pass, as ? that the common time of passing from London to Edinburgh 
 would be forty-six hours. At the recent opening (in 1825) of the railroad 
 near Darlington, a train of loaded carriages was dragged by one little steam- 
 engine a distance of twenty-five miles within two hours ; and in some parts 
 of the journey the speed was more than twenty miles an hour : the load 
 was equal to a regiment of soldiers, and the coal expended was not of the 
 value of a crown. An island with such roads would be an impregnable 
 fortress ; for in less time than an enemy would require to disembark on 
 any part of the coast, the forces of the country might be concentrated to 
 defend it. 
 
 " Strength depends on the magnitude, form and position of bodies, as well 
 as on the degree of cohesion in the material." (Head the Analysis, 
 page 84.) 
 
 The minute details connected with this branch of the subject belong to 
 the practical engineer, but there are some of the general truths which should 
 be familiar to every body. 
 
 Of similar bodies the largest is proportionally the weakest. 
 
 Suppose two blocks of stone left projecting from a hewn rock, of which 
 blocks one, as d r p. 120, is twice as long, and deep, and broad, as the other, 
 b. The larger one will by no means support at its end as much more weight 
 than the smaller, as its mass is greater, and for two reasons. 1st. Iii the 
 larger, each particle of the surface of attachment at c, in helping to bear 
 
120 MECHANICS. 
 
 the weight of the block itself, has to support hy 
 Fig. 64. its cohesion twice as many particles beyond it in 
 
 the double extent of projection, as a particle has 
 to support in the shorter block at a; and 2ndly, 
 both the additional substance, and anj-thing ap- 
 pended at its outer extremity, are acting with a 
 double lever advantage to break it, that is, to de- 
 stroy the cohesion at c. Hence, if any such mass 
 be made to project very far, it will be broken off 
 or will fall by its own weight alone. And what 
 is thus true of a block supported at one end, is 
 equally true of a block supported at both ends, and 
 indeed of all masses, however supported, and of 
 whatever forms, if they have projecting parts. It 
 is ,to be observed, also, that masses, like an abso- 
 lutely perpendicular cliff, which have no projecting 
 
 or overhanging parts, are still limited, as to size, by the degree of cohesive 
 force among their particles, for the upper part of such a mass tends to 
 crush or break down the lower. A lofty pillar cannot be formed of soft clay. 
 That a large body, therefore, may have proportionate strength to a 
 smaller, it must be still thicker and more clumsy than it is longer : and 
 beyond a certain limit, no proportions whatever will keep it together, in 
 opposition merely to the force of its own weight. 
 
 This great truth limits the size and modifies the shape of most productions 
 of nature and art; of hills, trees, animals, architectural or mechanical 
 structures, &c. 
 
 Hills. Yery strong or cohesive material may constitute hills of sublime 
 elevation, with very projecting cliffs and very lofty perpendicular precipices; 
 and such accordingly are seen where the hard granite protrudes from the 
 bowels of the earth, as in the Andes of America, the Alps of Europe, the 
 Himalayas of Asia, and the Mountains of the Moon in Central Africa. 
 But material of inferior strength exhibits more humble risings and more 
 rounded surfaces. The gradation is so striking and constant, from granite 
 mountains, down to those of chalk, or gravel, or sand, that the geologist can 
 often tell the substance of which a hill is composed by observing the pecu- 
 liarities of its shape. 
 
 Even in granite itself, which is the strongest of rocks, there is a limit to 
 height and projection ; and if an instance of either, much more remarkable 
 than now remains on earth, were by any chance to be produced again, the 
 law which we are considering would prune the monstrosity. The grotesque 
 figures of rocks and mountains, seen in the paintings of the Chinese or 
 actually formed in minature for the gardens, to express their notions of 
 perfect sublimity and beauty are caricatures of nature for which originals 
 can never have existed. Some of the smaller islands in the Eastern Ocean, 
 however, and some of the mountains of the chains seen in the voyage 
 towards China, along the coasts of Borneo and Palawan, exhibit, perhaps, 
 the very limits of possibility in singular shapes. In the moon, where the 
 weight or gravity of bodies is less than on earth, on account of her smaller 
 size, mountains of a given material might be many times higher than on 
 earth and observation proves that the lunar mountains are in fact very 
 high. 
 
STRENGTH OF MATERIAL. 121 
 
 By the action of winds, rains, currents, and frosts, upon the mineral masses 
 around us, there is unceasingly going on an undermining and wasting of 
 supports, so that every now and then immense rocks, or almost hills, are 
 torn by gravity from the station which they have held since the earth re- 
 ceived its present form, and fall in obedience to the law now explained. 
 
 The size of vegetables, of course, is obedient to the same law. We have 
 no trees reaching a height of three hundred feet, even when perfectly perpen- 
 dicular, and sheltered in forests that have been unmolested from tho 
 beginning of time : and oblique or horizontal branches are kept within 
 comparatively narrow limits by the great strength required to support them. 
 The truth, that to have proper strength, the breadth or diameter of bodies 
 must increase more quickly than the length, is well illustrated by the con- 
 trast existing between the delicate and slender proportions of a young oak or 
 elm, yet in the seedman's nursery, and the sturdy form of one which has 
 braved for centuries all the winds of heaven, and has become the monarch 
 of the park or forest. 
 
 Animals furnish other interesting illustrations of this law. 
 How massive and clumsy are the limbs of the elephant, the rhinoceros, 
 the heavy ox, compared with the slender forms of the stag, antelope, and 
 greyhound ! And unless the bones were made of stronger material than now, 
 an animal much larger than the elephant would fall to pieces owing to its 
 weight alone. The whale is the largest of animals, but feels not its enor- 
 mous weight because lying constantly in the liquid support of the ocean. A 
 cat may fall with impunity from a greater height than would suffice to dash 
 the bones of an elephant or an ox to pieces. 
 
 For the reason which we are now considering, the giants of the heathen 
 mythology could not have existed upon this earth; although, on our moon, 
 where, as already stated, weight is much less, such beings might be. In the 
 planet Jupiter, again, which is many times larger than the earth, an ordi- 
 nary man from hence would be carrying in the simple weight of his body a 
 load sufficient to crush the limbs which supported him. The phrase a little 
 compact man, points to the fact that such a person is stronger in proportion 
 to his size than a taller man. 
 
 The same law limits the heighth and breadth of architectural structures. 
 In the houses of fourteen stories, which formerly stood for protection close 
 under the Castle of Edinburgh, there was danger of the superincumbent 
 wall crushing the foundation. 
 
 Roofs. Westminster hall approaches the limit of width that is possible 
 without either very inconvenient proportions or central supports; and the 
 dome of the church of St. Peter in Rome is in the same predicament. 
 
 Arches of a bridge. A stone arch, much larger than those of the magni- 
 ficent bridges in London, would be in danger of crushing or splintering its 
 material. 
 
 Ships. The ribs or timbers of a boat have scarcely a hundredth part of 
 the bulk of the timbers of a ship only ten times longer than the boat. A 
 ship's yard of ninety feet contains, perhaps, twenty times as much wood as a 
 yard of thirty feet, and even then is not so strong in proportion. If ten men 
 may do the work of a three-hundred-ton ship, many more than three times 
 that number will be required to manage a ship three times as large. Very 
 large ships, such as the two built in Canada, in the year 1825, which carried 
 each nearly ten thousand tons of timber, are weak from their size alone ; and 
 the loss of these first two specimens of gigantic magnitude will not encourage 
 the building of others. 
 
122 
 
 MECHANICS. 
 
 a 
 
 II d, 
 
 The degree in which the strength of structure is dependent on the/orm and 
 position of their parts, will be illustrated by considering the two cases of 
 longitudinal and transverse compression. And the rule for giving strength 
 to any structure will be found to be, to cause the force tending to destroy 
 it, to act, as equally as possible, on the whole resisting mass at once, and 
 with as little mechanical advantage as possible. 
 
 In longitudinal compression, as produced by a body a, on the atoms of the 
 support 6, the weight, while the support remains straight, can only destroy 
 the support, by crushing it in opposition to the repulsion and impenetrability 
 
 of all its atoms. Hence a very small pillar, if 
 Fig. 65. kept perfectly straight, supports a very great 
 
 /^ ^ weight j but a pillar originally crooked, or 
 
 P *1 beginning to bend, resists with only part of its 
 
 strength j for, as seen in* c d, the whole weight 
 above is supported chiefly on the atoms of the 
 concave side, which are therefore in greater 
 danger of being oppressed and crushed, while 
 those on the convex side, separated from their 
 natural helpmates, are in the opposite danger of 
 being torn asunder. The atoms near the centre 
 in such a case are almost neutral, and might be 
 absent without the strength of the pillar being 
 much lessened. 
 
 Long pillars or supports are weaker than short 
 pillars of the same diameter, because they are 
 more easily bent; and they are more easily bent 
 
 L- ' because a very inconsiderable, and therefore 
 
 easily effected yielding between each adjoining 
 two of their many atoms, makes a considerable 
 
 bend in the whole ; while in a very short pillar there cannot be much bend- 
 ing without a great change in the relation 
 Fig. 66. of proximate atoms, and such as can be 
 
 effected only by great force. The weight 
 resting on any pillar, and bending it, may 
 be considered as acting (with obliquity 
 dependent on the degree of bending ) at 
 the end of a long lever which reaches 
 from the extremity to the centre of the 
 \ pillar, against the strength resisting always 
 \ directly at a short lever reaching from the 
 / side d to the centre j the strength of the 
 pillar, therefore, has relation to the differ- 
 ence between these levers and to the degree 
 of bending. Shortness, then, or any stay 
 or projection as a e b, which, by making 
 the resisting lever longer, opposes bending, 
 really increases the strength of a pillar. 
 
 A column with ridges projecting from 
 it, is on this account stronger than one 
 that is perfectly smooth. 
 
 A hollow tube of metal is stronger than the same quantity of metal as a 
 solid rod, because its substance standing farther from the centre resists bond- 
 
 I 
 j 
 e. \--c- 
 
 \ 
 
STRENGTH OF MATERIAL. 123 
 
 ing with a longer lever. Hence pillars of cast-iron are generally made 
 hollow, that they may have strength with as little metal as possible. 
 
 In the most perfect weighing-beams for delicate purposes, that there may 
 "be the least possible weight with the required strength, the arms, instead 
 of being of solid metal, are hollow cones, of which the substance is not much 
 thicker than writing paper. 
 
 Masts and yards for ships have been made hollow in accordance with the 
 same principle. 
 
 In Nature's work we have to admire numerous illustrations of the same 
 kind. 
 
 The stems of many vegetables, instead of being round externally, are ribbed 
 or angular and fluted, that they may have strength to resist bending. Many 
 also are hollow, as corn-stalks, the elder, the bamboo of tropical climates, 
 &c., thereby combining lightness with their strength." A person who has 
 visited the countries where the bamboo grows, cannot but admire the almost 
 endless uses to the inhabitants, which its straightness, lightness and hollow- 
 ness, fit it to serve. Being found of all sizes, it has merely to be cut into 
 pieces of the lengths required for any purposes, and nature has already been 
 the turner, and the polisher, and the borer, &c. In many of the Eastern 
 Islands it is the chief material, both of the dwellings, and of the furniture ; 
 there are the bamboo huts and bungallows, and then the fanciful chairs, 
 couches, beds, &c. ; flutes and other wind instruments there, are merely 
 pieces of the reed with holes bored at the requisite distance : conduits for 
 water are pipes of bamboo ; bottles and casks for preserving liquids are 
 single joints of larger bamboo with the natural partitions remaining; and 
 bamboo split into threads is twisted jnto rope, &c. 
 
 From the animal kingdom also we have illustrations of our present sub- 
 ject : as in the hollow stiffness of the quills of birds; the hollow bones of 
 birds ; the bones of animals generally strong and hard, and often angular 
 externally, with light cellular texture within, &c. 
 
 Transverse Pressure. 
 
 When a horizontal beam is sup- 
 ported at its extremities, as at a and 6, 
 its weight bends its middle down more 
 or less, as here shown, the particles 
 on the upper side being compressed, 
 while the parts below are distended; 
 and the bending and tendency to 
 
 break are greater, according as the beam is longer and its thickness and 
 depth is less. 
 
 The danger of breaking in a beam, so situated is judged of, by consider- 
 ing the destroying force as acting by a long lever reaching from an end of 
 the beam to the centre, and the resisting force or strength as acting only by 
 a short lever from the side d to the centre : while only a little of the sub- 
 stance of a beam on the under side is allowed to resist at all. This last cir- 
 cumstance is so remarkable, that the scratch of a pin on the under side of a 
 plank resting, as here supposed, will sometimes suffice to begin the fracture. 
 
 Because the resisting lever is small in proportion as the beam is thinner, a 
 plank bends and breaks more readily than a beam, and a beam resting on its 
 side bears less weight than if resting on its edge. Where a single beam 
 
124 MECHANICS. 
 
 cannot be found deep or broad enough to have the strength required in any 
 particular case, as for supporting the roof of a house, several beams are 
 joined together, and in a great variety of ways, as is seen in house-rafters, 
 &c., w.hich although consisting of three or more pieces, may be considered 
 as one very broad beam, with those parts cut out which would contribute 
 least to the strength. 
 
 The arched form, resting against immoveable abutments, bears transverse 
 pressure so admirably because by means of it the force that would destroy, 
 is made to compress not one side only, but all the atoms or parts of both 
 
 sides nearly in the same degree. By 
 
 Fig- 68. comparing this figure with the last, 
 
 d we see that the atoms on the under 
 
 side of an arch, must be compressed 
 about as much as those on the upper 
 side, and are therefore in no danger of 
 being torn, or overcome separately. 
 The whole substance of the arch therefore resists, nearly like that of a 
 straight pillar under a weight, and is nearly as strong. 
 
 An error, which has been frequently committed by bridge-builders, is the 
 neglecting to consider sufficiently the effect of the horizontal thrust of the 
 arch on its piers. Each arch is an engine of oblique force (see page 56,) 
 pushing the pier away from it. In some instances, one arch of a bridge 
 falling, has allowed the adjoining piers to be pushed down to.wards it, by the 
 thrust, no longer balanced, of the arches beyond, and the whole structure 
 has given way at once like a child's house or bridge built of cards. 
 
 It is not known at what time the arch was invented, but it was in com- 
 paratively modern times. The hint may have been taken from nature, 
 for there are instances in Alpine countries of natural arches, where rocks 
 have fallen. bet ween rocks, and have there been arrested and suspended, or 
 where burrowing water has at last formed a wide passage under masses of rock, 
 and has left them balanced among themselves as an arch above the stream. 
 Nothing can surpass the strength and beauty of some modern stone bridges ; 
 those, for instance, which spans the Thames as it' winds through London. 
 
 Iron bridges have been made with arches twice as large as those of stone ; 
 the material being more tenacious and easily moulded, is calculated to form 
 a lighter whole. The bridge of three fine arches lately built between the 
 city of London and Southwark, is a noble specimen, and compared with 
 those erected in the preceding century, appears almost a fairy structure of 
 lightness and grace. 
 
 The great domes of churches, as those of St. Peter's in Rome and St. 
 Paul's in London, have strength on the same principle as simple arches. 
 They are in general strongly bound at the bottom with chains and iron- 
 bars, to aid the masonry in counteracting the horizontal thrust of the 
 superstructure. 
 
 The jGotbic arch is a pointed arch, and is calculated to bear the chief 
 weight on its summit or key-stone. Its use, therefore, is not properly to 
 span rivers as a bridge, but to enter into the composition of varied pieces of 
 architecture. With what effect it does this, is seen in the truly sublime 
 Gothic structures which still adorn so many parts of Europe. 
 
 The following are instances, in smaller bodies, of strength obtained by the 
 arched form. A thin watch-glass bears a very hard push ; a dished or 
 arched wheel for a carriage is many times stronger to resist all kinds of shocks 
 than a perfectly flat wheel; a full cask may fall with impunity, where a 
 
STRENGTH OF MATERIAL. 125 
 
 strong square box would be dashed to pieces ; a very thin globular flask 
 or glass, corked and sent down many fathoms into the sea, will resist the 
 pressure of water around it, where a square bottle, with sides of almost any 
 thickness, would be crushed to pieces. 
 
 We have, from the animal frame, an illustration of the arched form giving 
 strength, in the cranium or skull, and particularly in the skull of man, 
 which is the largest in proportion to its thickness : the brain required the 
 most perfect security, and in the arched form of the skull has obtained it 
 with little weight. The common egg-shell is another example of the same 
 class : what hard blows of the spoon or knife are often required to penetrate 
 this wonderful defence of a dormant life ! The weakness of a similar sub- 
 stance not arched, is seen in a scale from a piece of freestone so readily 
 crumbling between the fingers. 
 
 To determine, for particular cases, the best forms and positions of beams 
 and joists, and of arches, domes, &c., is the business of strict calculation, 
 and belongs therefore to mathematics, or the science of measures. 
 
 It was a beautiful problem of this kind, which Mr. Sineaton, the English 
 engineer, solved so perfectly, in the construction of the far-famed Eddy- 
 stone .light-house. He had to determine the form and dimensions of a 
 building, which would stand firm on a sunken rock, in the channel of a 
 swift ocean tide, and exposed to the fury of tempests from every quarter. 
 Only the man who has himself been driven before the irresistible storm in 
 the darkness of night, and in the midst of dangers, and whose eyes have 
 watched the steady ray from the light-house which saved him, can appre- 
 ciate fully the importance of the studies which bring such useful results ; or 
 can feel how happy he is to have fellow-men, whose talents, although ex- 
 erted usually for individual good, are yet, by God's providence, made to 
 accomplish the most philanthrophic ends, and to bind the whole of human 
 kind into one great society of helping brotherhood. 
 
 [For Animal and Medical Mechanics, see Part V. Sect. 1.] 
 
126 HYDROSTATICS. 
 
 PAET III. 
 
 THE PHENOMENA OF FLUIDS.* 
 SECTION L HYDROSTATICS. 
 
 ANALYSIS OF THE SECTION. 
 
 The particles of a fluid mass are freely moveable among one another, so as 
 to yield to the least disturbing force ; and if bearing force at all, can be 
 at rest only when equally forced in all directions. Hence : 
 
 1. In a mass of fluid submitted to compression, the whole is equally affected^ 
 and equally in all directions. A given pressure, for instance, made by 
 a plug forced inwards upon a square inch of the surf ace of a fluid filling 
 a vessel, is suddenly communicated to every square inch of the vessel's 
 surface, however large, and to every inch of the surface of any body 
 immersed in the fluid. 
 
 2. In any fluid, the particles that are below bear the weight of those that 
 are above, and there is, therefore, within the mass, a pressure increasing 
 exactly with the perpendicular depth, and not influenced by the size, or 
 shape, or position of the containing vessel. 
 
 3. The open surface of a fluid is level; and if various pipes or vessels 
 communicate with each other, any fluid admitted to them will rise to the 
 same level in all. 
 
 4. A body immersed in a fluid displaces exactly its own bulk of it, which 
 quantity having been just supported by the fluid around, the body is 
 pressed upwards, or supported, with a force exactly equal to the weight 
 of the fluid displaced, and must sink or swim according as its own weight 
 is greater or less than this. By comparing, therefore, the weight of a 
 body with the force which holds it up in, a fluid, the comparative weight 
 or specific gravities are found. 
 
 Fluid." 
 
 It was explained in Part I., that the same atoms may exist in the form 
 of a solid or of a fluid ; and as a fluid, they may either constitute a dense 
 liquid like water, or a light elastic mass like air. A pound of ice, or a 
 pound of water, or a pound of steam, differs only in the particles being 
 more or less distant from each other, owing to the different quantities of 
 heat among them. In the ice, they are comparatively near, and are held 
 together by attraction, as if they were spitted or glued to each other ; in 
 the water, the repulsion of heat seems nearly to balance attraction, and to 
 leave the particles at liberty to glide about among each other almost without 
 
 * Read again the Synopsis, page 20. 
 
FLUIDS. 127 
 
 friction ; and in the steam, the repulsion altogether overcomes the attraction, 
 and the particles separate to a great distance, as if held apart by some bulky 
 elastic medium. The few facts not evidently reconcilable with the simple 
 and satisfactory explanation of so many phenomena, as that water in freez- 
 ing, and even in cooling down from forty ' degress to the freezing point, 
 increases in volume, instead of contracting, like things in general, and like 
 itself in cooling at other temperatures, and that baked clay, in proportion 
 as it is more heated, contracts instead of dilating, are treated of in other 
 parts of our work. 
 
 Whether matter be in the solid or fluid form, the properties of the indi- 
 vidual atoms remain unchanged, that is, the atoms always exist in accord- 
 ance with the " general truths ;" but as, in the chapter on Mechanics, we 
 found so many important modifications of effect produced by the circum- 
 stance of the attraction being in the degree which produces solid cohesion 
 among the particles, in this chapter on fluids we shall find as many 
 important results springing from the circumstances of non-cohesion or 
 fluidity. 
 
 In a liquid the particles, although comparatively near to one another, 
 seem not to be in actual contact j for the mass may be condensed indefinitely 
 by pressure. The force required, however, to change the volume of a liquid 
 in any sensible degree, is so great, that until improved means of experiment, 
 recently contrived, liquids were accounted absolutely incompressible. In 
 aeriform fluids, on the contrary each particle, under common circumstances, 
 has about two thousand times as much space to itself as when forming part 
 of a liquid or solid ; and hence it is that these fluids are so extensively com- 
 pressible and dilatable or elastic, as they are called. On account of this 
 elasticity, they exhibit so many important phenomena, in addition to those 
 of mere fluidity, that the consideration of them requires to be gone into 
 apart, and forms the branch of the subject called pneumatics, from a Greek 
 word, signifying "spirit" or "breath I" 
 
 " In a quantity of fluid submitted to compression, the whole mass is equally 
 affected, and similarly in all directions. A given pressure, therefore, 
 made upon an inch of the surface of a fluid confined in a vessel, as by a 
 plug forced inwards, is suddenly borne by every inch of the surface of 
 the vessel, however large, and by every inch of the surface of any body 
 immersed in the fluid.'' 
 
 This truth is of great importance, both from its explaining so many re- 
 markable phenomena of nature, and from the useful applications of it in the 
 construction of machinery. 
 
 When a man compresses in his hands a bladder full of air, he readily 
 conceives that the air immediately under his fingers is not at all more 
 compressed than that in every other part of the bladder ; and of course 
 that every part of the bladders' s surface must be pressing the air as much 
 as those parts of it on which his fingers rest, and must be bearing a reaction 
 or resistance of the air in an equal degree ; and that every single particle 
 of air must be acted upon equally on every side, so that if a small opening 
 were made in the bladder anywhere, the air would issue from it with 
 equal readiness. This is in accordance with the characteristic of fluidity, 
 " that the particles glide about among one another almost without friction, 
 so that a particle can never be at rest unless when equally urged in all di- 
 rections." 
 
128 
 
 HYDROSTATICS. 
 
 In like manner, if a close vessel B be filled with water, and into the top of 
 it a tube a c be screwed, and if then, by means of 
 
 Fig. 69. 
 
 1 
 
 a cork or moveable plug in the tube at c, the surface 
 of the water in the vessel be pressed upon with a 
 force of one pound, the water throughout the whole 
 will be squeezed or condensed in proportion to the 
 pressure, and every other portion of the" vessel B, 
 of equal surface with c, will be keeping up the con- 
 densation just as much as c, and will be bearing the 
 resistance or elasticity of the water to the extent of 
 one pound. And if there were another similar tube, 
 b, also with a plug, screwed into the top of the box B, 
 the force of one pound depressing the plug c would 
 be pushing up the plug b, with the same force. And if there were many 
 other similar tubes and plugs, by acting on one, all would be equ- illy affected ; 
 and a plug or piston of double size would be twice as much affected as the 
 smaller one ; and a plug d, of ten times the size, would bu lifted with a 
 force of ten pounds. Hence it appears that, -through the medium of con- 
 fined fluid, a force of one pound, acting upon an inch square of the fluid sur- 
 face in a vessel, may become a bursting force of ten, or a hundred, or a thous- 
 and pounds, according to the size of the vessel, or may be used as a mechani- 
 cal power to overcome a force much more intense than itself. It will be 
 explained below that the well-known hydrostatic press is merely a large plug 
 or piston as here described, forced up against the substance to be pressed by 
 the action of a smaller piston in another barrel. 
 
 If, in the above figure, the tube a were such as to contain just one pound 
 of water, on the plug c being withdrawn from it, and water being poured in 
 to fill it, the .same pressure or condensation would take place in the box B 
 as when the plug was pressed with the force of one pound } and of course 
 exactly the same effects would follow on the sides of the vessel and on the 
 other pistons ; and if, in the other tubes also, water were substituted for the 
 pistons, it is evident that, to effect a balance in all, it would require to stand 
 as high in every one as in the tube a c, producing the same level in all, 
 whatever their size 
 
 The fact that the weight of one pound of water, or any 
 other force of one pound similarly applied, may be made, 
 through the medium of extended fluid surface to produce a 
 pressure of hundreds or of thousands of pounds, has been 
 called the " hydrostatic paradox," yet there is nothing in 
 reality more paradoxical in it than that one pound at the long 
 end of the lever should balance ten pounds at the short end : 
 indeed it is but another means, like the contrivances usually 
 called mechanical powers, and described in the last chapter, 
 of balancing different intensities of force, by applying them 
 to parts of an apparatus moving with different velocities. 
 Here the tube a being ten times smaller than the tube e, the 
 piston a must descend ten inches to raise the greater piston 
 in e one inch. 
 
 This law of fluid pressure is rendered very striking in the 
 experiment of bursting a strong cask by the weight or action 
 of a few ounces of water. Suppose a cask a already filled 
 with water, and that a long small tube I c is screwed tightly 
 into its top, which tube will contain only a few ounces of 
 
 Fig. 70, 
 
 r 
 
 b 
 
FLUIDS. 
 
 129 
 
 Fig. 71. 
 
 water ; by pouring these few ounces into the tube, the cask will be burst. 
 In explanation it is unnecessary to say more than that if the tube have an 
 area of a fortieth of an inch, and contain, when filled, half a pound of water, 
 that water would produce a pressure of half a pound upon every fortieth of 
 an inch all over the interior of the cask, or nearly 2,000 Ibs. on- the square 
 foot, a pressure greater than a common cask can bear. 
 
 A similar effect is seen in what is called the hydrostatic bellows. This 
 consists of a long small tube a I, into which water is poured to enter the 
 body of the apparatus at c, which resembles the common bellows, in having 
 wooden boards above and below, and strong leather connecting them. If 
 the tube a b holds an ounce of water, and has itself only one-thousandth of 
 the area of the top of the bellows, an ounce of water in it will balance 
 weights of a thousand ounces placed on the top of the bellows at. d. If 
 mercury were substituted in this machine for water, the effect would be 
 fourteen times greater, because mercury is fourteen 
 times heavier in the same bulk. And if a man stand 
 on a large bellows of the kind, he may raise himself 
 by blowing into the tube with his mouth. 
 
 The annexed cut will give an idea of Mr. Braham's 
 singularly powerful and useful hydrostatic or hydrau- 
 lic press; which, if compared with the bellows, exhi- 
 bits merely a strong forcing pump instead of the 
 lofty tube, and a barrel with its piston instead of the 
 leather and boards. The letter e points out the pis- 
 ton of the forcing pump worked by the handle d, and 
 driving water along the horizontal tube into the space 
 f under the large solid piston c, which last, with its 
 spreading top, is urged against the object to be com- 
 pressed. If the small pump have only one-thousandth 
 of the area of the large barrel, and if a man by means 
 
 of its lever-handle d, press its piston down with a force of five hundred 
 pounds, the piston of the great barrel will 
 rise with a force of one thousand times five 
 hundred pounds, or more than two hundred 
 tons. Scarcely any resistance can withstand 
 the power of such a press ; with it the hand 
 of an infant can break a strong iron bar ; and 
 it is used to condense substances, as cotton 
 or hay for sea voyages, to raise great weights, 
 to uproot trees, to tear things asunder, &c. 
 
 The Dilater is a surgical instrument of extensive applicability, of which 
 the action depends on the principle of the communication of fluid pressure. 
 It was proposed by the author some years ago, and was brought to great 
 practical perfection by his brother Dr. James Arnott, (now superintendent 
 surgeon in the service of the Hon. East India Company,) in whose publica- 
 tion it is minutely treated of. Many professional men in this country 
 doubted of its power, from not being aware of the nature of fluid nature ; 
 but it is in reality a kind of hydraulic press, allowing the operator to act 
 with the most gentle or most energetic force. Further remarks are made 
 upon it in the medical Section which follows this chapter. 
 
 Fig. 72. 
 
130 HYDROSTATICS. 
 
 " In any fluid, the particles that are below, bear the weight of those that are 
 above, and there is therefore a pressure among them increasing in exact 
 proportion to the perpendicular depth, and not .influenced by the size, or 
 shape, or position of the containing vessels." 
 
 The atoms of matter having gravity, it is evident that the upper layer of 
 any mass of fluid must be supported by the second, and this with its load by 
 the third, and the third with its double load by the fourth, and so on. This 
 truth is experimentally proved by putting different heights of liquid into an 
 upright tube, of which the bottom is closed by a flap having a spring or lever 
 to support it, and to indicate the force acting on it. And what is true of the 
 entire column of water in the tube, may be considered true of any single line 
 of atoms ; just as it would be true of a line of bricks piled one above another. 
 
 A tube of which the area is an inch square, holds, in two feet of its length, 
 nearly a pound of water ; hence, the general truth, well worth recollecting, 
 that the pressure of water, at any depth, whether on the side of a vessel or 
 on its bottom, or on any body immersed, is nearly one pound on the square 
 inch for every two feet of depth. 
 
 The striking effects from the increase of pressure in a fluid, at great depths, 
 are, of course, most commonly exhibited at sea. The following instances will 
 illustrate them. 
 
 If a strong square glass bottle, empty, and firmly corked, be sunk in 
 water, its sides are generally crushed inwards by the pressure before it 
 reaches a depth of ten fathoms. 
 
 'A chamber of air similarly let down with a man it, would soon allow 
 him to be drowned by the water bursting in upon him ; as really happened 
 to an ignorant projector. 
 
 When a ship founders in shallow water, the wreck, on breaking to pieces, 
 generally comes to the surface, and is cast upon the beach ; but when the 
 accident happens in deep water, the great pressure at the bottom forces 
 water into the pores of the wood, and makes it so heavy that no part can 
 ever rise again to reveal her fate. 
 
 A bubble of air or of steam, set at liberty far below the surface of water, 
 is small at first, and gradually enlargens as it rises. 
 
 A man who dives deep, suffers much from the compression of his client, 
 as the elastic air within him yields under the strong pressure. This limits 
 the depth to which divers can safely go. 
 
 It is not known whether there is a limit to the pressure which fishes can 
 bear to impunity, but they are chiefly found living in the shallower waters 
 on coasts, or on banks in the midst of the ocean, such as the banks of 
 Newfoundland, the Dogger-bank, and other fishing stations out at sea. In 
 rounding the Cape of (rood Hope, at a considerable distance from land, 
 ships pass over the bank of Lagullas, where a hook let down with a bit of 
 red rag, or almost any thing as a bait, immediately secures its codfish. 
 
 By sending a vessel prepared for the purpose, down into the deep sea, 
 we can readily prove the compressibility of water. Suppose the vessel to 
 be made with only one entrance, and that a small round opening, into which, 
 instead of cork, a sliding rod has been closely fitted. If, then, when filled 
 with water, and having the rod inserted into the opening, it be allowed to 
 sink into the sea, the pressure around will push the*rod inwards, in a degree 
 proportioned to the yielding or compression of the water within ; and if 
 there be on the road a stiff sliding-ring, or other contrivance to indicate on the 
 return of the vessel how far the rod has been driven inwards, the apparatus 
 
PRESSURE AS DEPTH. 131 
 
 will show the degree of compression at the greatest depth to which it has 
 descended. Water a thousand fathoms below the surface is less bulky by 
 about one-twentieth part than when at the surface. 
 
 The following are proofs of the pressure of weight in an open fluid, opera- 
 ting in all directions, as any pressure does in the case of a confined fluid. 
 
 A bottle-cork carried far under water, is not flattened as if it were pressed 
 unequally, but is reduced in all its dimensions so as to appear a pbial-cork 
 of ^ie usual form. 
 
 If a corked empty bottle be sent down into the sea, the cork is generally 
 'forced inwards at a given depth, and equally so in whatever direction the 
 mouth of the bottle may happen to point. 
 
 If a vessel containing water have an opening in the side, covered by a 
 valve or flap so contrived as to tell the force required to keep it shut, we find 
 that the water tends to escape just as powerfully through such an opening 
 as it would through one in the bottom, with the same elevation of water 
 over its centre. And different equal openings in the side of a vessel require 
 to be closed with forces exactly proportioned to the heights of liquid above 
 their centres. 
 
 In an open square sided vessel full of water, the whole pressure on any- 
 upright side is just half the pressure on an equal extent of horizontal bottom ; 
 for the centre of the side being just half as deep as the bottom, the pres- 
 sure on any point there is only half as great as on a point at the bottom, and 
 on points above the level of the centre is just as much less than half, as, at 
 corresponding distances below, it is more than half, and so it amounts to an 
 exact half in the whole. Considering that the pressure on every point 
 below the central level is greater than on every point above it, we see the 
 reason why, to support a sluice or flood-gate by a single stay on the outside, 
 the point at which the pressure has to be made is below the central level. 
 Calculation discovers that this point, called the centre of pressure, is attme- 
 third from the bottom. The knowledge of such facts furnishes rules for the 
 construction of large vessels for liquids, canal embankments, &c. 
 
 The pressure on a given extent of the side of a narrow- vessel is just as 
 great as on the same extent of the side of a wide vessel, having the same 
 depth of fluid; because, as now expjained, it depends entirely on the extent 
 of surface acted upon and the depth of liquid. 
 
 Hence a flood-gate or sluice which shuts out the ocean, as in docks open- 
 ing to the sea, bears no more pressure than if it stood only against an equal 
 depth of lake or river ; or than if it were one of two such flood-gates become 
 the sides of a very narrow vessel, made to contain only a few hogsheads of 
 water. 
 
 Hence, again, the fear is unfounded which has been expressed with refer- 
 ence to the formation of a canal between the Red Sea and the Mediterra- 
 nean, that because the former, owing to the effects of easterly winds at its 
 mouth, &c., is twenty feet higher than the later, it might burst through the 
 flood-gates, and carry devastation along its course. 
 
 A deep crevice in a rock, when filled by a shower, is often the cause of 
 the rock being torn asunder, and of part being precipitated* 
 
 Extensive walls or faces of masonry, intended to confine banks of sand 
 or earth, if no openings were left for water to escape from behind them, 
 would be burst after a rain unless they had the strength of flood-gates of 
 the same size. Ignorance of this danger has led to some extraordinary 
 catastrophies. 
 
132 HYDROSTATICS. 
 
 Other examples of the pressure in fluids being in all directions, and pro- 
 portioned to the depth, are : the swelling and bursting of leaden pipes when 
 filled from a very elevated source: the tearing up of the coverings of sub- 
 terranean drains or water courses, when, during a flood, any accident chokes 
 them near their lower openings : the violence with which water escapes by 
 an opening near the bottom of any deep vessl, or enters by an opening or 
 leak near the keel of a deep-floating ship : the great strength required in 
 the lower hoops and securities of those enormous vessels of porter-brewers, 
 called vats, some of which contain man}' thousand barrels of liquid. 
 
 In speaking of the pressure of a fluid in all directions, some persons havc^ 
 difficulty in conceiving that there is an upward as well as a downward and a* 
 lateral pressure. But if, in a fluid mass, the particles below had not a ten- 
 dency upwards equal to the weight or downward pressure of the fluid over 
 them, they could not support that fluid, which entirely rests upon them. 
 Their tendency upward is owing to the pressure around them from which 
 they are trying to escape. Accordingly, if a long tube, open at both ends, 
 and with a sliding plug or piston in it near one end, be partially plunged into 
 water by the plugged end, the water is found to press the plug upwards with 
 force proportioned to the depth to which it is carried, and exactly equal to 
 the force with which water presses upon an equal extent of the bottom or 
 side of any other vessel having in it the same depth or, with which, in the 
 same vessel, it would press other plugs in other branches of the tube pro- 
 jecting in various directions. On removing such a plug altogether, the up- 
 ward pressure is visibly proved and measured by the column of water pushed 
 into the tube from below, and supported there to the level of the water around. 
 
 The pressure in a mass of fluid is proportioned to the perpendicular depth, 
 and is not at all influenced by the size, shape or position of the containing 
 vessel. 
 
 A body immersed in the water of a lake, one foot under the surface is 
 just as much pressed upon as if it were one foot under the surface of the sea, 
 and no more than if it were one foot under the surface of a small cistern. 
 Suppose vessels differing' from each other in form and capacity, as sketched 
 here at , 6, and c, but all having flat bottoms, 
 of exactly. the same area ; if fluid be poured 
 into all to the same level or perpendicular 
 height, as represented here by the dotted lines, 
 although the quantity be very different in each, 
 the pressure on the bottom will be the same in 
 all. This truth is easily proved experimentally, 
 by having the bottoms inoveable, and held to 
 their places by weights or springs capable of 
 measuring the pressure : or by letting the three 
 
 vessels all communicate with the same vessel of water below them, and then 
 observing that the water in all has still the same level. These results are 
 other exemplifications of the truths, "pressure equal in all directions" 
 tl pressure as depth," and "pressure as the extent of surface." For as a 
 column of the fluid, resting on the middle of each bottom, just presses with 
 its whole weight, and therefore, according to its altitude, this column could 
 not remain at rest if there were any greater or less pressure than its own 
 near it ; then as the fluid really is at rest in all the cases, and in all a 
 central column is of the same height, the pressure must be equal on all the 
 bottoms. The case of the largest vessel a, is in a degree illustrated by 
 
FLUID LEVEL. 133 
 
 supposing the water in it to be suddenly converted into smooth upright small 
 columns or rods of ice or glass ; Then evidently only those pieces which 
 rested on the bottom, could press on it while the others would be supported 
 by the oblique sides of the vessel, and by the lateral resistance of the pieces 
 around them. 
 
 " Level surface of a Fluid." (Read the Analysis.) 
 
 That the surface of a fluid must be level, follows from the facts of all the 
 particles being equally attracted towards the centre of the earth, and being 
 perfectly moveable among themselves. The particles forming the surface 
 may be regarded as the tops of so many columns of particles, supported at 
 any given level below, by a uniform resistance of pressure; for no particle 
 of an inferior level can be at rest unless equally urged in all directions, and 
 therefore all the particles at such a level, and which, by equally urging one 
 another, keep themselves at rest, must all be bearing the weight of equal 
 columns : thus a higher column must sink and a lower one must rise, until 
 just balanced by those around ; that is, until all become alike. Besides, 
 just as a ball rolls down a slope or inclined plane, so do the particles of 
 a fluid slide or move from any higher situation among themselves to any 
 lower unoccupied situation near them. The account now given explains 
 why any accidental elevation or depression of a fluid surface, usually called 
 a wave, continues to rise and fall, or to oscillate, for some time with gradu- 
 ally diminishing forces ; for when the mass is raised above the general level 
 it is not quite supported, and therefore'-soon sinks, but in sinking, like, a 
 falling pendulum, it acquires momentum which carries it below the general 
 level, until opposed and arrested by a resistance greater than its weight, it 
 then rises again, by acquiring a new momentum in its rise, it has to fall 
 again, again to rise, and this alteration continues, until the lateral sliding 
 ot the particles, and the friction among them, gradually destroy it. 
 
 A perfectly level surface on earth really means one in which every particle 
 is equi-distant from the centre of the earth, and it is therefore truly a 
 spherical surface ; but so large is the sphere, that if a slice of it of two miles 
 in diameter were cut off, and laid on a perfect plane, the centre of the slice 
 would only be four inches higher than the edges. Any small portion of it, 
 therefore, for all common purposes, may be accounted a perfect plane. 
 
 So truly smooth does a fluid surface become, that it forms a perfect 
 mirror; that is, it reflects or 'throws back the rays of light, which fall 
 upon it so exactly in the order which they had on leaving the object, that 
 an eye which receives them may fancy the object to be placed in the 
 direction of the mirror. It was over the glassy surface of the fountain or 
 the lake, that the shepherdess of the young world bent themselves, to learn 
 the charms which nature had bestowed on them. And a child contemplates 
 with wonder and delight, through the window of a still pool or gliding^ 
 stream, another sky below the ground, with its cloulds, and sun or stars; 
 and another landscape, with inverted wood and mountains, the supposed 
 dwelling of fairy beings. 
 
 In the cutting of canals, th^ making of railways, and in many other ope- 
 rations of engineering, it is of essential importance 
 to determine the level or horizontal direction at any Fig. 74. 
 
 place ; and this is usually done by a tube or glass, 
 a c, filled with spirit except one bubble of air b, ^ & -^ 
 
 and called a spirit level. When this tube is hori- " 
 
 zontal, the bubble has no tendency to move to either 
 
134 HYDROSTATICS. 
 
 end ; but if the tube inclines ever so little, the bubble rises to the end which 
 is highest ; or to speak more correctly, the denser spirit falls down to the 
 lower end, and forces the light bubble away from it. Such a tube properly 
 fixed in a frame, with a telescope attached to it, or simply with sight-holes 
 to look through, becoming the engineer's guide in many of his most 
 important operations. 
 
 A hoop surrounding the earth would bend away from a perfectly straight 
 line four inches in a mile. In cutting a level canal, therefore, which may 
 be considered as part of a hoop, there must be everywhere a falling from 
 the straight line, called by geometers a tangent, in the proportion now 
 described. All rivers also have the curvature of hoops applied to the surface 
 of the earth. 
 
 Canals leading from sea-ports to the interior of countries have generally 
 to ascend ; but as water cannot become stagnant in any channel which is riot 
 level, the canal is divided, by gates or sluices, into portions at different 
 levels, like steps of a stair, the rising .at the joinings being generally from 
 six to twelve feet. The boats are raised or lowered from one level to 
 another by the contrivance called a lock, which is merely a portion of the 
 canal, of sufficient capacity f6r the boat to lie in, furnished with high walls, 
 and with flood-gates at both ends; so that when the gates below are shut, 
 and water is gradually admitted from above, it becomes part of the high 
 level, ready as such to deliver a boat, or receive one ; and when the upper 
 flood-gates are shut, and the water is gradually allowed to escape from the 
 lock, it becomes a part of the low level, and a boat may enter it ; or leave it 
 by its lower gates. 
 
 The cutting of canals is one of the great items in the mass of modern 
 improvement, which both mark and hasten the progress of civilization. 
 Adverting to the importance of easy intercouse, as explained in a* former 
 section, we need only say here, that a horse which can draw only one ton 
 on our best roads, can draw thirty with the same speed in a canal-boat. 
 
 And what a glorious triumph to science and art it is, to be able to conduct 
 vessels of all kinds, even those originally intended for the ocean surge alone, 
 through the quiet valleys of -an interior country ! In Scotland, now, along 
 the Caledonian canal, a noble frigate may be seen, wandering as it were 
 among the inland solitudes, and displaying her grace and majesty to the 
 astonished gaze of the mountain shepherd ; and when she has traversed the 
 kingdom, and visited the lonely lakes, whose waters until lately had borne 
 only the skiff of the hunter, she descends again by the steps of her liquid 
 stair, and safely resumes her place among the waves. 
 
 It was lately in contemplation to lead a ship canal across the isthmus 
 which joins North and South America. The elevation to which the canal 
 must reach, to surmount the central ridge, is considerable, and will increase 
 the difficulty ; but such important consequences would follow the accomplish- 
 ment of the object, that, with the continuance of general peace, and the 
 increase of political wisdom, it will probably be attained. If so, the loaded 
 vessel, rising from the Atlantic, would soon be descried among the mountain 
 heights, and, a few hours after would be safely lodged in a port of the oppo- 
 site sea; having performed, by a near cut, a voyage which at present costs 
 months of delay and hazard, in a tedious navigation round the whole southern 
 continent. And if the Red Sea and Mediterranean were joined in the same 
 way, as has also been proposed, the operation would, in effect, bring India 
 nearer to Europe, and would more and more strengthen the bonds of mutual 
 utility and brotherhood among the nations of the earth. Then, indeed, might 
 
FL-U ID LEVEL. 135 
 
 it be said with truth, that the world is a vast garden, given to man for 
 his abode, of which every spot has its peculiar sweets and treasures j but 
 because the cultivator of each may exchange a share of its produce for shares 
 in return, the same general result follows as if every field or farm contained 
 within itself the climates and soils and capabilities of the whole. 
 
 In a canal, the least deviation from the true level would immediately cause 
 any water admitted into it to flow towards the lower end. This flux to a 
 lower situation is what is going on in the myriads of streams, which render 
 the face of the earth a scene of such varied beauty and incessant change. 
 
 As in the animal body, from every the minutest point a little vein endowed 
 with living power, takes the blood which has just brought life and nutriment 
 to the part, and delivers it into a larger vein, whence it passes into a larger 
 still, until at last, in the great reservoir of the heart, it meets the blood re- 
 turned from every part of the body, so, in this terraqueous globe, where the 
 magic moving power is simply fluid seeking its level, does the rain which 
 falls to sustain vegetable and animal life, and to renovate nature, glide from 
 every point of the surface into a lower bed, and from thence into a lower still, 
 until the countless streams, so formed, after every variety of course combine 
 to form the swelling rivers, which return the accumulated waters into the 
 common reservoir of the ocean. In the living body, the arteries carry back 
 the blood with renewed vitality to every point whence the veins had with- 
 drawn it, and so complete the circulation; and in what may be called the 
 living universe, the circulation is completed by the action of heat and. of the 
 atmosphere, which, from the extended face of the ocean raise a constant 
 exhalation of watery vapour of invisible purity, which the winds then carry 
 away and deposit as rain or dew on every spot of the earth. 
 
 A very slight declivity suffices to give the running motion to water. Three 
 inches per mile, in a smooth straight channel, gives a velocity of about three 
 miles per hour. The Ganges, which gathers the waters of the Himalaya 
 mountains, the loftiest in the world, is, at eighteen hundred miles from its 
 mouth, only eight hundred feet above the level of the sea that is, above 
 twice the height of St Paul's Church in London ; and to fall these eight 
 hundred feet, in its long course, the water takes nearly a month. The greater 
 river Magdalena, in South America, whose channel, for a thousand miles, is 
 between two ridges of the Andes, falls only five hundred feet in all that dis- 
 tance. Above the commencement of the thousand miles, it is seen descending 
 in rapids and cataracts from the mountains. The gigantic Rio de la Plata 
 has so gentle a descent to the ocean, that, in Paraguay, fifteen hundred miles 
 from its mouth, large ships arrive which have sailed against the current all 
 the way, by the force of the wind alone : that is to say, which on the beau- 
 tifully inclined plane of the stream, have been gradually lifted by the soft 
 wind, and even against the current, to an elevation greater than that of our 
 loftiest spires. 
 
 A small lake or extensive mill-pond, with uneven bottom, if suddenly 
 emptied by a sluice or opening at its lowest part, would exhibit a number of 
 pits or pools of various size and shape left among the inequalities. But sup- 
 posing rain to fall, and frequently to recur, the water seeking its level would 
 soon effect a very remarkable change. In consequence of each pool discharg- 
 ing over its lowest part, that is, sending out a streamlet either into another 
 lower pool, or into a channel leading directly to the sluice or opening, there 
 would be a constant wearing down of the part or side of the pool over which 
 the water was running, that is to say, a deepening of a breach or channel 
 there, and the surface of water in the pool would be consequently becoming 
 
136 HYDROSTATICS. 
 
 lower, while, at the same time, the bottom would be rising, owing to the 
 deposit of sand or mud washed down by the rain from the elevations around : 
 and these two operations continuing, the pool would at last altogether disap- 
 pear. And by this change going on in every pool through the whole of the 
 emptied mili-pond, the general bottom would at last exhibit only a varied or 
 undulated surface of dry land, with a beautiful arrangement of ramifying 
 water channels, all sloping with a precision unattainable by art, to the general 
 mouth or estuary. The reason that, in the supposed case, and in every other, 
 a water course soon becomes so singularly uniform, both as to dimensions 
 and descent, is, that any pits or hollows in it, are filled up by the sand and 
 mud carried along in the stream, and deposited where the current is slack ; 
 while any elevations are worn away by the action of the more rapid current 
 which accompanies shallowness. 
 
 The above paragraph describes, in miniature, what has been going on over 
 the general face of our earth ever since that convulsion of nature which 
 produced its present form. In many places the phenomenon is already com- 
 plete ; in others it is only in progress. The whole of what is now dry land, 
 has at some period been under water, and much of it has evidently been a 
 gradual deposition from water. By some extraordinary convulsion, there- 
 fore, our present continents and islands must have been thrown up from the 
 bottom of an ocean, or an ocean must have subsided away from them ; and 
 in either case the land must have merged as checkered and unsightly as the 
 bottom of the emptied lake above supposed. And it is the gradual opera- 
 tion of water seeking its level which has at last converted the earth into the 
 paradise which we now behold. 
 
 The marks of the former state of the world, and of the progressive 
 change, are every where most strikingly evident to the enlightened eye of 
 philosophy. The present kingdom of Bohemia, for instance, is the bottom 
 of one of the great lakes formerly existing over Europe. It is a basin or 
 amphitheatre, formed by a wall of mountains, and the only gate or opening 
 to it, is that remarkable one by which the water now escapes from it, and 
 which evidently has been gradually cut or formed by the action of the run- 
 ning stream. As the bottom became uncovered, owing to the sinking of the 
 water, and the formation of a regular sloping channel from every part, the 
 former lake was converted into a fine and fertile country, a fit habitation for 
 man ; and the continued drain from it of the rains which fall over its surface, 
 and either pass rapidly away, or sink into the earth, and ooze again more 
 gradually in the form of springs, is the beautiful river which we now call 
 the Elbe. 
 
 In Switzerland, many of the valleys which were formerly lakes, have the 
 opening for the exit of water so narrow, that, as happened in one of them a 
 few years ago, a mass of snow or ice falling into it, converts the valley once 
 more into a lake. On the occasion alluded to, the accumulation of water 
 within was very rapid; and although, from the danger foreseen to the coun- 
 try below, if the impediment should suddenly give way, every means was 
 tried to remove the water gradually, the attempt had not succeeded when the 
 frightful burst took place, and involved the inferior country in common ruin. 
 
 The magnificent Danube is the drain of a chain of basins or lakes, which 
 must, at one time, have discharged or run over one into another ; but owing 
 to the continued stream cutting a passage at last low enough to empty them 
 all, they are now regions of fertility, occupied by civilized man, instead of 
 the fishes which held them formerly. This operation is still going on in 
 all the lakes of the earth. The lake of Geneva, for instance, although con- 
 
FLUID LEVEL. 137 
 
 
 
 fined by hard rock, is lowering its outlet, and the surface has consequently 
 fallen within the period of accurate observation and records ] and as, at the 
 same time, the wearing of the neighbouring mountains, brought down by 
 the winter torrents, are filling up its bed, if the town of Geneva last long 
 enough, its inhabitants may have to speak of the river in the neighbouring 
 valley, instead of the picturesque lake which now fills it. Already several 
 towns and villages, which were close upon the lake a century ago, have fields 
 and gardens spreading between them and the shore. 
 
 Illustrating this subject, it is very interesting to observe the contrast 
 between the pure blue water of the Rhone issuing from the lake of Geneva, 
 and the turbid streams which join its course a little farther down. The 
 torrents which fall into the lake all around, are equally charged with the 
 debris or wearings of the mountains ; but having deposited all their load in 
 the still bosom, of the lake, the pure water alone escapes to form the river. 
 The streams, however, coining to the Rhone directly from the Alps, and 
 bringing with them their charge of broken-down earth, even after they have 
 joined it, are long distinguishable by their muddy waters. It is the mud 
 deposited as here described, which is gradually filling up all lakes, and which 
 has formed the vast regions of flat country seen about the: mouths of great 
 rivers. The greater part of Holland is deposition of this kind, the whole 
 of lower Egypt, a great part of Bengal, &c., &c. 
 
 There are some lakes on the face of the earth which have no outlet towards 
 the sea, all the water which falls into them, being again carried off by 
 evaporation alone and such lakes are never of fresh water, because every 
 substance, which, from the beginning of time, rain could dissolve in the re- 
 gions around them, has necessarily been carried towards them by their feed- 
 ing streams, and there has remained. The great majority of lakes, however, 
 being basins with the water constantly running over atone part towards the 
 sea, although all originally salt, have, in the course of time, become fresh, 
 because their only supply being directly from the clouds, or from rivers 
 and springs fed by the clouds, is fresh, while what runs away from them 
 must always be carrying with it a proportion of any substance that remains 
 dissolved in them. We thus see how the face of the earth has been gradu- 
 ally washed to a state of purity and freshness fitting it for the uses of man, 
 and why the great ocean necessarily contains in solution all the substances 
 which originally existed near the surface of the earth, soluble in water : 
 viz., all the saline substances. The city of Mexico stands in the centre of a 
 vast and beautiful plain, 7,000 feet above the level of the sea, and surrounded 
 by sublime ridges of mountains, many of them snow-capped. One side of 
 tne plain is a little lower than the other, and forms the bed of the lake, which 
 is salt for the reasons stated above ; but the lake will not long be salt, for 
 it now has an outlet. About 150 years ago, owing to unusual rains, an ex- 
 traordinary increase of the water took place, and covered the pavements of 
 the city. An artificial drain was then cut from the plain, at the distance of 
 about sixty miles from the city, to the lower external country. This soon freed 
 the city from the water, and since then, becoming every year deeper by the 
 wearing effects of the uninterrupted stream, it is still lowering the surface 
 of the lake, is daily rendering the water less salt, and is converting the vast 
 salt marshes, which formerly surrounded the city, into fresh and fertile fields. 
 
 The vast continent of Australasia, or New Holland, (as large as Europe,) 
 is supposed by some to have been formed at a different time from what is 
 called the Old World, so different and peculiar are many of its animal and 
 vegetable productions - } and the idea of a later formation receives counte- 
 
138 HYDROSTATICS. 
 
 
 
 nance from the existence of immense tracts of marshy or imperfectly drained 
 land discovered in the interior, into which rivers flow, but seem not yet to 
 have worn down a sufficient outlet or discharging channel toward the ocean. 
 Where the soil or bed of a country through which a water-track passes 
 is not of a soft consistence, so as to allow readily the wearing down of higher 
 parts, and the filling up of hollows by deposited sand, lakes, rapids and great 
 irregularities of current remain. We have, for instance, the line of the lakes 
 in North America, the rapids of the St. Lawrence, and the stupendous falls 
 of Niagara, where at one leap the river gains a level lower by a hundred and 
 sixty feet. A softer barrier than the rock over which the river pours, would 
 soon be cut through, and the line of lakes would be emptied. 
 
 The contemplation of the fact, that water in seeking its level is constantly 
 wearing where it rubs, and carrying the abraided portions down to lower 
 level, and ultimately to the bed of the ocean, brings irresistibly the awful 
 idea, that this earthly abode of ours, owing to natural causes already in 
 operation, can have but a limited existence in its present state. No shower 
 falls that does not send portions of mountains and plains into the depths 
 of the ocean, and thus cause a corresponding encroachment on the shores by 
 the rising water ; and with revolving ages, unless new convulsions of nature 
 disturb the progress, or art succeed, as in Holland and elsewhere, in shut- 
 ting out the ocean from extensive low tracts by means of sea dykes or em- 
 bankments, *the dry land must at last disappear, and another gradual deluge 
 embrace the globe. 
 
 There is, perhaps, nothing which illustrates in a more striking manner 
 the exact resemblance among nature's phenomena, or their accordance with 
 the few general expressions or laws which describe them all, than the perfect 
 level of the ocean as a liquid surface. The sea never rises nor falls in any 
 place, even one inch, but in obedience to fixed laws, and therefore its changes 
 may generally be foreseen and allowed for. For instance, the eastern trade- 
 winds and other causes force the water of the Indian Ocean towards the 
 African coast, so as to keep the Red Sea about twenty feet above the 
 general ocean level ; and the Mediterranean is a little below that level, 
 because the evaporation from* it is greater than the supply of its rivers, 
 causing it to receive an additional supply by the Strait of Gibralter ; but 
 in all such cases, the effect is as constant as the disturbing cause, and there- 
 fore can be calculated upon with confidence. 
 
 Were it not for this perfect exactness, in what a precarious state would 
 the inhabitants exist on the sea shores, and on the banks of low rivers ! Few 
 of the inhabitants of London, perhaps, reflect, when standing by the side of 
 their noble river, and gazing on the rapid flood-tide pouring* inland through 
 the bridges, that although sixty miles from the sea, the water there is, at the 
 moment, lower than the surface of the sea, which may at the time be heaving 
 moreover, in lofty waves, covered perhaps with wrecks and the drowning. 
 
 The horrid destruction that would follow any alteration of the level of 
 the ocean, may be judged of by the effects of occasional floods, produced by 
 rains and melting snow in the interior of countries, or by these combined 
 with winds and high tides on the coasts. The flood at St. Petersburg, in 
 1825, was dreadful, in which strong westerly winds had retarded the flow of 
 the Neva so much, that the water rose forty feet (the height of an ordinary 
 house) above its usual mark, covered all the low parts of the town, and 
 drowned thousands of the people. 
 
 In Holland, which is a low flat, formed chiefly by the mud and sand 
 brought down by the Rhine and neighboring rivers, much of the country 
 
FLUID LEVEL. 
 
 139 
 
 is really below the level of the common spring-tides, and is only protected 
 from daily inundations by artificial dykes or ramparts, made strong enough 
 to resist the ocean. On one occasion the water broke into such an enclosure, 
 and drowned more than sixty thousand people. What awful uncertainty 
 then would hang over the existence of the Dutch, if the level of the sea 
 were subject to change ; for while we know that its waters, owing to the 
 centrifugal force or of the earth's rotation, are seventeen miles higher at the 
 equator than at the poles, if the level, as now established, were from any 
 cause to be suddenly changed but ten feet, millions of human beings would 
 be the victims. 
 
 Where inundation is regularly periodical, as in the Nile and many other 
 rivers, 'the hurtful effects can be guarded against, and the occurrence may 
 even become useful, by fertilizing the soil. 
 
 Tracts of land in contact wi^i rivers, of which land, the surface lies be- 
 tween the levels of ebb and flood-tide, if surrounded with dykes, may be 
 kept constantly covered with water, by opening the sluices only at high water j 
 or may be kept constantly drained, by opening the sluices only at low water. 
 A vast extent of rice fields, near the mouths of rivers in India and China, is 
 managed in this way, the admission or exclusion of water being regulated by 
 the age of the rice plant. A great part also of the rich sugar plantations 
 of Demerara, Esequibo, &c., on the coast of South America, are in the same 
 predicament; and another advantage which these have over the plantations 
 on the West-India Islands, is the saving of the labour of transport effected 
 by the canals which intersect all the fields. 
 
 11 If various tubes and vessels communicate with one another, fluid admitted 
 into them will rise to the same level in all" (Read the Analysis, p. 84.) 
 
 The following sketch may represent a variety of tubes and vessels, fixed 
 upon and opening into the cistern or box Gr. Water poured into any one 
 would fill the box, and would then rise to the same level in all. The dotted 
 lines from a to/ may represent the surfaces of the fluid in the different 
 vessels. In the figure at p. 128, it 
 was seen why, in all upright cylin- 
 drical vessels, as a, b and c, the fluid 
 rises to the same level ; and the figure 
 at p. 132 explained why shape of the 
 vessel cannot affect the level. Al- 
 though in the oblique vessel c, re- 
 presented here, there is more water 
 than in a, still there is the same 
 pressure at the bottom of both, be-, 
 cause e supports part of the weight 
 of its contained fluid on the principle of the inclined plane. 
 
 If a tube twenty miles long, and rising and descending among the ine- 
 qualities of. a country, were filled with water, and could have its ends 
 brought together for comparison, it would exhibit two liquid surfaces having 
 precisely the same level; and on either end being raised, the fluid would 
 sink in it to overflow from the other. 
 
 An easy mode of determining a level line at any spot is to have an open 
 tube, bent up at its ends a and b, and nearly Fig. 76. 
 
 filled with liquid : by then looking along the 
 two liquid surfaces, or through floating sights 
 resting on them, an observer looks in a line 
 which is quite horizontal at the middle point between them. 
 
 f 
 
140 HYDROSTATICS. 
 
 If there were two lakes on adjoining hills of different heights, a pipe of 
 communication descending across the valley and connecting them, would 
 soon bring them to the same level ; or if one were much higher than the 
 other, would empty that one into the other. 
 
 A projector thought that the vessel of his contrivance, represented here, 
 was to solve the renowned problem of the perpetual motion. It was goblet- 
 shaped, lessening gradually towards the bottom until it became a tube, turned 
 upwards at c, and putting with an open extremity into the goblet again. He 
 reasoned thus : A pint of water in the goblet a must more than counterbalance 
 an ounce which the tube 6 will contain, and must there- 
 Fig. 77. fore be constantly pushing the ounce forward into the 
 vessel again, and keeping up a stream or circulation, 
 which will cease only when the water dries up. He 
 was confounded when^t trial showed him the same 
 level always in a and in b. 
 
 A glass tube inserted near the bottom of a cask or 
 cistern of any sort, not air-tight above, which tube is 
 then bent upwards, to appear on the outside like a 
 barometer tube, shows by the elevation of a fluid in it, 
 the height of the greater mass of fluid within. 
 
 In like manner a tube brought from a river into a neighbouring cellar or 
 pit, will indicate the height of water in the river. 
 
 A knowledge of the truth, that water in pipes will always rise again to 
 the height or level of its source, has enabled men in modern times to 
 construct those admirable systems of iron pipes, which distribute water in 
 great towns. The water brought to any elevated site, in or near the town, 
 may be delivered from a reservoir there, by the effect of gravity alone, to 
 every cistern which is under the level of the reservoir, the result not being 
 affected by the pipes having to rise over heights and to descend into valleys 
 many times in their course. 
 
 On the hill north of London, on which Pentonville stands, there is such a 
 reservoir to which water is brought from Hertfordshire, by a channel cut for 
 the purpose, upwards of thirty miles in length, and called the New River. 
 Another reservoir has lately been constructed by the West Middlesex Water 
 Company, at Primrose Hill, higher than any house in town. It is filled by 
 operation of steam-engines at thq Company's works, near Hammersmith, 
 five miles off. It will supply water to the summits of all the houses con- 
 nected with it, and is exceedingly useful in cases of fire. 
 
 Many persons have believed that the ancients were ignorant of the law, 
 that fluid in pipes rises to the level of its source, because, in all the ruins 
 of their aqueducts, the channel is a regular slope. Some of the aqueducts, 
 as works of magnitude, are not inferior to the great wall of China, or the 
 Egyptian Pyramids; yet, at the present day, a single pipe of cast-iron is 
 made to answer the same purpose, and even more perfectly. It is now 
 ascertained, however, that it was not ignorance of the principle, but want of 
 fit material for making the pipes, which cost our forefathers such enormous 
 labour. 
 
 The supply and distribution of water in a large city, particularly since the 
 steam-engine has been added to the apparatus, approach closely to the per- 
 fection of nature's own work in the circulation of blood through the animal 
 body. From the great pumps or a high reservoir, main pipes issue to the 
 chief divisions of the town ; these then send suitable branches to the streets, 
 which branches again divide for the lanes and alleys; and at lust subdivide 
 
FLUID LEVEL. 141 
 
 until every house has its small leaden conduit carrying its precious freight, 
 if required, even into the separate apartments, and yielding it anywhere to 
 the turning of a cock A corresponding arrangement of drains and sewers, 
 most carefully constructed in obedience to the law of level, receives the 
 water again when it has answered its purposes, and sends it to be purified 
 in the great laboratory of the ocean. And so admirably complete and 
 perfect is this counter-system of sloping channels, that a heavy shower may 
 fall, and after washing and purifying every superficial spot of the city, and 
 sweeping out all the subterranean passages, may, within the space of an 
 hour, form part of the river passing by. It is the recurrence of this almost 
 miracle, of extensive, sudden, and perfect purification, which makes modern 
 London the most healthy, while it is the largest city in the world. 
 
 English citizens have now become so habituated to the blessing of a 
 supply of pure water, more than sufficient for all their purposes, that it no 
 more surprises them than the regularly returning light of day or warmth of 
 summer. But a retrospect into past times may still awaken them to a 
 sense of their obligation to advancing art. How much of the anxiety and 
 labour of men in former times had relation to the supply of this precious 
 element ! How often, formerly, has periodical pestilence arisen from the 
 deficiency of water j and how often has fire devoured whole cities, which a 
 timely supply of water might have saved ! Kings have received almost 
 divine honours for constructing aqueducts, to lead the pure streams from 
 the mountains into the peopled towns. In the present day, it is he who has 
 travelled on the sandy plains of Asia or Africa, where a well is more prized 
 than mines of gold, or who has spent months on ship-board, where the fresh 
 water is often doled out with more caution than the most precious product 
 of the still, or who, in reading history, has vividly sympathized with the 
 victims of siege or shipwreck, spreading out their garments to catch the rain 
 from heaven, and then with mad eagerness, sucking the delicious moisture 
 it is he who can appreciate fully the blessing of that abundant supply 
 which most of us now so thoughtlessly enjoy. The author of this work 
 will long remember the intense momentary regret with which, at once 
 approaching a beautiful land after months spent at sea, he saw a stream of 
 fresh water gliding over a rock into the salt waves it appeared to him as 
 if a most precious essence, by some accident were pouring out to waste. 
 
 The subject of fluid level leads to a consideration of springs or wells, and 
 of the operation of boring for water. 
 
 The water which falls from the clouds, and which must all ultimately 
 return to the sea, may find its way to the rivers, either by running directly 
 along the surface of soils which refuse it admittance } or by first sinking into 
 porous earth, and again oozing out at lower situations in the form of springs. 
 If a spring be as low as the bottom of the porous earth from which it issues, 
 that is to say, as low as the surface of the impermeable clay or rock on 
 which at some depth all such earth rests, it may drain the whole ; but if not, 
 the water will stand at a certain level among the earth as it would among 
 bullets in a water-tight vessel. If a hole or pit be then dug in such earth, 
 reaching below the level of the water lying in it, the pit will soon be filled 
 with water up to the level, and will be called a well. In many places this 
 water-level is very far below the surface of the ground ; and in some places, 
 by reason of the water having an easy drainage towards the sea, or of the 
 superficial soil being altogether impermeable to it, there is none to be found 
 within an accessible depth. 
 
142 HYDROSTATICS. 
 
 A remarkable illustration of this subject occurred a few years ago, in Kent, 
 on the occasion of cutting between Rochester and Gravesend the canal called 
 the Thames and Medway canal. This canal consists of but one cut or 
 level, seven miles long, of which two are in a tunnel through the hill which 
 level is that of high water in the connected rivers ; the intention having 
 been to let the canal be filled always from the rivers at high water ; but as 
 the level of the subterranean water in the surrounding land, and therefore of 
 all the inhabitants' wells there, is, as might be anticipated, half-way between 
 the levels of high and low tides, the salt water from the rivers was no 
 sooner admitted to the canal than it spread into the land on either side, 
 where the resisting internal water-level was lower, and destroyed all the 
 wells. If the canal had been dug a few feet lower, the mischief would not 
 have occurred, and the company would have escaped paying the heavy 
 damages, which rendered their undertaking a very ungainful speculation. 
 
 All the wells and springs in the. world are merely the rain water which has 
 sunk into the earth, appearing again, and gradually escaping at lower places : 
 nature thus admirably making the bowels of the earth an ever-stored reservoir 
 of the substance most indispensable to the comfort and existence of man, and 
 of all living creatures. It is worthy of remark here, that high cultivation or 
 agricultural improvement of a country has a great effect on the quantity of 
 spring water in it. While the face of a country is rough the rain-water 
 remains long among its inequalities, slowly sinking into the earth to feed the 
 springs, or slowly running away from the surface as from bogs and marshes 
 towards the rivers. The rivers, hence, have a comparatively uniform and 
 regular supply, even when rain has not fallen for a long time : but in a 
 well-drained country, the rain, by a thousand prepared channels, finds its 
 way to the brooks and rivers almost immediately, producing often dan- 
 gerous floods or inundations of the neighboring low grounds. A friend of 
 the author had a waterfall and mill in Surrey, which he formerly let for a 
 rent of 1,200 a year; but after agricultural improvements in the district 
 from which the water came, the supply of water was generally either 
 superabundant or deficient, and the value of the mill was reduced to one-half. 
 
 The surface of our globe is formed of different strata or layers, as of clay, 
 chalk, sand, gravel, c., &c., which appear all to have been at former periods 
 horizontal, formed under water, and to have been afterwards thrown up, by 
 some convulsion or convulsions of nature, into every variety of position. In 
 particular situations, the upper surface is now concave or basin-shaped, the 
 different strata or layers, when water-tight, being like cups or basins placed 
 one within another ; and as water poured in, to fill the space between two 
 basins so placed, would spring out to the height of its upper or level surface, 
 through any hole made in the side of either, so on boring for water, through 
 an innermost or superior water-tight stratum or basin of earth, the water 
 often springs out and rises far above the surface of the ground. London 
 stands in a hollow of which the first-met layer is a basin of clay, placed 
 over chalk, and on boring through the clay (sometimes of three hundred feet 
 thickness,) the water issues and in many places will form a jet considerably 
 above the surface of the ground ] showing that there is a higher source or 
 level somewhere as among the hills of Surrey, or those north of London. 
 , When fluids of different kinds and of different weights under the same 
 bulk, are made to oppose or to balance each other in communicating vessels 
 as water, for instance, in one leg of the bent tube b d c,and oil in the other 
 the surfaces will not all rest or settle at the same height or level, but that 
 of the lighter fluid will be just as much higher than that of the other as it is 
 
FLUID SUPPORT. 
 
 143 
 
 Fig. 78. 
 
 'IV 
 
 lighter. Thus a column of oil must be of a 
 length as d o, to balance a column of water 
 d w : and alcohol, because lighter than oil, 
 to balance the same water, would have to 
 stand higher still, as at a ; while mercury > 
 because thirteen times weightier than water, 
 would stand only about m. The shape, size, 
 or position of the vessels in which the oppos- 
 ing fluids might stand, would have no in- 
 fluence on the relative heights of the surfaces ; 
 for if we suppose a larger vessel, such as is 
 represented here by the dotted lines between 
 the letters e f m, to be substituted for the 
 leg c d of the tube, the various -fluids to 
 balance the water in b d, would have to stand 
 just as high in it as in the smaller tube. 
 
 " A body immersed in a fluid, displaces exactly its own bulk of it, which 
 quantify having been just supported by the fluid around the body is held 
 up with force exactly equal to the weight of the fluid displaced, and must 
 sink or swim according as its own weight is greater or less than this" 
 
 A bladder full of air, and maintaining the bulk of a pound of water, 
 requires a force of one pound (except a few grains, the weight of the air,) 
 to plunge it under the water. The same bulk of gold is held up in water 
 with exactly the same force ; so that, if previously balanced at the end of a 
 weighing beam, it appears on immersion to have lost one pound of its 
 weight. 
 
 And a piece of wood, ivory, or any other substance, having exactly the 
 same bulk, is opposed on entering the fluid by the same resistance. 
 
 The reason of thi^i is obvious, for the immersed body takes the place of 
 water which weighed one pound and yet was supported, and whose pressure 
 was necessary for the equilibrium of the rest. In a vessel of water repre- 
 sented here by the figure a b, let us append to any 
 portion of the water, a single column of particles, 
 for instance, represented by the line c d : we 
 know that each column is steadily supported in 
 its place, because the particle of the liquid 
 immediately under it is tending upwards to 
 escape from the surrounding pressures, with force 
 exactly equal to the weight of the column ', and 
 what is true of a column of single particles, is 
 
 true of any other portion, such as the larger column represented by the 
 figure/ h g. If such portion weighed exactly a pound, the surface under it 
 would be tending upward with the force of a pound ; and if the portion, 
 without changing its bulk or form, were to become ice, it would still be 
 exactly supported by the surface below pressing upwards with a force of a 
 pound ; and farther, if a similar column of wood, or stone, or metal, were there, 
 the surrounding pressure would still be the same. Again, if we suppose 
 only half the column to be solidified, the portion h g for instance, it, would 
 be pressed upwards with a force of one pourfd at g ; but its own weight of 
 half a pound, and the weight of the half pound of water above it, would 
 produce an exact balance and maintain rest. 
 
 Fig. 79. 
 
 J? 
 
 cL 
 
144 HYDROSTATICS. 
 
 It is very important to have clear notions on this subject ; and as different 
 minds apprehend such matters with different degrees of facility, and ill 
 different ways, we shall state the same general truth in other words. 
 
 Let us consider a mass of fluid as consisting of a vast number of extremely 
 minute columns of single particles standing side by side, where every 
 particle supports those above it by tendency upwards which it requires 
 through the pressure of the fluid surrounding it. Now if we suppose the 
 particles of a portion of a fluid mass, of any shape, to stick together, or to 
 become ice without change of bulk or weight, that portion when solid would 
 still be between the same forces as when fluid, and therefore would be equally 
 supported, and would remain at rest. And if gold, or silver, or glass, or 
 wood, having the same bulk, were substituted for the supposed ice, such 
 new substance would still be sustained with the same force ; so that a 
 substance of exactly the same weight as the ice or water displaced, would 
 have no tendency either to rise or to fall more than the water itself had j 
 but a substance heavier would sink, and one lighter would swim, and in 
 either case with force exactly proportioned to the difference between its 
 weight and that of an equal bulk of water. 
 
 Few persons, in now reading the statement of this truth in appearance 
 so simple and obvious would imagine that it had remained so long unknown 
 and that the discovery of it may be accounted one of the most important 
 which human sagacity ever made, but such is the case. We owe the dis- 
 covery to one of the master-minds of antiquity that of Archimedes. He 
 caught the idea one day while his limbs were resting on the liquid support 
 of a bath : and as his God-like intellect darted into futurity, and perceived 
 many of the important uses to which the knowledge was applicable, he is 
 said to have become so moved with admiration and delight, that he leapt 
 from the water, and unconscious of his nakedness, pursued his way home- 
 wards, calling out " evgyxa, wgyxa," I have found it. He was thinking chiefly 
 of the ready means, thus obtained, of ascertaining in all cases what has since 
 been called the specific gravity of bodies, viz., the comparative weights of 
 equal bulks of different substances ; as of gold, or silver, or copper, or iron, 
 compared with water ; and in" the case of mixtures, as of gold with silver for 
 instance, of declaring at once the proportion present of each important 
 problems, which, until then, could not be correctly solved. 
 
 The hydrostatic law now explained, has since led to great advances in 
 various arts. It may be regarded as a chief foundation of chemistry, for by 
 it the chemist distinguishes one substance from another, distinguishes a 
 pure from an impure substance, and discovers the nature of many mixtures 
 or compounds. The merchant often judges by it of the worth of his merchan- 
 dize. In any case it enables an inquirer to ascertain at once the exact size 
 or solid bulk of a mass, however irregular even of a bundle of twigs. It 
 has become the cause of improvements in navigation, in marine architecture, 
 and in many other arts. 
 
 We shall now discuss more particularly the subject of comparative 
 weights or specific gravity. 
 
 11 The force with which a body is held up in a fluid, being the exact weight 
 of its bulk of that fluid, by ascertaining this force and comparing it with 
 the weight of the body itself the comparative weights or SPECIFIC GRAVI- 
 TIES are found." (Read the Analysis, p. 126.) 
 
 If any b6dy, c, a mass of gold, for instance, be suspended by a thread or 
 
FLUID SUPPORT. SPECIFIC GRAVITY. 
 
 145 
 
 Fig. 80. 
 
 hair from the bottom of one scale b 
 of a weighing-beam, and be balanced 
 by weights put into the other scale a, 
 and if a vessel of water be then lifted 
 under it so that the water shall sur- 
 round it, the body is pushed up or 
 supported by the water with force 
 equal to the weight of the water which 
 it displaces; the weights, therefore, 
 then required in the scale b to restore 
 the balance, show truly the exact 
 weight of the water displaced ; or of 
 water equal in bulk to the body ; and 
 the weights in the two opposite scales 
 show the comparative weights of the 
 body and of its bulk of water. In the 
 supposed case, whatever weight the 
 gold had in the air, it would seem to lose when the water surrounded it, 
 about a nineteenth part of such weight ; that is, the water would support it 
 with this force ; and gold would thus be proved to be about nineteen times 
 as heavy as water. 
 
 In making- a table of specific gravities, it was necessary to select a common 
 standard with which all other substances should be compared, and this has 
 been done in choosing water ; the reason of preference being, that water can 
 be so easily procured in a state of purity, and therefore of uniformity, in all 
 situations. When we say, therefore, that gold is of the specific gravity 19, 
 and copper 9, and cork ^, we mean that these substances are just so much 
 heavier or lighter than their bulk of pure water in its densest state, viz., at 
 the temperature of 40 degrees of Fahrenheit's thermometer. 
 
 As the substances in nature differ as to form and other qualities, cor- 
 responding differences have to be made in the manner of ascertaining their 
 specific gravities : the following cases are most important. 
 
 Solid bodies insoluble in water and heavier than it as the metals, &c., 
 are merely suspended by a thread or hair, having nearly the specific gravity 
 of water, to one scale of the hydrostatic balance (simply a good weighing- 
 beam with a water- vessel below one of the scales ;) and the body being first 
 balanced or weighed in the air, and then in water, as already described, the 
 weight and the loss, represented, if the operator chooses, by the weights in 
 the opposite scales, are the weights of equal bulks of the two substances ; 
 and by finding, through the arithmetical operation of division, how often 
 the weight of the water is contained in the weight of the solid, we find the 
 specific gravity of the solid, or how much it is weightier than its bulk of 
 water. It is almost superfluous to remark, that putting weights into the 
 scale, b or taking them out of the scale a, are equivalent operations. We 
 shall explain afterwards, that for very delicate purposes bodies must be 
 weighed first in a vacuum, instead of in air, or a suitable allowance must be 
 made ; for air itself supports a little any body immersed in it. 
 
 Solids lighter than water, as cork, are weighed in it by attaching to 
 them a mass of metal or glass heavy enough to sink them, and already 
 balanced in water for the purpose ; or by making the line which connect 
 them with the weighing beams pass under a small pully fixed at the bottom 
 of the vessel, so that the rising of the end of the beam to which they are 
 attached shall draw them down. 
 
 10 
 
146 HYDROSTATICS. 
 
 A solid soluble in water, as a chrystal of any salt, may be protected 
 during the operation of weighing in water, by previously dipping it in 
 melted wax, so as to leave a thin covering on it ; or it may be weighed in 
 some liquid which does not dissolve it, allowance being afterwards made for 
 the difference between the weight of such liquid and of water. 
 
 Powders insoluble in water, such as gold dust, are weighed in a glass cup 
 which has previously been balanced in water for that purpose. 
 Powders soluble in water, must be weighed in some other liquid. 
 Mr. Leslie, the highly endowed professor of natural philosophy in -the 
 University of Edinburgh, has lately suggested a novel and ingenious mode 
 of acertaining the specific gravity of pulverized or porous bodies j but as it 
 can be understood only by persons acquainted with the doctrines of 
 pneumatics, the consideration of it must come under that head. 
 
 Other liquids may be compared with water in several ways. 1st. If a 
 phial be made to hold exactly one thousand grains of distilled water, at 
 the temperature of 40, the weight of the same measure of any other liquid 
 is found, by simply filling the phial, and weighing it. Of sulphuric acid, 
 for instance, such a phial will contain nearly nineteen hundred grains, while 
 of alchohol it will receive only about eight hundred. 2d. A bulb of glass, 
 which loses one thousand grains when weighed in water, (which thousand 
 grains is therefore the weight of its bulk in water,) may be weighed in other 
 liquids, and the difference of loss marks the specific gravity,* as in the last 
 case. The bulb for this purpose may be of any size, but one which loses 
 in water exactly one thousand grains, is preferable, from the simplicity 
 thereby given to the calculations : This remark applies also to the phial 
 last mentioned. 3d. A contrivance which renders the beam and scales alto- 
 gether unnecessary, is a hollow floating bulb of glass or metal a, with a 
 slender stalk rising from it to support the little scale or dish b, and with 
 another stalk descending to carry the weight or weights at c, which serve as 
 ballast to it. The whole is so adjusted that when displacing one thousand 
 grains, or other known quantity of pure water, it shall float with a certain 
 mark upon the upper stalk just at the surface of the water. By then im- 
 mersing it in other liquids ancl finding how much weight must be added to, 
 or taken from it above or below, to make it float in them 
 at the same elevation, the comparative weights of these 
 other liquids and of water are found : or the difference of 
 weight which makes it float at different elevations in 
 water, having been previously ascertained, it will only be 
 necessary, in any other case, to note exactly its elevation ; 
 an inch of the slender stalk may be equivalent to a 
 difference of ten grains. This instrument is called an 
 hydrometer. There are generally printed tables and 
 directions, accompanying all forms of it, telling the exact 
 import of the several indications, and the allowances to 
 be made for temperature, &c. It may be used for weigh- 
 ing solids as well as liquids, for if any mass be put into the saucer b, weights 
 exactly equal to the mass must be taken out of the saucer b, or from below 
 at c, to restore the equilibrium of the instrument. The mass may be after- 
 wards placed at c, and weighed in water. 4th. The shortest mode of ascer- 
 taining the specific gravities of liquids, is to have a set or series of small 
 glass bubbles of different specific gravities, so that when they are thrown 
 into any liquid, those heavier than it will sink, and those lighter will swim, 
 while that one which marks its specific gravity will remain merely suspended. 
 
FLUID SUPPORT. SPECIFIC GRAVITY. 147 
 
 The bubbles must, of course be numbered, and the specific gravity of each 
 be previously known. 
 
 A common use of hydrometers is to ascertain the quality of the distilled 
 spirits brought to market, as of rum, brandy, gin, &c. All these consist of 
 alcohol more or less diluted with water; and' duty or tax is levied upon them 
 in proportion to their strength, or the quantity of alcohol which they con- 
 tain. A delicate hydrometer discovers this at once. 
 
 A shop-keeper in China sold to the purser of a ship, a quantity of 
 distilled spirit according to a sample shown ; but not standing in awe of 
 conscience, he afterwards, in the*privacy of his store-house, added a certain 
 quantity of water to each cask. 
 
 The spirit having been delivered on board^ and tried by the hydrometer, 
 was discovered to be wanting in strength. When the vendor was charged 
 with the intended fraud, he at first denied it, for he knew of no human 
 means which could have made the discovery ; but on the exact quantity of 
 water which had been mixed being specified, a superstitious dread seized 
 him, and having confessed his roguery, he made ample amends. On the 
 instrument of his detection being afterwards shown to him, he offered any 
 price, for what he foresaw might be turned to great account in his trade. 
 
 The specific gravity of aeriform substances is ascertainnd by means of 
 a glass flask of known size, furnished with a stop-cock. It is first weighed 
 when emptied by the air-pump, and afterwards when filled successively 
 with water and with different airs or gases. Comparison of the weights 
 gives the specific gravities, as already described. 
 
 The following table shows, in round numbers, the comparative weights 
 or specific gravities of some common substances. Water is the standard 
 kept in view, and any equal bulk of another substance is heavier or lighter 
 than water, according to the numbers severally attached to them. 
 
 Common Salt, . . .2 
 Brick .... 2 
 
 Alcohol . . . S 
 
 Cork . 
 
 Atmospheric Air 
 Hydrogen Gas . 
 
 Platinum . . . . 22 f 
 
 Gold 19i 
 
 Mercury . . . . 13 
 
 Copper .... 8f 
 Steel and Iron . . .8 
 
 Diamond . . . 3J 
 
 Glass 3 
 
 Common stones . . 2$ 
 
 Complete tables are found in systems of Dictionaries of Chemistry. 
 
 A cubic foot of water happens to weigh very nearly one thousand ounces 
 avoirdupois, or 62 ? pounds. Hence, in the foregoing table, the figures 
 denoting the specific gravities tell how many times a thousand ounces of 
 the different substances a cubic foot contains. Of gold, for instance, 
 a cubic foot contains more than nineteen thousand ounces, being worth in 
 money about 63,000 sterling. A cubic foot of common air contains only 
 a little more than one ounce ; and of hydrogen gas, the lightest of ponder- 
 able things, a cubic foot contains less than a drachm. 
 
 The following facts are also illustrations of the truth, that a body immersed 
 in a fluid is held up, or has its entrance resisted, with force equal to the 
 weight of the quantity of fluid which it displaces. 
 
 A stone which on land requires the strength of two men to lift it, may be 
 lifted and carried in water by one man. There are cases, therefore, where 
 
148 HYDROSTATICS. 
 
 the support of water thus rendered useful, is equivalent to the assistance of 
 additional hands. A boy will often wonder why he can lift a certain stone 
 to the surface of water, but no farther. 
 
 The invention of the diving-bell in modern tirade, having enabled men, in 
 the building of piers, bridges, &c., to work under water almost as freely as 
 above, many have experience of this influence of water: but workmen are- 
 generally surprised at first, to find that below they can move much larger 
 and heavier stones than they can in the air. Some had supposed the fact 
 accounted for by saying that the denser air of the diving-bell, when received 
 into the lungs gave greater strength. In recovering property from a sunken 
 ship by the diving-bell, everything is found to be lighter in the proportion 
 now stated. 
 
 This law explains also why stones, gravel, sand and mud, are so easily 
 moved by waves and currents. Many people expressed astonishment, in 
 March, 1825, to learn that at the Plymouth Breakwater, the storm had dis- 
 placed blocks of stone of many tons weight ; but we now see that the moving 
 water had only to overcome about half the weight of the stone. 
 
 When a person lies in a bath, the limbs are so nearly supported by the 
 water as to require scarcely any exertion on the part of the individual. When 
 this softest of all beds has been indulged in for half an hour or more, the 
 person, on first lifting a limb out of the water, feels surprise at its great 
 apparent weight. The workers about diving-bells always experience the 
 sensation now spoken of, on returning to the air. 
 
 The bodies of most fishes are nea'rly of the specific gravity of water, and, 
 therefore, if lying in it without making exertion, they neither sink nor rise 
 very quickly. When this subject was less understood, many persons 
 believed that fishes had no weight in water; and it is related as a joke at 
 the expanse of philosophers, that a king -having once proposed to his men 
 of science to explain this extraordinary fact, many profound disquisitions 
 came forth, but not one of the competitors thought of trying what really 
 was the fact. It was beneath the dignity of science in those days to make 
 an experiment. At last a simple man balanced a vessel of water in scales, 
 and on putting a fish into the water, showed its scale preponderating just as 
 much as if the fish had been weighed alone. 
 
 In the sense now explained, water is said to have no weight in water. 
 The least force will raise a bucket of water from the bottom of a well to the 
 surface; but if the bucket be lifted at all farther, its weight is felt just in 
 proportion to the part of it which is above the surface. 
 
 " A body lighter than its bulk of water will float, and with force propor- 
 tioned to the difference." (Read the Analysis, p. 126.) 
 
 * The reason"of this is clear. If any body, the cylinder abed for instance, 
 be partially immersed in water, we know that tHe 
 upward pressure of the water on the bottom c d, ia 
 exactly what served to support the water displaced 
 by the body, viz., water of the bulk, efc d. The 
 body, therefore, that it may remain out as far as 
 here represented, must have exactly the weight 
 of the water which the immersed part of it dis- 
 places ; and if it be lighter than this, it will rise 
 farther ; if heavier, it will sink farther until the 
 exact balance be produced. 
 
FLUID SUPPORT. SWIMMING. 149 
 
 Hence of any body which floats in water, a pound weight displaces just a 
 pound of water, whether the body be very light in proportion to its bulk, as 
 cork, or heavier, as a piece of dense wood. This is experimentally shown 
 by putting such bodies to float in a vessel originally full of water. The 
 water displaced by each must run over the. sides of the vessel, and may be 
 caught and measured. 
 
 Hence a porcelain basin weighing four ounces will sink in water only as 
 far as a similar wooden basin or bowl of the same weight ; and the weight of 
 either basin may be in the substance of which it is formed, or in anything 
 else put into it as a load. 
 
 Hence a boat made of iron floats just as high out of water as a boat of simi- 
 lar form and size made of wood, provided the iron be proportionately thinner 
 than the wood, and therefore not heavier on the whole. An empty metallic 
 pot or kettle is often seen floating with a great part of it above the surface of 
 the water. Prejudice for a long time prevented iron boats from being used, 
 although, for various purposes, they are superior to others : and there are still 
 people who would fear to go on board of a ship built of the strong and singu- 
 larly durable Indian teaks, because it is heavier than water, and, in the form 
 of a log, therefore, sinks in water. Many fine ships of the line, however, and 
 East-Indiamen of fifteen hundred tons or more, are now built of teak. 
 
 Hence a ship carrying a thousand tons weight will draw just as much 
 water, or float to the same depth, whether her cargo be of cotton or of lead : 
 and the exact weight of any ship and her cargo may be determined by find- 
 ing how much water she displaces. In canal boats, which are generally of 
 a simple form, this truth affords a ready rule for ascertaining the quantity 
 of their load. 
 
 The human body, in an ordinary healthy state with the chest full of air, is 
 lighter than water. 
 
 If this truth were generally and familiarly understood, it would lead to the 
 saving of more lives, in cases of shipwreck and in other accidents, than all 
 the mechanical life-preservers which man's ingenuity will ever contrive. 
 
 The human body with the chest full of air naturally floats with a bulk of 
 about half the head above the water, having then no more tendency to sink 
 than a log of fir. That a person in water, therefore, may live and breathe it 
 is only necessary to keep the face uppermost. The reason that in ordinary 
 accidents so many people are drowned who might easily be saved, are chiefly 
 the following : 
 
 1st. They believe that the body is heavier than water, and therefore, that 
 continued exertion is necessary to keep it from sinking; and hence, instead 
 of lying quietly on the back, with the face upwards, and with the face only 
 out of the water, they generally assume the position of a swimmer, in which 
 the face is downwards, and the whole head has to be kept out of the water 
 to allow of breathing. Now, as a man cannot retain this position but by con- 
 tinued exertion, he is soon exhausted, even if a swimmer, and if he is not, 
 the unskilful attempt will scarcely secure for him even a few respirations. 
 The body raised for a moment by exertion above the natural level, sinks as 
 far below it when the exertion ceases; and the plunge, by appearing the 
 commencement of a permanent sinking terrifies the unpractised individual, 
 and renders him an easier victim to his fate. To convince a person learn- 
 ing to swim of the natural buoyancy of his body, it is a good plan to throw 
 an egg into water about five feet deep, and then desire him to bring it up 
 again. He discovers that instead of his body with the chest full of air na- 
 
150 HYDROSTATICS. 
 
 turally sinking towards the egg, he has to force his way downwards, and is 
 lifted again by the water as soon as he ceases his effort. 
 
 2d. They fear that water entering by the ears may drown, as if it entered 
 by the nose or mouth, and they make a wasteful exertion of strength to pre- 
 vent it ; the truth being, however, that it can only fill the outer ear, as far 
 as the membrane of the drum, where its presence is of no consequence. 
 Every diver and swimmer has > his ears thus filled with water, and cares not. 
 
 3d. Persons unaccustomed to the water, and in danger of being drowned, 
 generally attempt in their struggle to keep their hands above the surface, 
 from feeling as if their hands were imprisoned and useless while below ; but 
 this act is most hurtful, because any part of the body held out of the water, 
 in addition to the face which must be out, requires an effort to support it, 
 which the individual is supposed at the time ill able to afford. 
 
 4th. They do not reflect, that when a log of wood or & human body is 
 floating upright, with a small portion above the surface, in rough water, as 
 at sea, every wave in passing must cover it completely for a little time, But 
 again leave its top projecting in the interval. The practiced swimmer chooses 
 this interval for breathing. 
 
 5th. They do not think of the importance of keeping the chest as full of 
 air as possible ; the doing which has nearly the same effect as tying a blad- 
 der of air to the neck, and without other effort, will cause nearly the whole 
 head to remain above the water. If the chest be once emptied, while from 
 the face being under water the person cannot inhale again, the body remains 
 specifically heavier than water, and will sink. 
 
 When a man dives far, the pressure of deep water compresses, or dimi- 
 nishes the bulk of the air in his chest, so that, without losing any of that air, 
 he yet becomes really heavier than water, and would not again rise, but for 
 the exertion of swimming. The author of this work once saw a sailor ( a 
 fine-bodied ^West India negro ) fall into the calm sea from a yard-arm eighty 
 feet high. The velocity on his reaching the water was so great, that he shot 
 deep into it, and, of course, his chest was compressed as now explained : 
 probably also the shock stunned him, for although he was an excellent 
 swimmer, he only moved his 'arms feebly once or twice, and was then seen 
 gradually sinking for a long time afterwards, until he appeared only as a 
 black and distant speck, descending towards the unknown regions of the 
 abyss. 
 
 Every person need not learn to swim ; but every one who makes voyages 
 should have practiced the easy lesson of resting in the water with the face 
 out. The head, from the large quantity of bone in it is a heavy part of the 
 body, yet, owing to its proximity to the chest, which is comparatively light, 
 a little action of adjustment with the hands, easily keeps it uppermost ; and 
 there is an accompanying motion of the feet, called treading the water, not 
 diffcult to learn, which suffices to sustain the entire head above the* surface. 
 Many of the seventy passengers who were swallowed up on the sudden sink- 
 ing of the Comet steam-boat near Greenock, in November, 1825, might have 
 been saved by the boats, which so soon went to their assistance, had they 
 known the truth which we are now explaining. 
 
 A. man having to swim far, may occasionally rest on his back for a time, 
 and resume his labor when he is somewhat refreshed. 
 
 So little is required to keep a swimmer's head above water, that many 
 individuals, although unacquainted with what regards swimming or floating, 
 have been saved after shipwreck, by catching hold of a few floating chips or 
 broken pieces of wood. An oar will suffice as a support to half a dozen 
 
FLUID SUPPORT. STABILITY. 151 
 
 people, provided no one of the number attempts by it to keep more than his 
 head out of the water; but often, in cases where it might be thus serviceable, 
 from each person wishing to have as much of the security as possible, the 
 number benefitted is much less than it might be. 
 
 The most common contrivances, called, life-preservers, for preventing 
 drowning, are strings of cork put round the chest or neck, or air-tight bags 
 applied round the upper part of the body, and filled, when required, by those 
 who wear them blowing into them through valved pipes. 
 
 On the great rivers of China, where thousands of people find it more con- 
 venient to live in covered boats than in houses upon the shore, the younger 
 children have a hollow ball of some light material attached constantly to 
 their necks, so that, in their frequent falls overboard, they are not in danger. 
 
 Life-boats have a large quantity of cork mixed in their structure, or of 
 air-tight vessels of thin copper or tin plate : so that, even when the boats 
 are filled with water, a considerable part still floats above the general surface. 
 
 Swimming is much easier to quadrupeds than to man, because the ordi- 
 nary motion of their legs in walking and running is that which best supports 
 them in swimming. Man is at first the most helpless of creatures in water. 
 A horse while swimming can carry his rider with half the body out of the 
 water. Dogs commonly swim well on the first trial. Swans, geese, and 
 water-fowls in general, owing to the great thickness of feathers on the under 
 part of their bodies, and the great volume of their lungs, and the hollowness 
 of their bones, are so bulky and light, that they float upon the water like 
 stately ships, moving themselves about by their webbed feet as oars. 
 
 A water-fowl floating on plumage half as bulky as its naked body, has 
 about half that body above the surface of the water ; and similarly a man 
 reclining on a floating mattrass, as in the hydrostatic bed afterwards to be 
 described, has nearly as much of his body above the level of the water- 
 surface, as he forces of the mattrass under it. His position, therefore, 
 depends on the thickness of the mattrass. 
 
 A man walking in deep water may tread upon sharp flints or broken glass 
 with impunity, because his weight is nearly supported by the water. 
 
 But many men have been drowned in attempting to waae across the fords 
 of rivers, from forgetting that the body is so supported by the water, and 
 does not press on the bottom sufficiently to give a sure footing against a very 
 trifling current. A man, therefore, carrying a weight on his head, or in his 
 hands held over his head, as a soldier bearing his arms and knapsack, may 
 safely pass a river, where, without a load, he would be carried down the 
 stream. 
 
 There is a mode practised in China of catching wild ducks, which requires 
 that the catcher be well loaded or ballasted. The light grain being first 
 strewed upon the surface of the water to temp them, a man hides himself 
 in the midst of it, under what appears a gourd or basket drifting with the 
 stream, and when the flock approaches and surrounds him, he quickly 
 obtains a rich booty by snatching the creatures down one by one adroitly 
 making them disappear as if they were diving, and then securing them 
 below. Each bird becomes as a piece of cork attached to his body. 
 
 Fishes can change their specific gravity, by diminishing or increasing the 
 size of a little air-bag contained to their body. It is because this bag is 
 situated towards the under side of the body, that a dead fish floats with the 
 belly uppermost. 
 
 Animal substances, in undergoing the process of putrefaction, give out 
 much aeriform matter. Hence the bodies of persons drowned and remaining 
 
152 HYDKOSTATICS. 
 
 in the water, generally'swell, after a time, and rise to the surface, again to 
 sink when the still increasing quantity of air shall burst the containing parts. 
 
 A floating body sinks to the same depth whether the mass of fluid 
 supporting it be great or small : as is seen when a porcelain basin is placed 
 first in a pond, and then in a second basin only so much larger than itself 
 that a spoonful or two of water suffices to fill up the interval between them. 
 One ounce of water in the latter way may float a thing weighing a pound 
 or more, exhibiting another instance of the hydrostatic paradox : And if 
 the largest ship of war were received into a dock, or case, so exactly fitting 
 it that there were only half an inch of interval between it and the wall or 
 side of the containing space, it would float as completely, when the few 
 hogsheads of water required to fill this little interval up to its usual water- 
 mark were poured in, as if it were on the high sea. In some canal locks, 
 the boats just fit the place in which they have to rise and fall, and thus the 
 expense of water at the lock is diminished 
 
 The preceding examples of floating are all illustrations also of the truth 
 that the pressure of a fluid on any immersed body is exactly proportioned 
 to the depth and extent of the surface pressed upon. The lateral pressures 
 just balanced one another, and the upward pressure has to be balanced by 
 the weight of the body. 
 
 Similar reasoning to that which proves that the whole weight of a body acts 
 as if lodged in the point called its centre of gravity, proves that the whole 
 buoyancy of a body, or the upward push of the fluid in which a body is 
 immersed, acts as if lodged in the point which was the centre of gravity 
 of the fluid displaced. This point, consequently, is called the " centre of 
 buoyancy." 
 
 A floating body, to be stable in its position, either must have its centre 
 of gravity below the centre of buoyancy in which case it resembles a 
 pendulum ; or it must have a very broad bearing on the water, so that any 
 inclination may rause the centre of gravity to ascend in which case*it 
 resembles a cradle or rocking-horse. 
 
 Hence arises, in the stowing of a ship's cargo, the necessity of putting 
 the heavy merchandise underneath, and generally of putting iron ballast 
 under all the merchandise. Hence, also, the danger of having a cargo or 
 ballast which is liable to shift its place. A ship loaded entirely with stones, 
 is sometimes lost by a wave making her incline for a moment so much that 
 the load ships to one side, which is then kept down. For a similar reason, 
 a cargo of salt or sugar has a peculiar danger attached to it, for if the ship 
 leak, the cargo may be dissolved, and then pumped out with the bilge water, 
 leaving her with altered trim. In a fleet coming home from India, in 1809, 
 four fine ships disappeared during a hurricane off the isle of France, and from 
 what happened to the other ships that were saved, the cause of the destruc- 
 tion was supposed to be, that the saltpetre of the cargoes had been dissolved 
 and pumped out, and that the ships in consequence became unmanageable. 
 
 Bladders used by beginners in swimming are dangerous, unless secured 
 so as not to shift towards the lower part of the body. 
 
 A great inventor (in his own estimation) published to the world, that he 
 had solved the important problem of walking safely upon the water ; and he 
 invited a crowd to witness his first essay. He stepped boldly upon the wave, 
 equipned in bulky cork boots, which he had previously tried in a butt of 
 water at home ; but it soon appeared that he had not pondered sufficiently on 
 
FLUID SUPPORT AMONG FLUIDS. 153 
 
 the centres of gravity and of floatation, for in the next instant all that was 
 to be seen of him was a pair of legs sticking out of the water, the movements 
 of which showed that he was by no means at his ease. He was picked up 
 by help at hand, and, with his genius cooled and schooled by the event, was 
 conducted home. Some soldiers once finding a few cork/ac&efe, among old 
 military stores, determined to try them j but mistaking the shoulder straps 
 for lower fastenings, they put them on as drawers^nd on then plunging in, 
 with the hope of being able to sit pleasantly on th *water, their heavy heads 
 went down, and they were nearly drowned 
 
 When, on the return of summer, the ice breaks up in the polar regions, 
 immense islands of it are set afloat, rising high into the air and sinking deep 
 into the sea. The melting process, in most cases, does not go on equally in 
 the water and in the air, and from the mass, consequently, changing form, its * 
 stability is often lost, and one of the grandest phenomena in nature follows 
 the overturning of a mountain the sudden subversion of an island pro- 
 ducing a tumult in the ocean around, felt often at the distance of many leagues. 
 
 The phenomena of pressure, floating, &c., in fluids, vary in proportion to 
 the weight or specific gravity of the fluid. 
 
 A ship draws less water, or swims lighter, by one thirty-fifth, in the heavy 
 salt-water of the sea than in the fresh water of a river : and for the same reason 
 a man swimming supports hmiself more easily in the sea than in a river. 
 
 Many kinds of wood that float in water will sink in oil. 
 
 A man floats on mercury as the lightest cork floats on water, and with 
 practice he might be able to walk upon mercury. 
 
 Had the water of our ocean been but a little heavier than it is, men after 
 shipwreck might have died of famine and cold, but would not have been 
 drowned 
 
 Oil floats on water, but sinks in alcohol or aether. The term proof spirit 
 means spirit light enough for oil to sink in it. The strength of spirit is 
 proportioned to its lightness. 
 
 Cream rises in milk, and forms a covering to it. 
 
 Blood, allowed to rest after flowing from the living body, separates into 
 parts or layers, which arrange themselves according to their specific gravities. 
 The buffy coat of inflammation (where this exists) is uppermost, forming 
 the surface of the general coagulum : towards the lower part of the coagulum 
 there is an accumulation of red globules; and-the whole of the solid part 
 floats in the serum, which is therefore lowest of all. When the red globules 
 escape from the coagulum, they fall to the bottom even of the serum. 
 
 Wine, if slowly and carefully poured on water, will float upon it. In a 
 vessel shaped like a common sand-glass, only with a larger 
 opening between the chambers at c, if wine be put into the Fig. 83. 
 under chamber, and water into the upper, the two liquids 
 will gradually change places : and if the lower half of the 
 glass be covered, so as to leave the upper half with the 
 appearance of a simple goblet, the water will seem to have 
 been changed into wine. The liquids are less mixed, and 
 change places sooner, when there is a tube 6 to carry the 
 water down to the bottom without touching the wine, and a 
 tube a to carry the wine directly to the top. 
 
 Mercury, water, oil, air, and some other fluids may all be 
 shaken together in the same vessel, and on standing will 
 separate again and arrange themselves in the order of their specific gravities. 
 
154 
 
 HYDROSTATICS. 
 
 When, in a mass of water, part of it is heated more than the rest, that 
 part, by its expansion, becomes specifically lighter than the rest, and rises 
 to the surface. Hence, wh^n heat is applied to the bottom of a vessel 
 containing water, a circulation is established, which goes on from the first 
 moment until the operation of heating finishes : water is always rising 
 from the hotter parts of the vessel, and descending over the colder parts. 
 
 In like manner, when a tall glass containing hot water is dipped into cold 
 water, a downward currem lakes place within the glass near the sides all 
 round, and there is an upward current in the middle. This motion may be 
 rendered very obvious by small portions of amber thrown into the water, for 
 these being nearly of the specific gravity of water, rise and descend with it. 
 On account of the current established in such cases, heat applied to the 
 bottom of a vessel of liquid is soon equally diffused over it ; but heat 
 applied at the top is there confined, because the heated and lighter fluid 
 does not descend. Water may be made to boil at its surface, while a piece 
 of ice lies at the bottom. The converse is impossible. 
 
 The current in a fluid, produced by local change of temperature, is an 
 important part of the following process, which the author deems applicable 
 to various useful purposes. Heat may be transferred from one liquid to 
 another, without mixing them, by making the hot liquid descend in a very 
 thin metallic tube, through the cold liquid rising around it in a larger tube, 
 Boiling water from the vessel e, for instance, may descend slowly by the 
 
 small tube ea bf, which is sur- 
 
 Fig. 84. rounded from a to b by cold 
 
 water ascending through the 
 tube c g. Then, as the tem- 
 perature of two liquids, brought 
 so nearly into contact with each 
 other, will not, after a very short 
 time, differ, in any one place 
 more than a few degrees, it fol- 
 lows that the water lately cold, 
 will on leaving the part of the 
 tube g, which is in contact with 
 the boiling water descending di- 
 rectly from e, be nearly boiling, 
 while the water lately hot will, 
 on leaving the tube at &, which 
 is in contact with cold water 
 just arrived from A, be itself 
 nearly cold j and thus equal 
 quantities of hot'and cold water will have exchanged temperatures. *The flux 
 of the hot water is to be regulated by a cock 6, and that of the cold water by 
 a cock h. The water in the part of the tube c g d rises, because it is hotter and 
 therefore specifically lighter than that in the part h c. The author believes 
 that an apparatus made on this principle, with an arrangement of many thin 
 flat tubes instead of a single large tube, for the descending fluid, and a spacious 
 box c g to contain these and the rising fluid, would be an excellent refrigera- 
 tor in a distilling apparatus, and for cooling the wort of brewers ; or would 
 serve as a means of diminishing the expense of warm baths, by transferring 
 the heat from the water lately used to pure water. In distilling, the wash or 
 low wines, about to enter the still, might be used as the cold condensing fluid 
 to surround the warm or vapor tubes, and thus, without expense, would be 
 
FLUID SUPPORT AMONG FLUIDS. 155 
 
 heated in its progress to the still. Half the original expense of a great 
 porter brewery is in the construction of the numerous water-tight floors on 
 which the hot wort is thinly spread to cool. The practice of warm bathing, 
 so conductive to health, is less common in this country, because the present 
 expense is so great. 
 
 It is a general truth in nature, that substances contract in size as they 
 cool. There is, however, in water, a curious exception to this rule, which, 
 operating through the principle of specific gravities, effects most important 
 purposes in the economy of nature. Water contracts only down to the 
 temperature of 40 deg., below which, towards 32 deg., or the freezing point, 
 it goes on dilating again, and as ice is much lighter than as a fluid. Ice, 
 therefore, floats on the surface of water, and being a very slow conductor of 
 heat, defends the water underneath from the cold air, and preserves it liquid, 
 and a fit dwelling for the finny tribe, until the return of the mild season. 
 And not only is the extreme of cold below thus prevented, but because very 
 cold water remains floating on the surface of a wintry lake, as cream floats 
 on milk, it preserves underneath that warmth which is agreeable to the 
 fishes, just as very hot water in summer remains uppermost, preserving 
 underneath an agreeable coolness. By the dilation of very cold water, then, 
 and the formation of ice, nature has prepared a winter garb for the inhabited 
 lakes and rivers, as complete and effectual as for terrestrial animals, by the 
 periodical thickening of their wool or fur. Had ice become heavier than 
 water, so that it must have fallen to the bottom, and have left the surface 
 without protection, a deep lake in European winters, would have been frozen 
 into a solid lifeless mass, which summer suns would no more have melted 
 than they now do the glaciers of Switzerland. But for this important ex- 
 ception, therefore, to a general law of nature, many of the now most fertile 
 and lovely portions of the earth's surface would have remained for ever barren 
 and uninhabited wastes. 
 
156 PNEUMATICS. 
 
 PART III. 
 
 THE PHENOMENA OP PLUIDS 
 
 SECTION IL--PNEUMATICS. 
 
 ANALYSIS OF THE SECTION. 
 
 In aeriform fluids, that is, in such as have their particles held far apart 
 by mutual repulsion, which yields, however, to any force applied, so that 
 the mass suffers great change of volume under different degrees of com- 
 pression. the phenomena are modified by the GREAT LIGHTNESS and 
 ELASTICITY of the fluids, but are still in strict accordance with the general 
 properties of fluids already explained, viz., PRESSURE EQUAL IN ALL 
 
 DIRECTIONS PRESSURE AS THE DEPTH LEVEL SURFACE, and FLUID 
 
 SUPPORT. The pressure of air, in all directions, and as the depth, may 
 be studied in the effects of our atmosphere on solids on liquids : or 
 ichen it concurs with heat, in producing the phenomena of boiling, evapo- 
 ration, clouds, rain, dew, &c. ; or when, by varying in degree, it allows 
 certain substances to exist sometimes in the liquid and sometimes in the 
 aeriform states. The fluid support in air is exemplified by ballons, the 
 ascent of flame, and smoke", winds, &c. 
 
 WHATa change has taken place in the degree of man's knowledge of nature, 
 since philosophers thought that air was one of four primary elements, 
 viz., air, fire, water, and earth, of which all things were composed, and each 
 of which was for ever distinct from the others. We now know that air or 
 gas is merely an accidental state, in which any body may exist, according to 
 the quantity of heat pervading it: the body being solid when the absence of 
 heat allows its atoms to obey freely their mutual attraction, and to cohere 
 as in ice, for instance ; being liquid, when so much heat is present as nearly 
 to balance the attraction, and to let them slide freely among each other as 
 they do in water ; and being aeriform when still more heat is added, causing 
 the atoms mutually to repel and dart asunder to a great distance as they do 
 in steam. But in any one of these three states, the various substances are 
 as much themselves as in the others, and at the command of the chemist 
 will assume any of the forms which he desires, As most substances in 
 nature have a different relation to heat, there are some which, at the medium 
 temperature of our earth are solid, some which are liquid, and some aeriform. 
 The solids, in general, are the heaviest under a given volume, and therefore 
 sink down and form the great mass or centre of the earth j the liquids follow 
 next in order, and float upon this solid centre, filling up its inequalities with 
 
AERIFORM FLUIDS. 157 
 
 a level surface, so as to constitute the ocean ; while the airs are the lightest 
 of all, and as a second ocean, rest above the sea and above the highest moun- 
 tain, to an elevation of about fifty miles. Among the substances whose 
 relation to heat causes them, when not restrained in certain combinations, to 
 assume the form of air at very low temperatures, there are two in particular, 
 viz., oxygen and nitrogen, which are very abundant in nature in such uncom- 
 bined state, and of these, therefore, the atmosphere chiefly consists ; but 
 smaller portions of almost every other substance are found in it. Water, 
 among the supplementary matters, is much more abundant than any of the 
 others, and in various states of cloud, mist, rain, dew and snow, it answers 
 a thousand useful purposes, and serves beautifully to vary the scenes of 
 nature. The atmosphere is about fifty miles high or deep, and therefore, 
 in relation to the bulk of the earth, is as a covering of one-tenth of an inch 
 in thickness to a common library globe of a foot in diameter. 
 
 The atmospheric ocean is the great laboratory in which most of the actions 
 of life go on, and on the composition of which they depend. A human 
 being requires for breathing a gallon of fresh air every minute, dying equally 
 if deprived of air, or if confined to the same. All other animals also require 
 fresh air, but in various proportions. And in the vegetable creation, the 
 beautiful green leaf and delicate flower are merely broad and tender expan- 
 sions of surface for the contact of the vivifying air. Animals give out to 
 the atmosphere a substance which vegetables absorb, and vegetables, by the 
 absorption, fit the air again for the use of animals ; so that, upon the whole, 
 in the various changes of nature, there is a perfect balancing of actions, 
 which preserves the atmospheric mass in a uniform state, constantly fit for 
 its admirable purposes. 
 
 While the ancients had that notion of air, which made them apply to it 
 vaguely, and almost indifferently, the names of air, ether, spirit, breath, life, 
 &c., they never* dreamed of making experiments upon it, with a view to prove 
 its relation to common matter : and one of the most beautiful portions of 
 the modern history of man's progress in knowledge, is that which exhibits 
 the light gradually breaking in upon this most interesting subject. Galileo 
 was led to conclude that air made a definite pressure upon things at the 
 surface of the earth ; Torricelli and Pascal proved that this was occasioned 
 by its weight, and hence, moreover, they deduced the height of the aerial 
 ocean ; Priestly, Black, Lavoisier, and others, discovered that air might be 
 united with a metal, so as to increase its weight, and to produce a compound 
 of totally new qualities, for they showed that many of the ores of our mines 
 are merely metals concealed, by being thus united with a substance which, 
 when set free, ascends as one of the ingredients of the atmosphere. They at 
 last analyzed the atmosphere itself, and exhibited its two ingredients as 
 distinct substances. And within a few years the nature of air or gas has 
 been so thoroughly investigated, that we can now take a little of many a 
 light, invisible, impalpable fluid such as we breathe, and squeezing the heat 
 out of it by strong pressure, can make its particles collapse from their 
 aeriform distances to assume the state of a tranquil fluid; which may 
 then be retained as such for ever, or may be decomposed and made solid in 
 combination with other bodies, or may be again set at liberty. 
 
 The suspicion once excited, that air was as much a material fluid as water, 
 only much less dense, by reason of a greater separation and repulsion of the 
 particles, it was easy to follow out the parallel, and to confirm the supposition 
 by .reference to the commonest facts. Thus, a leathern sack or pouch, opened 
 and dipped into water so far as to become full, if its mouth be then carefully 
 
158 PNEUMATICS. 
 
 closed, retains the water, and its sides cannot afterwards be pressed together : 
 a similar sack or bladder, opened out, and then closed in air, is found to 
 remain, in a corresponding way, bulky and resisting, and forms what is called 
 an air-pillow. The motion of a flat board is resisted in water : the motion 
 of a fan is resisted in the air. Masses of wood, sand, and pebbles, are rolled 
 along or floated by currents of water : chaff, feathers, and even rooted trees 
 are swept away by currents of air. There are mills driven by water ; and 
 there are mills driven by the wind. Oil set free under the surface of water, 
 or placed there in a bladder, rises to the surface : hot air or hydrogen gas 
 placed in a balloon, rises in the air. A fish moves itself by its fins in water : 
 a bird moves itself by its wings in the air ; and as on taking the water from 
 a vessel in which a fish swims, the creature falls to the bottom, gasps a few 
 moments, and dies, so, on exhausting the air from a vessel in which birds 
 or butterflies are enclosed, their useless wings may flap ; but they sink to 
 the bottom, and if the cruel experiment be continued, they soon become 
 motionless and forever. 
 
 We proceed now to prove that air or gas, as a fluid, differs from the 
 other fluids, which we call liquids, only in the two circumstances of great 
 lightness or rarity, and of being very extensively elastic, that is to say, the 
 particles being so related, that pressure brings them much more nearly into 
 contact, and on ceasing, allows them to regain their former distance. 
 
 Lightness of Air. 
 
 The lightness or rarity of atmospheric air, as it is found on the general 
 gurface of the earth, is such, that if, by the action of a pump, a bag of it 
 holding a cubic foot be emptied into the copper ball of an air-gun, the ball 
 weighs about an ounce and a quarter more than before. The same volume 
 of water weighs nearly a thousand ounces j so that common air is about eight 
 hundred times lighter than water. Other gases, or substances in the aeri- 
 form state, have their various specific gravities, just as the same substances 
 have when liquid or solid. Thus water in the form of air, that is to say, 
 when existing as steam, and of the common density, is little more than half 
 as heavy as the same bulk of common air \ hydrogen is only one-fourteenth 
 part as heavy : and carbonic acid gas, which is the air that rises out of soda- 
 water, brisk ale, champagne wine, &c., is so much heavier, that even in the 
 atmosphere it may be poured out of one open vessel into another, as a liquid 
 might, or, more exactly, as water might be poured out under oil. 
 
 Elasticity of Air. 
 
 A small bladder full of air may be pressed or squeezed between the hands 
 so as to be much reduced in size, but on being relieved from the 
 Fig. 85. pressure, it will immediately resume its former bulk. 
 
 d, If a metalic tube or barrel of perfectly uniform bore a b, be 
 
 fitted with a moveable plug or piston c, which is covered with 
 leather and oiled, so as to slide up and down without allowing 
 the air to pass by its sides, the air between the piston and the 
 close bottom b may be compressed to a hundredth or less of its 
 usual bulk ; but when allowed, will push the piston back again 
 with the same force as it opposed to the condensation, and will 
 recover the volume which it had before the experiment. 
 
 Again, if the plug, at the commencement of the experiment, 
 were only an inch from the bottom, enclosing air of the usual 
 density, on drawing it up to the top, the inch of air beneath it 
 
ELASTICITY OF AIR. 
 
 159 
 
 would expand so as to occupy the whole tube, having become, of course, 
 proportionally less dense. 
 
 To the question why the air, which admits of such various density, is 
 found to have that certain degree of it met with at the surface of the earth, 
 we answer, that as the water, in any place near the bottom of the ocean, is 
 pressed with force exactly proportioned to the quantity of water above it, so 
 the air at the surface of the earth bears the pressure of the superincumbent 
 mass of air, and on account of its extensive elasticity, suffers, like the lower- 
 most bags of cotton or wool in a great heap, that degree of compression 
 which the superincumbent mass is calculated to produce. We shall see 
 below that the density of the air near the earth is changing with every 
 circumstance which affects the weight of the atmosphere above, as winds, 
 clouds, rain, &c., and that it bears relation to the altitude of the place of 
 observation above the level of the sea. 
 
 The tube with its piston, described in the last page, becomes, according to 
 the position of its valves, either a syringe for injecting and condensing air, 
 or a pump for exhausting or removing it from any vessel ; both opera- 
 tions depending on the elasticity of air. 
 
 A barrel and piston is a condensing syringe, when in a passage of com- 
 munication between the bottom of the syringe and a receiving vessel, there 
 is a flap or valve allowing air to pass towards the receiver but not to return. 
 The piston, therefore, at each stroke, forces what the barrel contains of air 
 into the receiver. When the piston is lifted again after the stroke, air 
 re-enters the barrel from the atmosphere, either through a valve in the 
 piston, or through a small hole near the top of the barrel. That useful 
 contrivance, a valve, for whatever purpose used, and in whatever way formed, 
 is in principle merely a moveable flap, placed on an opening, against which 
 it is held by its weight, or by some other gentle and yielding force. Such 
 a flap, it is evident, will allow fluid to pass only in one direction, viz., out- 
 wards from the opening, for any fluid tending inwards must shut the flap, 
 and press it the closer, the greater the tendency. 
 
 To convert a forcing syringe or pump into an exhausting syringe or pump, 
 commonly called an air-pump, it is only necessary to reverse the position of 
 the valves ; then, on the descent of the piston, all the air between it and the 
 bottom, instead of entering the vessel or receiver, as in the last case, escapes 
 
 by a valve in the piston itself towards 
 the atmosphere, and on the rising of 
 the piston, a perfect vacuum would be 
 left under it, but that the valve below, 
 then opened by the elasticity of the air 
 in the receiver, allows a part of that 
 air to follow it. Thus, at each stroke, 
 a quantity of the air, proportioned to 
 the size of the pump, is removed from 
 the receiver. In a good air-pump, 
 there are two similar pumping barrels, 
 as a and 6, to quicken the operation 
 of exhausting ; and both are worked 
 at the same time by the reciprocating 
 winch or handle f } with jts pinion e, 
 acting on the teeth of the piston rods d 
 
 Fig. 86. 
 
160 PNEUMATICS. 
 
 and c. This double construction has the farther advantage, that the atmo- 
 spheric pressure, if fifteen pounds per square inch on the upper surface of 
 either piston, and which for a single piston would have to be overcome by 
 the worker in lifting it, as here balanced always by the corresponding 
 pressure on the other piston. Both pumps communicate with the tube g h, 
 which at h rises tightly through the round plate of the machine to i. This 
 flat plate is so smooth, that a glass bell or receiver &, with a smooth 
 ground lip, when placed upon it, forms an air-tight joining. On working 
 the pump, such a bell is exhausted of its air, and fitted for showing the 
 many interesting phenomena which the air-pump can display, and which 
 will pass under review as we proceed. The supporting frame-work of the 
 pump is not shown here. 
 
 The law of the elasticity of air is, that its spring, or resistance to compres- 
 sion, increases exactly with its density or the quantity of it collected in a 
 given space. Hence, by finding in any case either the density of the air, 
 or the spring, or the compressing force, we know all the three. 
 
 It has been ascertained by experiments described a few pages farther on, 
 that in the atmospheric ocean surrounding the earth, there are nearly fifteen 
 pounds of air above every square inch of the surface of the earth ; and that 
 the air nearest the earth, and bearing this superincumbent weight or pressure, 
 has the density of an ounce and a quarter of weight to a cubic foot of volume. 
 We further find that such air is reduced to half its bulk, or becomes of what 
 is called double atmospheric density, by an additional pressure of fifteen 
 pounds on the inch, and of triple density, by tripple pressure, and so forth ; 
 and on the other hand, that it dilates to double bulk, if the pressure be 
 diminished to half, and to any greater bulk, even beyond a thousand-fold, if 
 the pressure be diminished in a corresponding degree ; and any air bearing 
 a given force or pressure, is always acting as a spring of that force on 
 whatever it touches. 
 
 It is very important to be familiar with this truth or law, for it holds very 
 nearly with respect to all aeriform fluids as well as common air, and throws 
 light, therefore, on the action of steam-engines, air-guns, pneumatic machines 
 generally. It also explains the condition of our atmosphere as to density at 
 various elevations ; telling us, for instance, that when a balloon has risen 
 through half of the atmospherical mass, the air around it will be of only 
 half the density which exists at the surface of the earth. 
 
 We know not exactly to what extent the rarefaction of air may go on the 
 removal of pressure ; in other words, at what distance the gravity of the 
 particles becomes just a balance to their mutual repulsion ; and therefore we 
 know not exactly -what the degree of rarity is at the top of our atmosphere ; 
 but we know that it must be exceedingly great, from the fact that the air 
 left in the receiver of an air-pump has still spring or elasticity enough to lift 
 the valve of the pump, when less remains than the thousandth part of the 
 original quantity. In the most perfect air-pumps, that the exhaustion may 
 be as complete as possible, the machine itself is made to raise the valve. 
 
 The expansion of air is well illustrated by a bladder, having a very little 
 air in it, placed under the receiver of an air-pump. On exhausting the 
 receiver, the bladder gradually swells, with force sufficient to lift a moderate 
 weight laid upon it, and at last appears quite full, and may even be burst. 
 A shriveled apple treated in the same way becomes,plump. The explana- 
 tion of such* phenomena is, that at first the air in the bladder or apple is in a 
 
ELASTICITY OF AIR. 
 
 161 
 
 Fig. 87. 
 
 state of condensation, like all air, at the surface of the earth under the pres- 
 sure of the superincumbent atmosphere; but that its volume increases as that 
 pressure is diminished by the air-pump : it is rarefied in the same propor- 
 tion as the air which remains in the receiver surrounding it. 
 
 The curious instrument called the air-guji has a strong globular vessel of 
 copper attached under the lock, into which air is usually forced to be thirty 
 or forty times as dense as the air in the atmosphere around : hence the 
 pressure or elasticity tending outwards is thirty or forty times fifteen pounds 
 on the inch, and when the valve is opened for an instant by the action of 
 the lock, a portion of the air issues and propels the charge with this force. 
 The effect of air thus condensed nearly equals that of gunpowder, and one 
 charge of the ball suffices for many shots, the force, however, becoming less 
 for every successive discharge. 
 
 If a bottle or vessel a b } partly filled with water, have a 
 tube c d passed tightly through the cork to near the bottom 
 of the water ; and if more air be then forced through this 
 tube in any way, so as to accumulate in the upper part of the 
 vessel above the water surface a b; on turning the cock c, 
 which opens the tube, the elasticity of the condensed air 
 will press the water out as a beautiful jet, to a height pro- 
 portioned to the condensation, and gradually diminishing as 
 the condensation diminishes. Or if such a vessel, with air 
 of common density, be placed under a tall air-pump receiver, 
 on working the pump so as to diminish the density of the 
 air in the receiver, the jet of water will equally rise. A 
 table-lamp,, by the force of condensed air, may be supplied 
 with oil from a reservoir far below the wick : and lately an 
 enema syringe and a shower-bath have been constructed on 
 the same principle. 
 
 The elasticity of air is rendered very serviceable in con-- 
 nection with great water-pumps, such as those used for the 
 supply of cities. A pump throws its water by a distinct 
 gush at each stroke, while the current through the pipe 
 towards the city should be uniform. Now uniformity is 
 attained by causing the gushes from the pump a to enter by 
 the passage b at one side of a large vessel c, of which the. 
 upper part is full of the condensed air, and from the other side of which at d the 
 water issues on its way. The air in this vessel 
 (called the air-vessel) is condensed, as a spring, by 
 the entering water, and its resisting elasticity, both 
 immediately, and afterwards during the interval of 
 the strokes, forces the water along the pipe d. Each 
 entering gush has only the effect of compressing 
 the air a little more for the time, while the flow in 
 the great pipe continues nearly uniform. The 
 pump itself is made to take in a little air at each 
 stroke, so that not only is the vessel always sup- 
 plied, but some air is constantly passing on with 
 the water, and effecting the highly useful purpose 
 of giving an elasticity to the whole contents of the 
 pipe and its ramifications. 
 
 The same object is attained by the same means in the fire-engine used to 
 check conflagration. In it there are generally several water-pumps working 
 
 11 
 
 Fig. 88. 
 
162 PNEUMATICS. 
 
 together, which throw their interrupted supply into an air-vessel whence it 
 passes in a nearly uniform jet to the point desired. 
 
 The compressibility and corresponding spring of air are remarkably exhi- 
 bited in that singular contrivance of modern times, the diving-bell, in which 
 men now descend with safety to, considerable depths in the ocean, there to 
 reside and labor, attaining many objects of high importance to them : they 
 recover sunken treasures, they are enabled to pursue works of submarine 
 architecture, as in constructing light-houses and noble harbours, where 
 formerly no foundations could have been laid, &c. The diving-bell, in 
 point of utility, has proved a remarkable contrast to its sister invention, the 
 balloon, which, although so wondrously bearing man aloft to the regions of 
 the clouds, has brought him as yet little advantage to compensate for the 
 many fatal accidents which its use has occasioned. 
 
 The diving-bell is a large heavy open-mouthed vessel, with accommoda- 
 tion in it for one or more persons. It is let down into the water with its 
 mouth undermost, from a crane to which it is suspended, and which rests .on 
 a suitable carriage either on the shore, or on the deck of a ship, or barge 
 fitted for its service. On first entering the water it appears full of air; but 
 air being compressible, according to the law now explained, and the pressure 
 of the water around the descending bell increasing with the depth, the volume 
 of the air gradually diminishes, and at thirty-four feet is reduced to half. 
 The bell, then, unless more air be supplied, will of course be half full of 
 water, and a person breathing in it, at each inspiration will receive twice as 
 much air into the lungs as when breathing at the surface. A constant supply 
 of fresh air is sent down to the bell by a forcing-pump above : and the heated 
 and contaminated air, which has served for respiration, and which rises to 
 the top of the bell, is allowed to escape by a cock placed there for the pur- 
 pose. The men who work at a distance from the bell have tubes of com- 
 munication with it, by which they inhale the air required ; and they allow 
 the used air to rise through the water above them. A man cannot breathe 
 comfortably by such a tube if he be either much above or much below the 
 level of the water in the bell ; for if above, the air in the bell is more com- 
 pressed than his chest, and is forced towards him so as to require an effort 
 to control its admission; and if below, his chest is bearing greater pres- 
 sure than the air in the bell, and he must therefore act strongly with the 
 muscles of the ribs to draw the air down to him. A phenomenon similar 
 to this takes place when two bladders of air are connected by a long tube, 
 and immersed in water to unequal depths, the air being always strongly 
 forced from the lower into the upper one, because the lower one is more 
 pressed. The difficulty of pumping air down to the diving-bell increases, 
 of course, with the depth to which it has ascended : for if the bell be so low 
 that the water is pressing on the air in it with a force of fifteen pounds per 
 inch, ( which would happen at thirty-four feet, ) it is evident that a syringe 
 or pump can not inject more air unless it act with a force greater than this. 
 Divers might often, if not always, more conveniently receive their supply 
 of air through tubes from an air vessel kept charged to the necessary density 
 in a boat over them, or on the shore, than from a bell below. If they 
 would, moreover, dress in India-rubber cloth, and use a hood of metal with 
 windows for the head, they might work under water without wetting any 
 part but their hands. 
 
 It is remarkable, when the use of the diving-bell has become so familiar, 
 that a kindred and still more simple contrivance of the same class has not 
 been introduced for certain purposes, particularly of sudden emergency, such 
 
ELASTICITY OP AIR. 163 
 
 as to aid in the recovery of the bodies of drowning persons. A ten-gallon 
 cask, or vessel of any kind, filled with air, and made heavy enough just to 
 sink in water, with a breathing tube from it like that of a diving-bell, would 
 be a provision of air for a man below water for ten minutes ; and a man with 
 it under his arm, might instantly descend from a boat or walk from the shore, 
 into water of any depth, to recover the body of a fellow-creature lately sunk, 
 and in time probably to save the life, which a few minutes wasted in waiting 
 or in unsuccessful dragging would suffer to be lost. The author would pro- 
 pose this as an addition to the apparatus of the Humane Society for the 
 recovery of persons apparently drowned. It shows the remoteness from 
 common trains of thinking of the truths connected with the constitution of 
 our atmosphere and sea, when a means so simple and easily procured should 
 never have been thought of or tried in any way by pearl-fishers, or by per- 
 sons who gain their bread by diving to recover things dropped overboard in 
 harbours or anchoring stations ; all of whom have hitherto been limited to 
 the single gulp of air taken on descending. In the case of a man working 
 under water, cask after cask of air might be sent down, to enable him to 
 remain as long as necessary. 
 
 There is an exceedingly beautiful philosophical toy, of which the action 
 depends chiefly on the elasticity of air ; and as it moreover 
 illustrates most of the laws of fluidity, it is deemed worthy of Fig. 89. 
 description here. It is a small balloon or thin globe of glass 
 c, having an opening at the bottom, and its little car or basket 
 hanging to it. If put to float in water while the globe con- 
 tains air only, it is so light that half the globe remains above 
 the surface ; but water may be introduced to adjust the specific 
 gravity of the whole, until it becomes only a little less than 
 that of water. If the balloon be then placed in a tall jar of 
 water a b, the moutn of which is closely covered by bladder- 
 skin or India-rubber tied upon it, on pressing such covering 
 with the hand, the balloon will immediately descend in the 
 water, to rise again when the pressure ceases, and will float 
 about, rising, or falling, or standing still, according to the 
 pressure made. The reason of this is, that pressure on the 
 top of the jar first condenses the air between the cover and the 
 water surface ; this pondensation then presses upon the water below, and by 
 influencing it through its whole extent, compresses also the air in the balloon 
 globe, forcing as much "to ore water into this as to render the balloon heavier 
 than water, and therefore heavy enough to sink. As soon as the pressure 
 ceases, the elasticity of the air in the balloon repels the lately entered water, 
 and the machine, becoming as before, lighter than water, ascends to the top. 
 If the balloon be adjusted to have a specific gravity too nearly that of water, 
 it will not rise of itself after once reaching the bottom, because the pressure 
 of the water then above it will perpetuate the condensation of the air which 
 caused it to descend. It may even then, however, be made to rise again by 
 inclining the water-jar to one side, so that the perpendicular height of water 
 over it shall be diminished. 
 
 This toy proves many things the materiality of air r by the pressure of 
 the hand on the top being communicated to the water below through the air 
 in the upper part of the j-ar the compressibility of air, by what happens in 
 the globe just before it descends the elastic force of air shown in expansion, 
 when, on the pressure ceasing, the water is again expelled from the globe 
 the lightness of air ; in the buoyancy of the globe : -it shows, also, that in a 
 
164 
 
 PNEUMATICS. 
 
 Fig. 90. 
 
 fluid the pressure is in all directions, because the effects happen in whatever 
 position the jar be held it shows that pressure is as the depth, because less 
 pressure of the hand is required the farther that the globe has descended in 
 the water and it exemplifies many circumstances of fluid support. A 
 young person, therefore, familiar with this toy, has learned the leading 
 truths of hydrostatics and pneumatics, and has had much amusement as 
 well as instruction. 
 
 On the same principle as the balloon now described, 
 three or four little figures of men may be formed of 
 glass, hollow within, and having each a minute open- 
 ing at the heel, by which water may pass in or out. 
 If these be placed in a jar as the balloon was, and be 
 adjusted by the quantity of water admitted into them, 
 so that in specific gravity, they shall differ a little from 
 each other, and if then, a gradually increased pressure 
 be made on the cover of the jar, the heaviest figure 
 will descend first, and the others will follow in suc- 
 cession , and they will stop or return to the surface 
 in reverse order when the pressure ceases. A person 
 while exhibiting these figures to spectators who do not 
 understand them, may appear only carelessly to rest 
 his hand on the cover of the jar while he is making the 
 required pressure, and he will seem to have the power 
 of ordering their movements by his will. If the jar 
 containing the figures be inverted, and the cover be 
 placed over a hole in the table, through which, unob- 
 served, the exhibitor can act by a rod rising through 
 the hole and obeying his foot, he may produce the most 
 amusing and surprising evolutions among the little 
 men, in perfect obedience to his word of command. 
 
 The beautiful fountain, called the fountain of Hiero, 
 by which water is made to spout far above its source, 
 depends for its action upon the resisting elasticity 
 of compressed air. The vessel d is first filled with 
 water, while b and a contain air only. On then 
 pouring water into a, the water of d darts upwards 
 through the jet-pipe e,to an elevation nearly equal to 
 the length of the tube from a to b. The reason is, 
 that the water from a descends by the tube to b, and 
 compresses the air at c ; which compression conveyed 
 along the other tube from c to 5, acts on the water in 
 the vessel r/, and causes it to jet. As the pressure 
 is produced by the column of water a b, the jet is 
 proportioned to the length of that column. This 
 kind of fountain may have its parts concealed under 
 a variety of forms as here exemplified, and may thus 
 become a beautiful ornament among flowers in a sum- 
 mer drawing-room. It may be made of size to play 
 for an hour or more, and it will always recommence 
 on the water being shifted from the low to the high 
 reservoir. The useful table-lamp, consisting of a 
 simple column or pillar with the oil rising to the 
 flame from far below, is a Hiero's fountain, only the 
 
 Fig. 91. 
 
PRESSURE AS THE DEPTH. 165 
 
 oil, instead of being allowed to jet out, rises in a tube to the flame. The 
 contrivance for maintaining the two columns always of the same length, not- 
 withstanding the expenditure of oil has to be explained some pages hence. 
 
 Having now explained the two peculiarities which distinguish aeriform 
 from other fluids, viz., their lightness and extensive elasticity, we proceed to 
 show that they have the four other properties already described under 
 hydrostatics, as belonging to fluids generally : and first, 
 
 " Pressure in all directions." ( Read the Analysis, at pages 140 and 172.) 
 
 A quantity of air or gas shut up in any vessel and compressed, is equally 
 affected throughout, and its tendency to escape from the pressure is equal ill 
 all directions, as is proved by the force necessary to keep similar valves close 
 wherever placed. Hence the hydrostatic press and hydrostatic bellows 
 described in the last section, which depend for their action on this law, may 
 be worked by air or gas as by a liquid. 
 
 Owing to this law, air, when allowed, will always rush from where there 
 is more pressure to where there is less. The actions of the common fire- 
 bellows, and of the animal chest in breathing, blowing, sucking, &c., are so 
 many instances. 
 
 The suddenness with which any compression made on part of a confined 
 aeriform fluid is communicated through the whole, is strikingly seen in the 
 simultaneous increase or burst of all the gas-lights over an extensive build- 
 ing or even in a long street, at any instant when the force supplying the gaa 
 is augmented. 
 
 Many very interesting illustrations of the fluid pressure of air being in, 
 all directions, will occur under the next head, joined with proofs of the 
 atmospheric pressure being as the depth. 
 
 "Pressure as the depth" 
 
 On first approaching this subject, a person is naturally surprised to hear 
 the depth or height of the atmosphere spoken of as something perfectly 
 ascertained, although nobody can ever have approached the surface to mea- 
 sure it ; but science often furnishes means of reaching precise truth, in. 
 cases where ignorance would not' even dream of the possibility of making 
 an approximation. It may facilitate the apprehension of this point as regards 
 air, to describe first some parallel cases in which water is concerned. 
 
 The bottom of a lake evidently supports all the water in the lake, and 
 each portion bears just the weight of the water directly over it : a means 
 then of ascertaining the weight or pressure of water on any portion of the 
 bottom would tell how much water stood over that portion, and by the 
 known rej^,tion of the weight and bulk of water would tell also the depth 
 at that part. In like manner the ocean of air which surrounds the globe 
 rests with its whole weight upon the surface of the globe, and each portion 
 of the surface bears its share : if we ascertain, then, the pressure of the 
 atmosphere on a given extent of the surface, we find how much air is stand- 
 ing directly over it; in other words, the weight of a column of air resting 
 on such surface as its base, and reaching to the top of the atmosphere. 
 Having then the weight of the whole column, and finding the weight of a 
 given bulk of it at the botton (ascertained as described at page 158,) and 
 knowing the law of aerial elasticity (explained at page 158, ) we determine 
 the depth or height of the column by a simple calculation. Now accurate 
 
166 PNEtJM ATICS. 
 
 experiments show that there are nearly fifteen pounds of air over every 
 square inch of the earth's surface ; producing the same pressure as would 
 be made by a depth of water of thirty-four feet, or by a depth of quicksilver 
 of thirty inches ; and from this fact and the ascertained lightness and elasti- 
 city of air, we know that its depth on earth must be nearly fifty miles, 
 which, as already stated, is about as much in relation to the size of the garth 
 as the tenth of an inch is to a globe of one foot in diameter. The remaining 
 part of this section has chiefly to trace the effects of this mass of matter 
 resting upon the earth's surface, and as a fluid embracing and compressing 
 every object placed there. 
 
 Water is a substance much more obvious to the human senses than air, 
 and which is constantly under observation ; yet many of its most important 
 agencies escape the notice of common observers. Few persons, for instance, 
 of themselves discover the law explained in the last section, of the pressure 
 in water being proportioned to the depth : but when made to observe that a 
 piece of cork plunged deep into it is compressed to much smaller bulk, and 
 that strong empty vessel of glass, or even of metal under the same circum- 
 stances, are crushed or broken inwards, and that pieces of sunken wood are, 
 at great depths, filled with water through all their pores, so as to become 
 nearly as heavy as stone, &c., their minds are roused to a sense of the import- 
 ant fact that a fluid presses, and in proportion to its depth. If the truths 
 of hydrostatics thus escape notice, we need not wonder that those of pneu- 
 matics escape still longer. 
 
 If a piece of bladder-skin or a pane of glass be laid at the bottom of a 
 vessel, holding water, the bladder or glass exhibits no sign of being pressed 
 upon, although it bears on its upper side the whole weight of the water 
 directly above it: the reason being that water beneath the bladder resists 
 just as strongly as the water above presses, in the same way that one stone 
 in a pillar resists those above it : but if the bladder be tied closely over the 
 mouth of a common drinking glass or tumbler filled with water, and placed 
 at the bottom of the vessel, and if then, by means of a syringe or pump, 
 the water be extracted from within the glass, the bladder itself has to bear 
 the whole pressure of the water above it, ( independently of a pressure of 
 air, to be explained afterwards, ) and will probably be torn or burst. The 
 degree of pressure, and consequently the depth of the water, in such a case, 
 might be ascertained by placing some support, of which the action could 
 be measured, under the bladder to sustain it after the removal of the 
 interior water. Now this case may be closely copied in our atmosphere or 
 sea of air. A glass held in the hand is immersed in the fluid air, and is full 
 of it as the other glass was supposed full of water : its mouth may be covered 
 over with bladder, and no external pressure will be apparent, because there 
 is a resistance of the air within, just equal to the pressure of the air on the 
 outside : but if the air be extracted from under the covering bj^rneans of 
 an air pump, the bladder is first seen sinking down and becoming hollow 
 from the weight of the air over it, and at last bursting inwards with a great 
 noise or crack. By placing a circular piece of wood under the bladder-skin, 
 for it to rest on, and a spring of known force to support the wood, we may 
 ascertain very nearly the weight and pressure of the air over it. This 
 mode, however, of ascertaining the weight of the atmosphere, is not that 
 commonly used, but is described here as a good illustration of the present 
 subject; the problem being solved much more elegantly and accurately by 
 means of the barometer described farther on. The phenomenon of atmos- 
 pheric pressure is often exhibited by placing the hand on the mouth of a 
 
ATMOSPHERIC PRESSURE ON SOLIDS. 167 
 
 glass so as to. cover it closely, and then extracting the air from underneath 
 the hand : the weight of the atmosphere holds the hand down on the mouth 
 of the glass with the force of fifteen pounds to the inch. 
 
 As should follow, from the pressure of fifteen pounds per inch thus detected 
 at the surface of the earth, being the weight of our superincubent atmo- 
 sphere, we find that exactly as we rise from the earth, and leave part of the 
 atmosphere beneath us, the pressure diminishes. This fact now furnishes 
 the readiest means of ascertaining the heights of mountains and of balloon 
 ascents, as will be explained in considering the barometer. 
 
 After the many explanations here given of fluid pressure being equal in 
 all directions, it is almost superfluous to remark, that the downward weight 
 of the atmosphere becomes a pressure in all directions. This is seen in the 
 fact of the bladder seen above, being as readily burst if turned sideways 
 as if turned directly upwards. Every body or substance, therefore, on the 
 surface of the earth, dead or living, solid or fluid, is compressed with this 
 force. In general the pressure on one side of a body, is just balanced by the 
 equal pressure on the other, so that no sensible effect follows; and it is on 
 this account that philosophers were so long in discovering it at all, and that 
 half-informed persons are still disposed to doubt its existence ; but the proofs 
 offered on all sides to the now awakened attention are irresistible. We shall 
 speak first of 
 
 " Atmospheric pressure on solids." 
 
 The atmosphere, then, presses on the two sides of a plate of glass or metal, 
 with force of fifteen pounds on the inch. Under ordinary circumstances, no 
 sensible effect follows, because the opposite pressures counterbalance ; but if 
 two plates of smooth glass or metal be laid against each other, and the air 
 be prevented from entering between them, they cannot be separated by less 
 force than fifteen pounds per inch of their surface. 
 
 In like manner, to draw down the piston of a syringe from the bottom of 
 its barrel, while no air is allowed to enter between them, requires force of 
 fifteen pounds to the square inch of surface of the piston. But if the 
 experiment be made in the exhausted receiver of an air-pump, the piston 
 falls by its own weight. It is pushed back immediately on re-admitting 
 the air. Wherever a vacuum is produced at the surface of the earth, there 
 is an external pressure, of the force stated, seeking admittance all round. 
 
 An air-pump receiver of five inches diameter has nearly twenty square 
 inches of surface in its upper part or roof, and bears a weight or pressure of 
 atmosphere, of twenty times fifteen, or three hundred pounds. W T hile it has 
 air within it, this pressure is exactly balanced, and is not sensible ; but when 
 exhausted on the plate of the air-pump, it is pressed against the plate with 
 this force. As the atmospheric pressure is in all directions, the pump-plate, 
 of course, is equally pressed upwards against the receiver, and the sides of 
 the receiver are pressed towards each other. This explains why air-pump 
 receivers must be made arched or of dome-shape to withstand the great 
 pressure. A flat piece of glass of great thickness, laid upon the upper 
 mouth of a receiver, so as to form an air-tight cover to it, is broken 
 instantly by exhausting the air beneath ; and a bottle or receiver with flat 
 side, when exhausted suffers in the same manner. 
 
 Illustrative of this pressure on solids there is the experiment of the Magde- 
 burgh hemispheres, as it is called. Two hollow half globes of metal a and 
 
168 
 
 PNEUMATICS. 
 
 the world. 
 
 b, are fitted to each other, so that their lips wjien touching 
 may be air-tight. While there is air between them or within, 
 resisting the pressure of the outward air, they can be separated 
 from each other without difficulty ; but when the air is 
 exhausted from within by the air-pump, a force is required to 
 separate them of as many times fifteen pounds as there are 
 square inches in the area of the mouth. The air is extracted 
 by unscrewing one of the handles at b, and then connecting 
 the remaining stalk (which is hollow and has a stop-cock) 
 with the air-pump. This experiment merits recollection, 
 because it was one of the first which drew attention to the 
 material nature and properties of the air; and it astonished 
 Otto Guericke, Burgomaster of Magdeburgh, the inventor, had 
 hemispheres made of three feet in diameter, and once when he exhausted 
 them, on the occasion of a public exhibition, twenty coach-horses of the 
 emperor were unable to pull them asunder. There being no air-pump when 
 Guericke began his experiments, although he himself invented it afterwards, 
 he originally emptied the balls of their air by first filling them with water, 
 and then extracting the water by a common pump or syringe applied to the 
 bottom. 
 
 It is a phenomenon of the same kind as the last described, when a boy 
 with his foot presses a circular piece of wet leather as a, against a flat-faced 
 stone as b, and then lifts the stone by pulling at a cord c, rising from the 
 centre of the leather. , If the leather be so close in its texture that air cannot 
 pass through it, and stiff enough not to be puckered or drawn together, he 
 must exert a force before detaching it, of as many times fifteen pounds as 
 there are square inches of surface covered by it for 
 such is the weight or pressure of the air over it, while 
 there is no counterbalancing pressure underneath nearer 
 than on the other side of the stone. The weight of the 
 stone that may be lifted is thus determined by the 
 size of the leather. The contrivance has been called a 
 sucker, or pneumatic tractor. A very large sucker 
 applie 1 upon a rock or wall, would resist the pull of 
 horses like the Magdeburgh hemispheres. 
 
 This contrivance seems suited to some purposes of 
 surgery. It might assist, for instance, in raising de- 
 pressed portions of a fractured skull, and might thus 
 sometimes save the operation of trepanning : for such 
 a purpose it would be preferable to the small cupping- 
 glass sometimes used, from its being perfectly inactive, 
 except during the instants when pulled at; whereas the 
 cupping-glass, by keeping up a continual flow of blood 
 to the part, might do injury. There is another surgical application spoken 
 of in the last section of the present part, which the professional reader may 
 consult immediately. 
 
 It is from having feet that act on the principle of the tractor, that the 
 common fly and other insects can move along ceilings, and even polished 
 surfaces of glass or metal with their bodies hanging downwards ; and there 
 are many marine animals which attach themselves to rocks, or other objects 
 by a similar action. 
 
 If two pneumatic tractors be applied to each other, men pulling opposite 
 ways, to separate them, must act with a force of fifteen pounds to the square 
 
 Fig. 93. 
 
ATMOSPHERIC PRESSURE ON LIQUIDS. 169 
 
 inch of the surface of contact, as if they were separating the Madgeburgh 
 hemisphere. 
 
 The case of the pneumatic tractor may be well illustrated by an experi- 
 ment made in a vessel containing a liquid. If a body with a flat surface be 
 applied to the bottom of the vessel so as perfectly to exclude the liquid, the 
 body bears the whole weight of liquid directly over it, and cannot be detached 
 without force equal to this.. The case is striking when a flat piece of cork is 
 pushed against the smooth bottom or side of a vessel containing mercury, 
 and is found not to rise again when the hand is withdrawn from it, but to 
 be firmly held down by the weight of the mercury. We have to remark 
 that in such experiments made in vessels open to the air, the weight of the 
 atmosphere on the liquids adds a pressure of fifteen pounds on every inch of 
 the surface of a body immersed in it. 
 
 " Atmospheric pressure on liquids" 
 
 The pressure of the atmosphere on liquids produces many important 
 effects, and now that we comprehend them, we wonder that they should have 
 been so long misunderstood. We have familiar examples of it in the work- 
 ing of pumps and syphons. All such phenomena, in former times, were 
 referred to what was called nature's horror of a vacuum, or to an obscurely 
 imagined principle of suction. It was not until the time of Galileo that 
 their true nature began to be detected. The discovery has led to many very 
 important results in the arts. 
 
 Persons may at first have a difficulty in conceiving that a fluid so rare and 
 subtle as air should affect or resist u dense liquid like water : but the action 
 or resistance of air in contact with water, is familiarly shown in the facts 
 that a glass does not become full of water when plunged, with its open mouth 
 downwards, from the air into water; and that when a tube, open at bol^i 
 ends, has been partially immersed in water, and, therefore, partially filled, 
 the water can be forced out of it by blowing air in at the upper end, to 
 return only when the blowing ceases. Then it may be recollected that a 
 hundred pounds of feathers are as great a load as a hundred pounds of lead. 
 
 That there are fifteen pounds of air above every square inch of the earth's 
 surface, is confirmed by the effects above described of the atmospheric pres- 
 sure on solids; and we now proceed to show that many of the phenomena 
 among liquids, which long appeared so mysterious, are merely the necessary 
 consequences of the same pressure upon them. It will facilitate the com- 
 prehension of these effects, if we first view them as they may be produced 
 by more visible agents, viz., by one liquid pressing upon another ; and for 
 this purpose the author has contrived the apparatus represented in the next 
 page, in which a layer of oil rests upon a layer of water, or upon a layer of- 
 mercury. 
 
 It has already been shown, that an ocean of oil, spread over the earth, to 
 have the same weight as our atmosphere, requires to be about thirty-seven 
 feet .deep A vessel, then, a b c, with water in it up to the level W, and 
 with thirty-seven feet of oil above this, up to the level 0, is fitted to illus- 
 trate many of the phenomena of atmospheric pressure on liquids. The 
 following are the seven principal cases. 
 
 1st. The weight of the oil pressing with a force of fifteen Ibs. per inch on 
 the water at W, would not at all disturb the level surface of the water. 
 Neither does the weight of the atmosphere o*f fifteen Ibs. per inch disturb 
 any liquid surface. 
 
170 
 
 PNE UN ATICS . 
 
 Fig. 94. 
 
 2. If the oil were gradually poured into 
 the vessel a b c, over the water, the water 
 would rise in the tube i w, as already ex- 
 plained by the figure at page 143 ; so that 
 when there were thirty-seven feet in height, 
 or fifteen pounds in weight of oil on the 
 inch, the water i w would stand thirty-four 
 feet above its level in the large vessel. If 
 these thirty-four feet of water were then 
 lifted out of the tube by a plug or piston 
 drawn up from the bottom of it at i, a second 
 equal quantity would be pressed up by the 
 oil, to be removed, if desired, in the same 
 way as the first, and the tube and piston 
 would constitute a pump. Now when the 
 atmosphere, instead of the oil, is allowed 
 to press upon a water surface in such a 
 vessel, but is excluded from the tube, the 
 water rises in the tube thirty-four feet, as 
 in the last case } and if this quantity be lift- 
 ed out of the tube by a piston, a second equal 
 quantity is pressed up, and the tube and 
 piston become a complete example of the 
 common lifting or sucking pump. We have 
 to describe it more particularly hereafter. 
 
 3d. If there were a quantity of mercury or of quicksilver at the bottom 
 of the vessel ale, filling it up to the level M, and if a tube i m issued from 
 under this level, the mercury pressed upon by thirty-seven feet of oil would 
 r\se in this short tube as the water did in the larger j but by reason of its 
 greater specific gravity, it would only reach a height of thirty inches above 
 its level, the water having stood at thirty-four feet. Now thirty inches of 
 mercury is the height of column which the atmospheric pressure, acting 
 in the same way really produces, as is seen in a similar apparatus made 
 expressly for measuring that pressure, and called a barometer or measure ' 
 of weight. 
 
 4th. If a tube d, of an inch square and open at both ends, were plunged 
 into the. oil, it would of course always be full up to the level of the oil on 
 the outside of it ; and if it were pushed low enough to touch the water at 
 W, it would just contain fifteen pounds of oil resting on an inch square of 
 the water-surface at its mouth j which surface would therefore be bearing a 
 weight of fifteen pounds like every inch of the surface around, but would 
 not yield, owing to the force with which it tended upwards to escape from 
 the pressure corresponding to its depth in the oil. Then if the tube were 
 pushed a little farther down, and if, by a piston or plug in it, the fifteen 
 pounds of oil were lifted out of it, water would rise into it until enough had 
 entered to reproduce the pressure of fifteen pounds on the surface below 
 as before ; that is to say, the water would rise thirty-four feet, as in the 
 external tube w i. This internal tube and piston again would form &pump. 
 In like manner, when a tube open at both ends is plunged from the air into 
 water, the air presses on the surface of the water within the tube, as on the 
 surface around it, with a force of fifteen pounds to the inch, and the two 
 surfaces are not affected by the* equal pressures ; but if, by a piston, we lift 
 the air out of the tube, as we suppose the oil lifted in the last experiment, 
 
ATMOSPHERIC PRESSURE Off LIQUIDS. 
 
 171 
 
 the water will then rise, following the piston to the altitude of thirty-four 
 feet. This arrangement of parts is the most usual for the lifting or house- 
 holt pump. 
 
 5th. If a common bottle or vessel of any shape, as the bent tube e, were 
 filled with water, and placed under the oil with its mouth or mouths reach- 
 ing below the water surface at the level W, it would remain full of water, 
 owing to the pressure of the oil surrounding it. For a similar reason, any 
 such vessel or tube, surrounded only by air, when filled with water, and 
 placed with its mouth or mouths under the surface of water, remains full ; 
 and if such a bent tube has one of its ends in another vessel lower than the 
 first, a current is established in it; the contrivance being then called a 
 syphon. 
 
 6th. A fish in the water below the level W, would be bearing the pres- 
 sure of the oil from to W, as well as the pressure of the water. So a 
 fish in water open to the air, is bearing the atmospheric pressure of fifteen 
 pounds per inch, in addition to that of the water itself. This is proved by 
 extracting the air from over water in which a fish is swimming : for then 
 the air-bag of the fish, situated near its under side, as already described, 
 immediately dilates and turns the fish upon its back. x 
 
 7th. To separate the Magdeburgh hemispheres, or to produce a vacuum 
 in any way, under the water level W, would require force proportionate to 
 the weight of oil above, in addition to that required on account of the 
 water ; and to separate the Magdeburgh hemispheres under any water- 
 surface pressed upon by the atmosphere, a force is required of fifteen pounds 
 per inch beyond what would balance the effect of the water itself. 
 
 Fig. 
 
 The following remarks illustrate more minutely some of the objects which 
 we have just been explaining. 
 
 The common lifting-pump (or sucking-pump as it used to be called,) is 
 then merely a barrel a b, with a close-fitting moveable plug or piston in it c. 
 When the lower end b } is plunged into water, and the piston is drawn up 
 from the bottom, the atmosphere being prevented from pres- 
 sing on the surface of the water within the tube, the pressure 
 on the surface external to the tube, drives the water up 
 after the piston. That the water which thus rises ma^y not 
 fall again, there is a valve or flap at the lower part of the 
 pump-barrel b, which opens only to water passing upwards ; 
 and* that the piston may be allowed to pass downwards 
 through the water in the barrel, to repeat its stroke, there 
 is in it a similar valve. The piston, in rising during a 
 second or succeeding stroke, causes all the water above it to 
 run over at the spout d. Formerly a lifting-pump was 
 said to act by sucking the water up from the well beneath 
 it ; the true meaning of which phrase we now perceive to 
 be, that the piston merely lifts or holds off the air which 
 was pressing on the water within the barrel, and allows the 
 water to rise in their obedience to the external pressure of the 
 air around. The reason is apparent, then, why, in the 
 lifting pump, the water will only follow the piston to a 
 certain elevation, viz., until its weight balances the external 
 pressure of the atmosphere. 
 
172 
 
 PNEUMATICS. 
 
 Fig. 96. 
 
 When the piston of a pump is solid, or without a 
 valve, as at c, the machine is called a forcing-pump. 
 The water rises beneath the piston, as already ex- 
 plained for the lifting-pump, but then as it cantiot 
 pass through the descending piston, as in the lifting- 
 pump, it is forced into any other desired direction, 
 as to d. A forcing-pump can bring water from only 
 thirty-four feet below the piston, but can send it to 
 any elevation. In forcing-pumps, it is usual to make 
 the water enter an air-vessel d a (already explained 
 at page 161,) from which it is again urged by the 
 elastic air, through the pipe &, in a, nearly uniform 
 stream. 
 
 The animal action of sucking is an approximation 
 to what we have described in the lifting-pump. The 
 difference is that the chest or mouth can make only 
 a partial vacuum, and therefore cannot raise a liquid 
 very far. 
 
 A syphon remains full of liquid, although partially 
 
 raised above the general surface of the liquid, as explained above. For com- 
 mon purposes, a syphon is made of the form here represented, viz., a bent 
 tube c b a, with one end longer than the other. To use it, the end c is first 
 immersed in liquid, and the end a being then stopped for the time by the 
 finger or a cock, the air is extracted by the mouth or otherwise, through the 
 small tube a d, and the atmosphere immediately fills the whole tube with 
 liquid from c. If the instrument be then left to act, the liquid will run from 
 the longer leg, because a long column of liquid overbalances a short one, 
 until the shorter has drunk up all within its reach. Whether the external 
 
 extremity be in the air only, or immersed 
 in liquid, makes no difference, except that 
 the immersion shortens so much the de- 
 scending column. If both extremities be 
 immersed in liquid, and in different vessels, 
 by alternately lifting one vessel or the 
 other, the liquid will be made to pass and 
 repass, and will come to rest in the syphon 
 only when the surfaces in the two vessels 
 are at the same level. Thus the same leg 
 becomes alternately the long and the short 
 leg, according to the height of the liquid 
 in which it is immersed. A syphon is 
 sometimes made with both legs equal and 
 turned up, as here represented, so that it 
 remains full of liquid although lifted away 
 from the vessel, and therefore is always 
 ready for action. As it is the same cause 
 which lifts the water in a pump and in a syphon, the top of a syphon must 
 evidently be within thirty-two feet of the water-surface below. In the syphon 
 as the cases of balancing liquids, described at page 131 (which see,) the 
 comparative diameters of the legs are of no importance, nor their oblique 
 length, provided the perpendicular heights of the two columns have the neces- 
 sary relation : even an inverted tea-pot may be used as a syphon. This 
 truth is well exemplified in what may be called the syphon-paradox an exact 
 
PRESSURE OF LIQUIDS. SYPHON. 
 
 173 
 
 Fig. 98. 
 
 counterpart of the paradox of the " Hydrostatic Bellows," 
 already explained. If the apparatus of the bellows be filled 
 with water in the ordinary way (see page 130,) and be then 
 reversed or turned so that the tube becomes like the long leg of 
 a syphon, the little stream of water issuing from it at a will lift 
 as great a weight suspended from the board d, as the same 
 slender column in the standing position can lift upon the board. 
 As farther illustrative of the atmospherical pressure exerted in 
 producing this effect, and in rendering a syphon active, we m^r 
 advert to the striking fact, that a long small tube of water 
 screwed into the side or bottom of a close cask of water so 
 as to communicate with it, and then allowed to discharge like 
 the long leg of a syphon, will cause the cask to be crushed 
 inwards, just as the same tube screwed into the top of the cask, as repre- 
 sented at page 131, causes the cask to burst outwards. 
 
 The syphon is very useful for drawing off liquids, where there is a sedi- 
 ment that should not be disturbed, or where it is desirable not to make an 
 opening in the lower part of the vessel. A large syphon would empty a 
 lake or mill-pond over its bank without injuring the bank. To fill a large 
 syphon that it- may act the most convenient way is, instead of pumping out 
 the air from it, to close the two ends for the time, and to pour in water 
 through a cock at the top. 
 
 There is a pretty syphon-toy,"called a Tantalus-cup, having in it a stand- 
 ing human figure which conceals a syphon. The short branch of the syphhon 
 rises in one leg of the figure to reach the level of the chin, and the long 
 branch descends in the other leg to pierce the bottom of the cup towards a 
 reservoir below. On pouring water into the cup, the syphon begins to act 
 as soon as the water reaches 'the chin of the figure, and the cup is then 
 emptied as if by magic. 
 
 Among the infinitely varied water-drains or courses in the bowels of the 
 earth, some are syphons, and produce what are called intermitting wells or 
 fountains. These may alternately run and cease for longer or shorter 
 periods, according to the comparative magnitudes of the collecting reservoir 
 and the' drain. The reservoir may be an internal cave of a mountain, 
 receiving a regular supply of water by a slow filtering of moisture from 
 above, and the drain is a syphon-formed channel, which^ like that of the 
 Tantalus-cup, begins to act only when the water in the reservoir has reached 
 the level of the top of the syphon, and then carries off the water faster 
 than it is supplied. There are some fountains that flow constantly, but at 
 regular intervals have a remarkable increase. In them a common spring is 
 joined with a syphon-spring. 
 
 The author has suggested an application of the syphon, which obviates a 
 strong objection to the high operation for stone, as explained in the next 
 medical section. 
 
 The following facts have close relation to those now explained, as farther 
 illustrative of atmospheric pressure on liquids. 
 
 A long glass of jelly, if inverted and placed with its mouth just under 
 the surface of warm water, will soon be found to have lost the jelly, but to 
 be full of water in its stead. The jelly is heavier than water, and when 
 melted by the heat sinks down, and is replaced by water from below, sent 
 up by the atmospheric pressure. 
 
174 PNEUMATICS. 
 
 The slaves in the West Indies steal rum, by inserting the long neck of a 
 bottle full of water through the top aperture of the rum-cask. The water 
 falls out of the bottle into the cask, while the lighter rum ascends in its stead. 
 The common water-glass for bird-cages has its only opening near the bot- 
 tom through the neck b ; yet no water. can escape 
 from it but when the level of the water at c, in the 
 open part, becomes low enough for some air to pass 
 into the body of the glass by the channel b. When 
 a bubble of air does pass in, an equal bulk of water 
 comes out, and by raising the water level in c, pre- 
 vents the passage of more. An ink-glass made on 
 this principle preserves the ink well, because there 
 is so small a surface exposed to the air; if made too 
 , . large, however, the accidental expansion of the air 
 O ]C i n it by heat may cause it to overflow. 
 
 In the common Argand or fountain-lamp, the 
 
 provision of oil is in a vessel like an inverted bottle, higher than the flame, 
 and with its mouth immersed in a small reservoir of oil, nearly on a level 
 with the flame, then no oil can escape from above, but as the flame consumes 
 the free oil from the' small reservoir, which supply is thus maintained 
 always at the same elevation. In the Hiero's fountain lamp, mentioned at 
 page 164, that the two balancing columns of oil may be always of the same 
 height, the oil is suppplied to them from high reservoirs, with the mouths 
 dipping in them as above described, and keeping their tops, therefore, 
 always at the same level; and that the descending column may not be 
 shortened by the rising of the oil in the low reservoir c. the tube containing 
 it is turned up at the bottom like an end of the " ever ready syphon/ 7 and 
 discharges near the top of c. 
 
 We have hitherto been contemplating only the direct weight or downward 
 pressure of the atmosphere on liquids : in the following instances we have 
 proof of the same pressure acting upon them in all directions. 
 
 If a bottle or cask be filled with liquid, and closely corked, and if a small 
 hole be then drilled in the bottom or side, the liquid will not escape by it, 
 because of the existing pressure of the atmosphere, and of there not being 
 room in the opening for a current of air to enter while the current of water 
 escapes : but if a second hole be drilled in the top, a jet from the lower 
 opening will follow immediately, because then the air will press on the upper 
 surface of the liquid as well as on the lower, and the weight of the liquid 
 will be free to act : thus, a cask, of beer or wine cannot be emptied by a 
 cock near the bottom, unless what is called a vent-hole be made at the top. 
 If the lower opening, however, in any case be so large, that the air may 
 enter by one side of it, as is seen in decanting a bottle of wine. In such a 
 case, it is the interrupted entrance of the air which causes that gurgling 
 sound so delightful to the ear of the drunkard, instead of allowing the 
 smooth stream which fall from a funnel. 
 
 Even a large opening at the bottom of a vessel which is close above, may 
 be prevented by the pressure of the air, from discharging liquid if any 
 mutual passing of the two currents of air and liquid be rendered difficult. 
 An inverted bottle of water will not discharge, if a piece of paper simply be 
 applied against its mouth. Even a wineglass filled with water may be 
 
ATMOSPHERIC PRESSURE. BAROMETER. 175 
 
 inverted, and yet will spill none, if the piece of paper, laid loosely upon its 
 mouth, be held to it during the turning, the pressure of the atmosphere 
 against the paper keeping it in its place, and supporting the water above it. 
 Any vessel or tube of water of less height than thirty-four feet may be kept 
 closed at the bottom in the same way. 
 
 The animal body is made up of solids and fluids, and is affected by the 
 atmospheric pressure accordingly. 
 
 There is a difficulty at first in believing that a man's body should be 
 bearing a pressure of fifteen pounds on every square inch of its surface, while 
 he remains altogether insensible of it : but such is the fact, and the reason of 
 his not feeling the fluid pressure is its being perfectly uniform all around. 
 If a pressure of the same kind be even many times greater, such, for instance, 
 as fishes bear in deep water, or as a man supports in the diving-bell, it equally 
 passes unnoticed. Fishes are at their ease in a depth of water where the 
 pressure around will instantly break or burst inwards almost the strongest 
 empty vessel that can be sent down; and men walk on earth without dis- 
 covering a heavy atmosphere about them, which, however, instantly crushes 
 together the sides of a square glass bottle emptied by the air-pump, or even 
 of a thick iron boiler, left for a moment by any accident, without the coun- 
 teracting internal support of steam or air. 
 
 The fluid pressure on animal bodies, thus unperceived under ordinary cir- 
 cumstances, may be rendered instantly sensible by a little artificial arrange- 
 ment. In water, an open tube partialy immersed becomes full to the level 
 of the water around it, and the water contained in it is supported, as already 
 explained, by that which is immediately below its mouth : now a flat fish 
 resting closely against the mouth of the tube, would evidently be bearing on 
 its back the whole of this weight, perhaps one hundred pounds; but the fish 
 would not thereby be pushed away, nor would it even feel its burden, because 
 the upward pressure of the water immediately under it would just counter- 
 balance the weight, while the lateral pressure around would prevent any 
 crushing effect of the upward and downward forces. But if, while the fish 
 continued in the situation supposed, the hundred pounds of water were sud- 
 denly lifted from off its back by a piston in the tube, the opposite upward 
 pressure of one hundred pounds would at once crush its body into the tube. 
 At a less depth, or with a smaller tube, the effect might not be fatal, but 
 there would be a bulging or swelling of the substance of the fish into the 
 mouth of the tube. In air and on the human body a perfectly analogous 
 case is exhibited. A man without pain or any peculiar sensation, applies 
 his hand closely to the mouth or opening of a tube, or of any vessel contain- 
 ing air, but the instant that the air is withdrawn from within the tube or 
 vessel, the then unresisted pressure of the external air fixes the hand upon 
 the opening, causes the flesh to swell or bulge into it, and makes the blood 
 ooze from any crack or puncture in the skin. 
 
 These last lines describe closely the surgical operation of cupping ; the 
 essential circumstances of which are, the application of a cup or glass, with a 
 smooth blunt lip, to the skin of any part of the body, and the extraction by a 
 syringe, or other means, of a portion of the air from within the cup. To 
 some minds the exact comprehension of this phenomenon may be facilitated, 
 by considering the case of a small bladder or bag of India-rubber full of any 
 fluid and pressed between the hands on every part of its surface except one : 
 at that one part it would swell, and even burst if the pressure were strong 
 enough. So in cupping, the whole body, except the surface under the cup, 
 
176 PNEUMATICS. 
 
 is squeezed by the atmosphere, with a force of fifteen pounds to the square 
 inch, while in that one situation the pressure is diminished according to the 
 degree of exhaustion in the cup, and the blood consequently accumulates 
 there. The application of a cup with exhaustion only, constitutes the opera- 
 tion called dry-cupping. To obtain blood, the cup is removed and the tumid 
 part is cut into by the simultaneous stroke of a number of united lancets : 
 and the cup is then applied again as before and exhausted, so that the blood 
 may rush forth under the diminished pressure. 
 
 The partial vacuum in the cup may be produced either by the action of a 
 syringe, or by burning a little spirit in the cup and applying it while the 
 momentary dilation effected by the heat has driven out the greater part of 
 the air. The human mouth applied upon any part becomes a small cupping 
 apparatus, and formerly, in cases of poisoned wounds, was used as such. 
 Our present perfect cupping glasses, of stronger and more permanent opera- 
 tion, are not yet always used, as they might be, to assist in removing the 
 poison after the bites of rabid or venomous animals. 
 
 The author has suggested an extension and modification of the operation 
 of the dry cupping, which he believes will prove an important remedy in the 
 hands of the medical practitioner. It is intended as a substitute for bleeding 
 in certain cases where blood can ill be spared, and as a more sudden and 
 effectual check than even bleeding itself, in certain cases of inflammatory 
 disease. It is explained in the next medical section of this work. 
 
 The atmospheric pressure on living bodies produces an effect which is 
 rarely thought of, although of much importance, viz., keeping all the parts 
 about the joints, firmly together, by an action similar to that exerted on the 
 Magdeburgh hemispheres. The broad surfaces of bone forming the knee- 
 joint, for instance, even if not held together by ligaments, could not, while 
 the capsule surrounding the joint remained air-tight, be separated by a force 
 of less than about a hundred pounds ; but on air being admitted to the 
 articular cavity, the bones at once fall to a certain distance apart. In the 
 loose joint of the shoulder, this support is of great consequence. When the 
 shoulder or other joint is dislocated, there is no empty space left, as might 
 be supposed, but the soft parts around are pressed in to fill up the natural 
 place of the bone. When a thigh bone is dislocated, the deep socket called 
 thee actabulum instantly becomes like a cupping- glass,* and is filled partly with 
 fluid and partly with the soft solids. In all joints, it is the atmospheric 
 pressure which keeps the bones in such steady contact, that they work 
 smoothly and without noise. 
 
 The barometer we have seen at page 170, is a column of fluid supported 
 in a tube by the pressure of the atmosphere, and therefore indicating most 
 exactly the degree of that pressure. It is an instrument now of such import- 
 ance, both in a scientific point of view and in the business of common life, 
 that for the sake of minds which conceive such subjects with difficulty we 
 shall add here the two following farther illustrations of its nature. 
 
 If mercury be poured into a bent tube open at both ends, it will stand at 
 the same level in the two legs, as at a and 6, and the air will be pressing on 
 the two surfaces at a and b with equal force of 15 Ibs. per square inch. If the 
 air be then removed from one leg a, by a piston or otherwise, while it continues 
 to press in the other leg b } the mercury will be pushed down in b, until the 
 growing height of a column in a produces a weight so much greater than 
 that in 6, as just to counteract the pressure : now this balance takes place, in 
 fact, when the mercury in a stands about thirty inches higher than in b; that 
 being the height of a column of mercury weighing 15 Ibs on the square inch. 
 
ATMOSPHERIC PRESSURE. BAROMETERS. 
 
 177 
 
 Fig. 100. 
 
 cl 
 
 If the top of the tube a were then closed permanently, the mer- 
 cury would for ever remain elevated in it, marking most perfectly 
 the atmospheric pressure; now this construction, only with the 
 empty and useless part of the tube above d cut off or wanting, 
 forms a common barometer. The exact altitude of the mercury 
 in it is known by observing how much the surface near c is 
 higher than that near d. Often, in such a barometer, a little mass 
 of metal is placed to float on the mercurial surface at d, and as 
 it rises and falls, is caused, by a thread passing from it over a 
 wheel or pulley, to move an index like the hand of a clock con- 
 nected with the wheel, and this index tells the degree of eleva- 
 tion. This modification is called the wheel barometer. 
 
 Again, as water at a, in the bottom of a closed pump-barrel, if 
 pressed upon by the piston b c, of which the rod d were hollow 
 or tubular, would rise in the rod to a height proportioned to the pressure made 
 by the piston, so, in a straight exhausted barometer-tube, which is as this 
 hollow piston-rod, the mercury or water rises, because the atmospheric pres- 
 sure around it is as the piston forcing the fluid up. To make a barometer 
 of this kind it is only necessary to procure a glass tube more than thirty inches 
 long, and close at one end, and then having filled it with 
 mercury, to plunge its mouth (stopped by the finger 
 while turning) into a small cup or basin of mercury; 
 the fluid falls away a little from the top of t]ie tube, 
 leaving a vacuum there, and stands at the elevation 
 which the atmospheric pressure is fitted to maintain. 
 We know from the law of hydrostatics already explained 
 that it is of no importance, in such a case, what the shape, 
 or inclination, or size of the tube may be, as only the 
 perpendicular height can measure or be measured by the 
 pressure. This fact enables us to construct barometers 
 with the upper part of the tube bent obliquely, PO that 
 for one inch rise of mercury in a perpendicular tube, 
 there shall be an advance of several inches in the oblique top, rendering 
 any change of elevation so much more apparent. 
 
 G-alileo had found that water would rise under the piston of a pump to a 
 height only of about thirty-four feet. His pupil Torricelli, conceiving the 
 happy thought, that the weight of the atmosphere might be the cause of the 
 ascent, concluded that mercury, which is about thirteen times heavier than 
 water, should only rise under the same influence to a thirteenth of the eleva- 
 tion ; he tried and found that this was so, and the mercurial barometer was 
 invented. Pascal then, to afford farther evidence that the weight of the 
 atmosphere was the cause of the phenomena, carried the tube of mercury to 
 the tops of buildings and of mountains, and found that it fell always in 
 exact proportion to the portion of the atmosphere left below it; and he 
 found that water-pumps in different situations varied as to sucking power, 
 according to the same law. 
 
 It was soon afterwards discovered, by careful observation of the mercurial 
 barometer, that even when remaining in -the same place, it did not always 
 stand at the same elevation ; in other words, that the weight of the atmosphere 
 over any particular part of the earth was constantly fluctuating ; a truth which 
 without the barometer could never have been suspected. The observation 
 of the instrument being carried still farther, it was found, that in serene dry 
 weather the mercury generally stood high, and that before and during storms 
 
 Fig. 
 
 101. 
 <l 
 
 
 L 
 
 c 
 
178 PNEUMATICS. 
 
 and rain it fell :- The instrument, therefore, might serve as a prophet of the 
 weather, becoming a precious monitor to the husbandman or the sailor. 
 
 The reasons why the barometer falls before wind and rain will be better 
 understood a few pages hence ; but we may remark here, that when water 
 which has been suspended in the atmosphere, and has formed a part of it, 
 separates as rain, the weight and bulk of the mass are diminished: and that 
 wind must occur when a sudden condensation of aeriform matter, in any 
 situation, disturbs the equilibrium of the air; for the air around will rush 
 towards the situation of diminished pressure. 
 
 To the husbandman the barometer is of considerable use, by aiding and cor- 
 recting the prognostics of the weather which he draws from local signs familiar 
 to him; but its great use as a weather-glass seems to be to the mariner who 
 roams over the whole ocean, and is often under skies and climates altogether 
 new to him. The watchful captain of the present day, trusting to this extra- 
 ordinary monitor, is frequently enabled to take in sail and to make ready for 
 the storm, where, in former times, the dreadful visitation would have fallen 
 upon him unprepared. The marine barometer has not yet been in general 
 use for many years, and the author of this work was one of a numerous crew 
 who probably owed their preservation to its almost miraculous warning. It 
 was in a southern latitude ; the sun had just set with placid appearance, closing 
 a beautiful afternoon, and the usual mirth of the evening watch was proceed- 
 ing, when the captain's order came to prepare with all haste for a storm. The 
 barometer had began to fall with appalling rapidity. As yet, the oldest sailors 
 had not perceived even a threatening in the sky, and were surprised at the 
 extent and hurry of the preparations : but the required measures were not 
 completed, when a more awful hurricane burst upon them than the most 
 experienced had ever braved. Nothing could withstand it; the sails already 
 furled and closely bound to the yards, were riven away in tatters : even the 
 yards and masts themselves were in great part disabled ; and at one time the 
 whole had nearly fallen by the board. Such, for a few hours, was the rnin 
 gled roar of the hurricane among the rigging, of the waves around, and of 
 the incessant peals of thunder, that no human voice could be heard, and 
 amidst the general consternation, even the trumpet sounded in vain. In 
 that awful night, but for the little tube of mercury which had given the 
 warning, neither the strength of the noble ship, nor the skill and energies 
 of the commander, could have availed anything, and not a man would have 
 escaped to tell the tale. On the following morning the wind was again at 
 rest, but the ship lay upon the yet heaving waves, an unsightly wreck. 
 
 The marine barometer differs from that used on shore, in having its tube 
 contracted in one place to a very narrow bore, so as to prevent that sudden 
 rising and falling of the mercury which every motion of the ship would else 
 occasion. 
 
 Civilized Europe is now familiar with the barometer and its uses, and 
 therefore, that Europeans may conceive the first feelings connected with it, 
 they almost reouire to witness the astonishment or incredulity with which 
 people of other countries still regard it. A Chinese once conversing on the 
 subject with the author, could only imagine of the barometer, that it was a 
 gift of miraculous nature, which fche God of Christians gave them in pity, 
 to direct them in the long and perilous voyages which they undertook to 
 unknown seas. 
 
 A barometer is of great use to persons employed about those mines in 
 which hydrogen gas, or fire-damp, is generated and exists in the crevices. 
 When the atmosphere becomes unusually light, the hydrogen being relieved 
 
ATMOSPHERIC PRESSURE. BAROMETERS. 179 
 
 from a part of the pressure which ordinarily confines it to its holes and 
 lurking-places, expands or issues forth to where it may meet the lamp of 
 the miner, and explode to his destruction. In heavy states of the atmos- 
 phere, on the contrary, it is pressed back to its hiding-places, and the miner 
 advances with safety. 
 
 We see from this, that any reservoir or vessel containing air would itself 
 answer as a barometer if the only opening to it were through a long tubular 
 neck, containing a close-sliding plug ; for then, according to the weight and 
 pressure of the external air, the density of that in the vessel would vary, and 
 all changes would be marked by the position of the movable plug. A baro- 
 meter has really been made on this principle, by using a vessel of glass, with 
 a long slender neck, in which a globule of mercury is the movable plug. 
 
 The state of the atmosphere, as to weight, differs at different times in the 
 same situation, so as to produce a range of about three inches in the height 
 of the mercurial barometer; that is to say, from twenty-eight to thirty-one 
 inches. On the occasion of the great Lisbon earthquake, however, the mer- 
 cury fell so far in the barometers, even in Britain, as to disappear from that 
 portion at the top usually left uncovered for observation. The uncovered 
 part of a barometer is commonly of five or six inches in length, with a 
 divided scale attached to it, on which the figures 28, 29, &c., indicate the 
 number of inches from the surface of the mercury at the bottom to the 
 respective divisions : on the lower part of the scale the words wind and 
 rain are generally written, meaning that when the mercury sinks to them, 
 wind and rain are to be expected ; and on the upper part, dry and fine 
 appear, for a corresponding reason : but we have to recollect, that it is not 
 the absolute height of the mercury which indicates the existing or coming 
 weather, but the recent change in its height ; a falling barometer usually 
 telling of wind and rain ; a rising one of serene and dry weather. 
 
 The barometer answers another important purpose, besides that of a weather- 
 glass in enabling us to ascertain readily the height of mountains, or of 
 any situation to which it can be carried. 
 
 As the mercurial column in the barometer is always an exact indication of 
 the weight or pressure of air above its level, being, indeed, as explained in 
 the foregoing paragraphs, of the same weight as a column of the air of equal 
 base with itself, and reaching from it to the top of the atmosphere- the 
 niercury must fall when the instrument is carried from any lower to any 
 higher situation, and the degree of falling must always tell exactly how much 
 air has been left below. For instance, if thirty inches barometrical height 
 mark the whole atmospheric pressure at the surface of the ocean, and if the 
 instrument be found, when carried to some other situation, to stand at only 
 twenty inches, it proves that one-third of the atmosphere exists below the 
 level of the new situation. If our atmospheric ocean were of as uniform 
 density all the way up as our watery oceans, a certain weight of air thus left 
 behind in ascending would mark everywhere a change of level nearly equal, 
 and the ascertaining any height by the barometer would become one of the 
 most simple of calculations : the air at the surface of the earth, being between 
 eleven and twelve thousand times lighter than its bulk of mercury, an inch 
 rise or fall of the barometer would mark everywhere a rise or fall in the 
 atmosphere of nearly twelve thousand inches or one thousand feet. But 
 owing to the elasticity of air, which causes it to increase in volume as it 
 escapes from pressure, the atmosphere is rarer in proportion as we ascend, 
 
180 PNEUMATICS. 
 
 so that to leave a given weight of it behind, the ascent must be greater, the 
 higher the situation where the experiment is made : the rule, therefore, of 
 one inch of mercury for a thousand feet, holds only for rough estimates near 
 the surface of the earth. The precise calculation, however, for any case, is 
 still very easy ; and a good barometer, with a thermometer attached, and 
 with tables, or an algebraical formula, expressing all the influencing circum- 
 stances, enables us to ascertain elevations much more easily, and in many 
 cases more correctly, than by trigonometrical survey. 
 
 The weight of the whole atmospherical ocean surrounding the earth being 
 equal to that of a watery ocean of thirty-four feet deep, or of a covering of 
 mercury of thirty inches, and the air found at the surface of the earth being 
 828 times lighter than water, if the same density existed all the way up, the 
 atmosphere would be 34 times 828 feet high, equal to about five miles and 
 a half. On account of the greater rarity, however, in the superior regions, 
 it really extends to a height of nearly fifty miles. From the known laws of 
 aerial elasticity, explained at page 158, we can deduce what is found to hold 
 in fact, that one-half of all the air constituting our atmosphere exists within 
 three miles and a half from the earth's surface ; that is to say, under the 
 level of the summit of Mont Blanc. A person unaccustomed to calculation, 
 would suppose the air to be more equally distributed through the fifty miles 
 than this rule indicates, as he might at first also suppose a tube of two feet 
 diameter to be only twice as capacious as a tube of one foot, although in 
 reality it is four times as capacious. 
 
 In carrying a barometer from the level of the Thames to the top of St. 
 Paul's Church, in London, or of Hampstead Hill, the mercury falls about 
 half an inch, making an ascent of about five hundred feet. On Mont 
 Blanc it falls to half of the entire barometric height, making an elevation 
 of fifteen thousand feet ; and in Du Lucts' famous balloon ascent it fell to 
 below twelve inches, indicating an elevation of twenty-one thousand feet, 
 the greatest to which man has ever ascended from the surface of his earthly 
 habitation. 
 
 The extreme rarity of the air on high mountains must, of course, affect 
 animals. A person breathing on the summit of Mont Blanc, although 
 expanding his chest as much as usual, really takes in at each inspiration 
 only half as much air as he does below exhibiting a contrast to a man in 
 a diving-bell, who, at thirty-four feet under water, is breathing air of double 
 density, at sixty-eight feet of triple, and so on. It is known that travellers, 
 and even their practised guides, often fall down suddenly as if struck by 
 lightning, when approaching lofty summits, on account, chiefly, of the 
 thinness of the air which they are breathing, and some minutes elapse before 
 they recover. In the elevated plains of South America, the inhabitants 
 have larger chests than the inhabitants of lower regions furnishing another 
 admirable instance of the animal frame adapting itself to the circumstances 
 in which it is placed. It appears from all this, that although our atmos- 
 phere be fifty miles high, it is so thin beyond three miles and a half, that 
 mountain ridges of greater elevation are nearly as effectual barriers between 
 nations of men, as islands or rocky ridges in a sea are between the finny 
 tribes inhabiting the opposite coasts. The intense cold which appertains to 
 high situations, and forms another obstacle to human approach, remains to 
 be considered in our next division. 
 
 A barometer connected with an air-pump, indicates exactly the progress 
 and degree of exhaustion in the receiver. When the mercury falls to half its 
 height, it shows that half of the air is extracted ; and so for all other propor- 
 
ATMOSPHERIC PRESSURE. BAROMETERS. 181 
 
 Fig. 102. 
 it 
 
 tions. A barometer then is a necessary appendage to a complete air-pump ; 
 but as its chief purpose is to mark when the exhaustion is carried nearly to 
 completion, a very short tube, corresponding to the bottom of a common 
 barometer, is all that is generally provided, and it is usually made of syphon 
 form. 
 
 The ingenious method, mentioned at page 145 of ascertaining the specific 
 gravity of the solid material forming any porous mass or powder, includes 
 the agency of a barometer. It proceeds upon this reasoning. The inter- 
 stices of a porous or pulverized mass are filled with air of the density of the 
 surrounding atmosphere, and if the atmospheric pressure on which that den- 
 sity depends be diminished upon the mass in any given degree, an exactly 
 corresponding proportion of the air will issue from the pores, and if measured 
 will declare the whole quantity, and, therefore, the amount of interstices 
 or pores in the solid mass. Now if the substance were inclosed at the end 
 or bottom of a syringe, the pressure of the atmosphere might be held off 
 from it in any degree by drawing at the piston, and the air would issue 
 from the pores as described, and would follow the piston ; but as, owing to 
 the friction or a solid piston, it would be difficult to measure the precise 
 action, the liquid piston of a mercurial column has been substituted, of which 
 the force is always proportioned to the length. The operator 
 takes an open glass tube, a e, of known dimensions, aud pre- 
 pare a part of its top, a b } as a receptacle for the substance 
 under trial, by affixing a partition at b, which shall support the 
 substance, but allow passage to air. Having then filled a b 
 with the substance, he gradually immerses the tube in a vessel 
 of mercury df, until the mercury stand both inside arid outside 
 of the tube at the level of 6, the air from the tube having 
 passed out through the substance in a b. It is evident that 
 on then closing the tube at a in the air-tight manner, and 
 lifting the tube, a column of mercury will remain standing in 
 it, above the lev^l of the external mercury at d, and will be 
 acting as a piston pulling down from b with force proportioned 
 to its height. If the tube be lifted until such mercurial 
 column c d be just of half the length in a column in a common 
 barometer, the air in the pores of the substance will be relieved 
 from half of the atmospheric pressure, and will dilate to double 
 bulk : so that while half of the air will remain in the pores, 
 the other half will have issued forth to occupy a space, as b c, 
 between the surface of the mercury at the partition at b. This 
 space b c, therefore will be exactly equal to the amount of 
 the pores or interstices ', and as it may be measured and com- 
 pared with the whole space a b, its ascertained magnitude will 
 solve the problem. It has been found in this way that char- 
 coal, which is usually said to be only half as heavy as its bulk 
 of water, is really formed of matter nearly four times as heavy ; 
 proving, in a new way, the identity of charcoal and diamond, 
 and that light pumice-stone consists of matter heavier than granite or 
 marble. This very ingenious application of the barometer may lead 
 ultimately to many useful results: and the contrivance merits consideration 
 here, as exhibiting, under a new and interesting aspect, the rationale of 
 barometric action aad the elasticity of air. 
 
 ci 
 
182 PNEUMATICS. 
 
 Atmospheric pressure determining the liquid or aeriform state of certain 
 substances. (See the Analysis, page 156.) 
 
 It has already been stated that the permanent gases or substances usually 
 in the aeriform state may be reduced to the liquid, or even solid form, by 
 simple pressure, and abstraction of the heat which is combined with them 
 in the aeriform state. Carbonic acid, the common coal gas, &c., have been 
 treated in this way. Now it becomes an interesting question whether many 
 of the substances known as liquids on the face of the earth, where they are 
 bearing the pressure of the atmosphere, would not appear as airs if that 
 pressure did not exist. 
 
 On investigating this subject by experiment, we accordingly find, that 
 sether, alcohol or ardent spirits, volatile oils, &c., and even water itself, are 
 known to us here as liquids, only because their particles are kept together 
 by the weight and pressure of a superincumbent atmosphere Any of these 
 substances, relieved by art from such pressure, quickly becomes an air or 
 gas, just as a common gas, which has been kept in the state of liquid by 
 any great pressure, becomes air again on being relieved. 
 
 In our first chapter we explained the dependence of the three forms which 
 any body may assume, viz., of solid, liquid, or air, on the quantity of heat 
 diffused among the particles ; we now see, however, that to understand the 
 subject completely, we must consider also the effect of accidental pressure ; 
 for, while heat is the power separating the atoms in the changes mentioned, 
 it has to overcome both the mutual attraction of the atoms and the additional 
 force of the atmosphere pressing them together. The combined influence 
 of these forces is fully displayed in the two phenomena called boiling and 
 evaporation, which exhibit the progress or the change of a liquid into an 
 aeriform fluid. We now proceed to examine these phenomena. 
 
 Boiling. If water be placed in a suitable vessel over a common fire, or 
 over the flame of a lamp, it is gradually heated to a certain degree ; and 
 then small bubbles of aeriform matter, viz., water, in the slate called steam, 
 are seen forming at the bottom of the vessel ; and successively rising to the 
 surface, where they disappear by mixing with the atmosphere; and the 
 operation being continued, the quantity of water diminishes with every 
 bubble, until the whole vanishes under the new form of air. 
 
 This change takes place in water, under common circumstances at the 
 degree of heat mark 212 on Fahrenheit's thermometer, and called, on that 
 account, the boiling point of water; at which therefore, the repulsive power 
 among the particles is just sufficient to overcome both their natural attrac- 
 tion, and the compressing force of the atmosphere of fifteen pounds on the inch. 
 But a less degree of heat suffices if the pressure of the atmosphere be lessened 
 or removed : and a greater degree is required if pressure be increased. Water 
 on the top of Mount Blanc boils at 180, because relieved from the pressure 
 of the air that is below the level of the mountain's summit; and at all inter- 
 mediate heights in descending to the level of the sea, and beyond that into 
 mines, there is a corresponding increase of the boiling temperature. So 
 exactly is this the case, that we now find it to be a good method of ascertain- 
 ing the heights of places, merely to observe the heat of boiling water at 
 them. To many persons the information here given that boiling water is not 
 equally hot in all places, will appear extraordinary; and they will not under- 
 stand that even in the same place, at different times, when the barometer is 
 high or low, there will be corresponding differences. Again, near the bottom 
 of a boiler, the water is hotter than above, because it is bearing an additional 
 
ATMOSPHERIC PRESSURE. BOILING. 183 
 
 pressure proportioned to the depth and does not, therefore, give out the 
 steam which it would part with if a little higher up. In very large and deep 
 boilers, therefore, such as are used in great porter breweries, the liquor is 
 much more heated that it can be in smaller vessels ; a circumstance which 
 probably has an influence on its ultimate quality. 
 
 While water, under common atmospheric pressure, or when the barometer 
 stands at thirty inches, boils at 212, other substances, with other relations 
 to heat, have their boiling points higher or lower : aether, for instance, at 
 98 ; spirit or alcohol at 174 ; fish-oil and tallow at about 600 ; mercury 
 at 650. 
 
 It is in consequence of the different temperatures at which the paticles of 
 different substances requires repulsion enough to rise against the atmospheric 
 resistance that we are enabled to perform the operation called distilling. If 
 a mixture of spirits and water, for instance, -be heated up to 180 deg., the 
 spirit will pass off in the aeriform state, leaving the water behind, and may 
 be caught apart and cooled to condensation in any fit receiver. Distillation 
 is the best means we possess of separating many substances from each other, 
 as spirit from wine and other fermental liquor ; various acids from water ; 
 water itself from its common impurities; and even the separation of 
 mercury from silver or gold which it has been used to dissolve from among 
 the rubbish of a mine or river-bottom, is merely a distillation. 
 
 We must call to mind here what was mentioned in a former part of the 
 work, that a large quantity of heat combines with every substance during the 
 change of form from solid to liquid, or from liquid to air; a quantity which, 
 from not remaining sensible to the thermometer, has received the name of 
 latent or concealed heat. The whole of this is given out again in the contrary 
 change. In the conversion of water into steam, the heat which thus disap- 
 pears is about 1,000 degrees, or six times as much as is required to raise the 
 cold water to the boiling point ; this is proved by the time and fuel expended 
 in boiling any quantity to dryness, and by the fact that a pint of water in 
 the form of steam will combine instantly with six pints of cold water, rais- 
 ing the whole to boiling heat. f 
 
 But for the fact of latent heat, the conversion of a liquid into air would 
 not be the gradual process of boiling which we now see, but a sudden and 
 terrible explosion : for when any quantity of water were raised to the boiling 
 heat one degree more would be sufficient to convert the whole into steam. 
 And but for the same reason, the thawing of winter snow would always be 
 a sudden and frightful inundation ; the whole load of a mountain or plain 
 becoming -at once as a lake bursting from its enclosing barriers. On the 
 other hand, if water in freezing had not to give out again its latent heat, after 
 any quantity were at once cooled down to the freezing point, the abstraction 
 of one degree more would instantly convert the'whole into a solid mass. 
 Thus, then, by an arrangement effecting most important purposes in nature 
 and art, all changes from solid to liquid and from liquid to air, and the 
 converse changes, are very gradual. 
 
 If a little heat be abstracted from steam, a part of the steam proportioned 
 to the abstraction is immediately condensed into water. What is called steam, 
 in common language as the vapour which becomes visible at a little distance 
 from the spout of a boiling kettle or the chimney of a tea-urn is not truly 
 steam, but small globules of water already condensed by the cold air and 
 mixed with it. Steam is as dry and invisible as air itself; but the instant 
 that it comes in contact with air or other bodies colder than itself, it becomes 
 water. 
 
184 PNEUMATICS. 
 
 By means of the exhausting air-pump on one hand, and of the condensing 
 syringe on the other, all the above-mentioned phenomena, depending on 
 the atmospheric pressure, and its increase or diminution, may be strikingly 
 shown. 
 
 Thus, to exhibit the effect of diminished pressure, water not heated by 
 several degrees to the boiling point of ordinary low situations, but which 
 would be boiling at the top of Mont Blance, is caused to boil instantly by 
 placing it under the receiver of an air-pump, and making a few strokes of the 
 piston ; if the exhaustion be rendered nearly complete, the water will boil, 
 even when colder by 20 degrees than the blood of animals ; and at degrees of 
 temperature still much lower, it will rapidly assume the form of air, although 
 not with force sufficient to produce the violent agitation of boiling. Other 
 liquids, as spirits, aether, &c., from requiring inferior degrees of heat to 
 separate their particles to aerfcform distances, boil under the receiver of an 
 air-pump at very low temperatures j aether, for instance, when as cold as 
 freezing water. 
 
 On the other hand, to exhibit the effect of increasing pressure, if we con- 
 fine the particles of a liquid still more than by a common atmospheric or 
 equivalent pressure, degrees of heat higher than the common boiling point 
 will be required to separate them. In a diving-bell, the boiling point of water 
 is higher than 212 deg. in proportion to the depth which the bell has reached : 
 and if at the surface of the earth, we heat water in a close vessel into which 
 air is forced so as to press thirty pounds on the inch instead of fifteen, as the 
 atmosphere does } or from which we prevent the steam's escaping until it 
 has acquired the force of a double atmosphere, before making the liquid 
 boil, we shall have to raise the heat, in a corresponding proportion beyond 
 212 deg. Under a very strong pressure, water may be rendered almost red- 
 hot, but the force with which its particles are then tending to separate is 
 almost that of inflamed gunpowder. Even then, however, if a gradual issue 
 were allowed, only a certain quantity of the water would absorb and render 
 latent the existing excess of heat above 212 deg., and would become common 
 steam, leaving behind a considerable portion as boiling water of the ordinary 
 temperature. 
 
 The fact that liquids are driven off, or made to boil at lower degrees of heat 
 when the atmospheric pressure is lessened or removed, has recently been 
 applied to some very useful purposes. 
 
 The process of refining sugar is to dissolve impure sugar in, water, and 
 after clarifying the solution, to boil off or evaporate the water again, that 
 the dry crystallized mass may remain. 
 
 Formerly this evaporation was performed under the atmospheric pressure, 
 and a heat of 21.8 or 220 was required to make the syrup boil; by which 
 degree of heat, however, a portion of the sugar was discoloured and spoiled, 
 and^ the whole product was deteriorated. The valuable thought occurred to J 
 Mr. Howard, that the water might be dissipated in boiling the syrup in a 
 vacuum, or at least a place from which air was nearly excluded, and therefore, 
 at a low temperature. This was done accordingly ; and the saving of sugar 
 and the improvement of quality were such, as to make the patent-right, 
 which secured the emoluments of the process to him and other parties, worth 
 many thousand pounds a year. The syrup, during this process, is not more 
 heated than if in a vessel merely exposed to a summer sun. 
 
 In the preparation of many medicinal substances, the process of boiling in 
 
ATMOSPHERIC PRESSURE.- BOILING. 
 
 185 
 
 Fig- 103. 
 
 var.no is equally important. Many extracts from vegetables, have their 
 virtues impaired, or even destroyed, by a heat of 212; but when the water 
 used in making the extract is driven off in vacuo, the temperature need 
 never be higher than blood-heat, and all the activity of the fresh plant 
 remains in the extract. 
 
 In the same manner, in the process of distillation, which is merely the 
 receiving and condensing again in' appropriate vessels the aeriform matter 
 raised by heat from any mass, substances which are changed and injured 
 by an elevated temperature may be obtained of admirable quality by carry- 
 ing on the operation in a vacuum. The essential oils of lavender, pepper- 
 mint, &c., never had the natural flavour and virtues of the plants until 
 within the last few years, since this plan has been adopted. 
 
 The influence on the human system of vegetable medicines obtained in 
 the old or in the new way, is so different, that the prescriber should carefully 
 advert to the circumstance. 
 
 The apparatus for evaporating and distilling in vacuo consists of vessels 
 strong enough to bear, when quite empty, the external atmospheric pressure 
 and which are therefore generally of arched form. The vacuum is produced 
 and maintained by air-pumps driven by a steam engine or otherwise; or by 
 first admitting steam to expel the air, and then condensing the steam into 
 water. 
 
 The author has suggested a very simple contrivance to answer, in certain 
 cases, the purpose of such air-pun\,ps and steam-engines or 
 apparatus. It is merely to establish a communication 
 between a close boiler, as a, and the vacuum at the top of 
 a water barometer, as b, To produce that vacuum, the 
 strong vessel b forming the top of the barometer, and 
 thirty-six feet of tube below, reaching to d are first filled 
 with water through a cock c at the top ; this cock being 
 then shut, and another cock d at the bottom, which was 
 shut, being opened, the water will sink down out of the 
 vessel 6, until the column in the tube be only thirty-four 
 feet high as at/, that being the height which the atmos- 
 phere will support. On then opening a communication 
 between the boiler a and the vacuum in b, the operation 
 will go on as desired, and the steam rising from a may be 
 condensed in b by a little stream of cold water allowed con- 
 stantly to run through from above. This water, it is evi- 
 dent, would always pass downwards to form part of the 
 column below, without filling up or impairing the vacuum. 
 If air should find admittance in any way. the original degree 
 of vacuum could easily be reproduced as at first ; and to 
 prevent interruptions, it might be convenient to have two 
 vessels like 6, of which one could always be in action while 
 the other was being emptied of air. The author planned this as a simple 
 apparatus for the preparation of medical extracts ; and it appears well suited 
 also for the manufacture of sugar in the colonies, where air-pumps and nice 
 machinery can with difficulty be either obtained or managed. On many 
 sugar estates there is a fall of water, which would supply the barometer with- 
 out the trouble of pumping. The tube d c need not be perpendicular, pro- 
 vided it be longer in proportion to its obliquity ; and it may be very small ; 
 some yards of common lead-pipe would answer. 
 
 When it was understood that, at common temperatures, water and many 
 
186 PNEUMATICS. 
 
 fe 
 
 other liquids would be existing in the form of air, but for an atmospheric 
 pressure opposing the separation of the particles, it became of great import- 
 ance in many of the arts, and for comprehending certain phenomena of 
 nature, to ascertain, very exactly, with respect to some of these liquids, the 
 degrees of expansive force belonging to them at different degrees of tempera- 
 ture. The subject, as far as water is concerned, has been investigated with great 
 care, and the following table shows part of the results. The left-hand column 
 marks temperature from 32 deg. of Fahrenheit's thermometer, or the freez- 
 ing point of water, to 290 deg. ; and the right-hand column marks the cor- 
 responding degrees of force with which the water tends to expand into the 
 state of steam, and therefore also the force and density of the steam existing 
 in any vessel above the water which it contains. One ounce and a half 
 per square inch, is the force exerted on the sides of any containing vessel by 
 steam rising from freezing water, that is to say, the force with which freezing 
 water seeks to dilate into steam or air ; and sixty pounds per inch is the force 
 of water heated to 290 deg. 9 To many readers the idea will be quite new 
 and surprising, that if some freezing water, or even ice, be placed in a bladder 
 containing nothing else, and the bladder be then placed in the exhausted 
 receiver of an air-pump or other vacuum, the bladder will quickly be dis- 
 tended with steam strong enough to support one ounce and a half on every 
 square inch of its upper surface. 
 
 At 32 force of steam is 
 50 
 
 1| oz. per inch. 
 
 O 3. f\rj 
 
 100 
 
 4 U/ . 
 
 IS 
 
 
 J.O 
 
 4- Ibs 
 
 180 
 212 
 
 7 Ibs'. 
 15 Ibs. 
 
 250 30 Ibs. 
 
 290 60 Ibs! 
 
 "* In this table we have to remark how much more rapidly the tendency 
 to become steam increases than the temperature of the water : for a rise of 
 eighteen degrees, viz., from 32 to 50, at the beginning of the scale, only 
 increases the dilating force one ounce and a quarter on the inch, while an 
 equal rise at the top of the scale, viz., from Ii72 deg. to 290 deg., increases 
 it fifteen pounds. It is most important to distinguish, however, between the 
 tendency to form steam at any temperature, and the bulk or quantity of 
 steam formed by a given quantity of heat ; for the matter imperfectly under- 
 stood has led to many vain schemes for improving the steam-engine. The 
 truth is, that high-pressure steam is merely condensed steam, as high-pres- 
 sure air is condensed air; in other words, the density of steam is greater, 
 or there must be more of it, exactly as its force is greater according to the 
 rule explained at page 160 : and the heat absorbed in its formation being 
 proportioned to the quantity of steam in a given space, or the density, the 
 force and the cost in fuel have always nearly the same relation to each other. 
 In one pint of steam, at 29t) deg., having an elastic force of sixty pounds on 
 the inch, there are very nearly four times as much water and four times as 
 much latent heat as in one pint of steam at 212 deg., which has a force of 
 fifteen pounds on the inch ; indeed, the one pint, at 290 deg., may be 
 changed into four pints at 212 deg., or the contrary, by merely changing 
 the degrees of pressure. It does not accord with the plan of the present 
 
STEAM ENGINE. 187 
 
 work to enter farther into the details of this subject, but they may be found 
 in various modern treatises. 
 
 Seeing the rapid increase of the expansive force in the preceding table, 
 we have the explanation of the terrible effects occasionally produced by con- 
 fined water when overheated. A boiler of any kind completely closed, and 
 having no safety valve, if heated to a certain degree, will explode as if 
 charged with gunpowder. Unhappily the instances are too numerous where 
 the incautious or ignorant use of steam has produced explosions, which have 
 shattered buildings and destroyed whole neighborhoods. 
 
 The prodigious force generated by heating water would at first only sur- 
 prise and terrify men, but in the course of time would lead inventive 
 minds to enquire whether it might not be turned to use ; in other words, 
 whether some* mechanism, to be called a steam-engine, might not be con- 
 trived to enable men to make it aid them in their various labours. To this 
 inquiry, after numerous less successful attempts, a glorious answer has been 
 given in our own day by the illustrious WATT ; and to this part of our 
 work it belongs to consider what he has accomplished, viz., to describe 
 
 The Steam- Engine, 
 
 which, in the few years since the genius of WATT carried it to its present 
 state of perfection, has changed the direction, of human industry, and may 
 almost be said to have elevated man in the scale of existence. 
 
 The name of steam enyine, to most persons, brings the 
 idea of a machine of the most complex nature, and hence to Fig. 104. 
 be understood only by those who will devote much time to 
 the study of it ; but he who can understand a common pump, 
 may understand a steam-engine. It is in fact only a pump 
 in which the fluid passing through it is made to impel the [~r ~~t 
 
 piston instead of being impelled by it, that is to say, in which 
 the fluid acts as the power instead of being the resistance. 
 It may be described simply as a strong barrel or cylinder c d, 
 with a closely fitting piston in it, here shown at b, which is 
 driven up and down by steam admitted alternately above and 
 below it from a suitable boiler ; while to the end of the pis- 
 ton rod a, at which the whole force may be considered as . 
 concentrated, there is attached in any convenient way the 
 work which is to be performed. The power of the engine is of course pro- 
 portioned to the size or area of the piston, on which the steam acts with a 
 force, according to its density, of from 15 to 100 or more pounds to each 
 square inch. In some of the Cornish mines there are cylinders and pistons 
 of more than ninety inches in diameter, on which the pressure of the steam 
 equals the efforts of six hundred horses. 
 
 In one place this wonderful piston-rod may be seen acting upon the end 
 of a great vibrating beam, to the other end of which capacious water-pumps 
 are attached, whose motion causes almost a river to gush up from the bowels 
 of the earth. In another place, it is seen working a crank, and urging com- 
 plicated machinery. One steam-engine four miles from London is at the 
 same instant filling all the water reservoirs, and baths, and fountains of the 
 finest quarter of the town. One engine stretching long arms over a great 
 barrack, or manufactory, keeps in one quarter, thousands of spinning-wheels 
 in motion, while in another it is carding the material of the thread, and in 
 another weaving the cloth. In like manner, one steam engine, in a great 
 
188 
 
 PNEUMATICS. 
 
 metropolitan brewery, may be seen at the same time grinding the malt, pull- 
 ing up supplies of all kinds from wagons around the building, pumping cold 
 water into some of the coppers, sending the boiling wort from others up to 
 lofty cooling-pans, over which it is turning the fans, perhaps also working 
 the mash-tub, drawing water from the deep wells under ground, and loading 
 the drays in a word, performing the offices of a hundred hands. Again, 
 there are manufactories where this resistless power is seen with its mechanic 
 claws seizing masses of iron, and in a few minutes delivering them out again 
 pressed into thin sheets, or cut into bars and ribbons, as if the iron had be- 
 come to it like soft clay in the hands of the potter. And now for some 
 years, over nearly the whole world, has this wonderful piston-rod, working at 
 its crank, been turning the paddle-wheels of innumerable steam-boats, there- 
 by sitting at defiance the violence of the winds and waves, aifd the currents 
 of the fleetest rivers, while it carries men and civilization into the remote 
 recesses of all the great continents. To wherever a river leads, the region, 
 although concealed perhaps since the beginning of the world, is now by the 
 steam-engine called as it were from its solitude, to form a part of the great 
 garden which civilized man is beautifying. Such are a few of the prodigies 
 which this machine is already performing, and every day is witnessing new 
 applications of its utility. 
 
 The following account of the parts of the steam-engine is intended, with- 
 out entering into minute practical details, still fully to explain the principle 
 or general nature of the machine. It should serve to render very interesting 
 to an attentive reader, a visit to any place' where a steam-engine is in use : 
 and it should make evident the folly of many of the modern schemes for 
 improving the engine. To avoid complexity in the figure, the parts which 
 the reader can easily conceive are not here sketched. 
 
 Fig. 105. 
 
 1st. The part which first claims attention is the great barrel c d, already 
 spoken of as the centre or main portion of the machine, in which the piston 
 P is moved up and down by the action of steam entering alternately above 
 and below it, through the pipes e c and e d. The barrel or cylinder is 
 bored with extreme accuracy, and the piston is padded round its edge with 
 hemp or other soft material so as to be perfectly air or steam-tight. Lately 
 
STEAM ENGINE. 189 
 
 pistons have been made altogether of metal, and, in some cases, from work- 
 ing with less friction, these answer even better than the others. 2d. The 
 next part to be mentioned is the boiler B, which is made of suitable size and 
 strength. 3d. The steam passes from the boiler along the pipe to e, and there 
 by any suitable cock or valves, worked by the' engine itself, is directed alter- 
 nately to the upper and under part of the barrel; and while it is entering to 
 press on one side of the piston, the waste steam is allowed to escape from 
 the other side, either to the atmosphere, for high-pressunj engines, or into 
 4th, the condenser at C, for those of low-pressure ; the condenser being 
 always kept at a low temperature by cold water running into it arid pumped 
 out again by the piston k, 5th. The supply of steam from the boiler to the 
 cylinder is regulated by a valve placed somewhere in the pipe B 6, and made 
 obedient to what is called 6th, the governor, a contrivance not represented 
 here, but already described at page 52, to illustrate centrifugal force. We 
 may recall it by saying, that it consists of two balls hanging by jointed rods 
 like the legs of a tongs, from opposite sides of an upright spindle, which is 
 made to revolve by connection with some turning part of the machinery : 
 when the spindle turns at all faster than with the desired speed, the balls fly 
 more apart and are made to affect the steam valve so as to narrow the passage ; 
 and, on the contrary, when it turns more slowly than is desired, they collapse, 
 and by so doing open the valve wider. 7th. The supply of water to the 
 boiler is regulated by afloat on the surface of the water in the boiler; which 
 float, on descending to a certain point, by reason of the consumption of 
 water, opens the valve to admit more. 8th. There is a safety valve in the 
 boiler, viz., a well-fitted flap or stopper, held against an opening by a weight, 
 but loaded so as to open before danger can arise from the over-heating of the 
 water. 9th. The rapidity of the combustion, or force of the fire, is exactly 
 regulated by the state of the boiler and the wants of the machine, thus : 
 there is a large open tube (not represented here) rising from near the bottom 
 of the boiler, through its top, to the height of several feet, and when the 
 water in the boiler is too hot, and the steam therefore too strong, part of the 
 water is pressed up into this tube, and by the agency of a float which rests 
 on its surface, it shuts the chimney valve or damper : the draught is then 
 diminished and the fuel saved," until a brisker fire is again required. 10th. 
 In this figure a i g marks the place of the great beam, turning on an axis 
 at i, and transmitting the force of the piston to the remote machinery. 
 When the object is to raise water, the pump rods are simply connected with 
 the end g of the beam ; but when any rotatory motion is wanted, the end g 
 is made to turn llth. A crank I n by the rod g I; and uniformity of 
 motion is obtained by the influence of 12th, the great fly wheel m, fixed 
 to the axis of the crank. 
 
 The smallest and simplest steam engine, and therefore the cheapest, is 
 that called the high-pressure engine. In it steam is used of great density, 
 and consequently of great force, as of 50 Ibs. or more to the inch; and while 
 the fresh steam is admitted to press on one side of the piston, the steam 
 which has already worked, is allowed to escape, or is driven out to the air, 
 from the other side. The atmospheric resistance to the issue of the steam 
 diminishes the working force of the piston just 15 Ibs. per inch. The sim- 
 plicity of this form of engine recommends it, but the danger of a large 
 boiler of overheated water, always, like inflamed gunpowder, seeking to 
 escape, bas ? by numberless fatal accidents, been proved to be so great, that 
 the use of such an engine is limited to certain situations. Notwithstanding 
 all the ingenious securities recently contrived against the danger, and which 
 
190 PNEUMATICS. 
 
 wijl suffice for small engines, such as are required for steam carriages, the 
 high-pressure engine is not employed in a single English passage-vessel.* 
 
 In the low-pressure engine, the steam is used generally of force not 
 exceeding 20 Ibs. on the inch) which force is only 5 Ibs. more than the atmos- 
 pheric pressure, and is insufficient to burst a common boiler, or to do serious 
 mischief :f but as the interior of the low-pressure engine is kept in a state of 
 
 * In this country, a^o, what is called the low pressure engine, is at present employed in 
 all the steamboats on our eastern waters, but we have reason to believe that few, if any, of 
 them are worked with a less pressure than 25 Ibs. on the square inch, and we have seen 
 many of them worked with a much higher pressure. It is stated, that the steam guage on 
 board the Pulaski, just before the fatal explosion, indicated a pressure of 26 Ibs., which was 
 considered by the engineer a safe working force. The British Ocean Steamers work with a 
 pressure of only 3 to 4i Ibs. to the square inch. On our western waters, the low pressure 
 engine has been discarded in favour of the high pressure. AM. ED. 
 
 f Our author must be understood as saying, only, that explosions do not take place in low 
 pressure engines whilst working under a pressure of only 20 Ibs. to the square inch, But 
 from various circumstances, the elastic force of the steam in a low pressure engine may be 
 
 freatly increased, and instead of its ordinary power of 20 Ibs., it may acquire one of 100 or 
 00 Ibs., and if the boiler, as is often the case, is unable to bear this pressure, it will burst. 
 In this way explosions have repeatedly taken place in low pressure engines. Several of 
 thesewill be found related in an interesting memoir by M. Arago, originally published in 
 ihe" Annuaire du Bureau des Longitudes," and which has been translated and published in 
 that useful periodical, " The Journal of the Franklin Institute of Pennsylvania." M. Arago, 
 in this memoir, remarks, " I ought not to conclude so long a paper on the subject of the 
 explosion of steam boilers, without explaining why I have not separated the examples of 
 the explosion of high pressure boilers from those of the low pressure ; it is because I think 
 there is no reason to make such distinction. Every one must, in fact, admit, that at the 
 time of an explosion, all boilers contain high pressure steam." 
 
 The belief that low pressure boilers are not liable to burst, or do mischief, has led, as 
 has been already observed, to their exclusive use in passage vessels ; this belief, however, 
 is founded in error. M. Arago, in the memoir already quoted, observes, ' it does not 
 appear established by any means, that explosions take place more frequently in hi<rh than 
 in low pressure boilers; the contrary has been maintained by different engineers, among 
 whom may be classed Messrs. Perkins, Oliver Evans, Ac." Indeed, a little reflection will 
 show that high pressure boilers ought not to be more liable to explosion than the low. 
 Boilers may be made of either iron or copper of sufficient strength to resist a much greater 
 force than that of the steam ever employed in high pressure engines. Now boilers are 
 always constructed of a strength proportionate to the pressure they are to sustain. Thus, 
 in a low pressure engine working with a force of 20 Ibs., the boiler is made of strength 
 calculated to support from 3 to 5 times that pressure," in a high pressure engine destined 
 to work with a pressure of 150 Ibs., the boiler is constructed so as to resist from 3 to 5 times 
 that pressure. It will immediately be asked, why cannot the boiler of a low pressure 
 engine be made of the same strength as if they were for a high pressure engine ? In answer 
 to this it may be remarked, that independent of many difficulties to be overcome before we 
 can exceed certain limits in the thickness of boilers, the weight and cost of the low presure 
 engine are already so great that it would be impossible to persuade owners of steamboats 
 to incur any addition to these particulars and even would they do so, perfect safety would 
 still not thus be obtained. We have already observed, that at the time of an explosion, all 
 boilers contain high pressure steam, and as we know no limits to the force of this steam, 
 however strong the boiler may be, it may burst, unless this be prevented by other means. 
 
 It was long ago known, that if a vessel, however strong it might be, containing water, 
 be placed over a fire, it will burst, unless an opening is provided for the escape of the 
 steam as fast as produced. The temperature which will cause the rending of a vessel, 
 must depend upon its form and dimensions, and upon the tenacity and thickness of the 
 material of which it is made. If we could keep the heat of our furnace below a certain 
 limit, no other precaution would be required to prevent explosions. But it is evident that 
 this cannot be done, we must, therefore, resort to some other expedient, and the safety 
 valve, invented by Papin, would seem to answer this purpose. We must be allowed to 
 anticipate the subject a little, in order to explain the nature of this valve. 
 
 The safety valve consists of a hole, say of an inch square, made in the upper part of the 
 boiler, upon which is placed a metal plate loaded with a certain weight. It is evident 
 that the hole will remain closed as long as the pressure of the steam within the boiler is 
 less that the weight of the valve, together with that of the atmosphere, upon the square 
 inch, and that as soon as the pressure within shall exceed this, the valve will be raised 
 and give a free vent to the steam. 
 
 It would lead us too far to explain how it has happened that so simple and apparently 
 
STEAM ENGINE. 191 
 
 vacuum, except where the steam is acting, the whole pressure of 20 Ibs. is 
 made available, and the engine has the same power, if of equal size, as a 
 high pressure engine working with steam of 35 Ibs. on the inch The re- 
 quired vacuum is preserved by means of a separate vessel or box, represented 
 at C. called the condenser, into which cold water is constantly running to 
 condense the steam, and is afterwards pumped out with the condensed 
 steam, and with any little air that may have entered : the pump is repre- 
 sented at k in the figure. Steam, on coming into contact with a cold body, 
 is condensed almost with the rapidity of an explosion ; and therefore the 
 instant that the opened valves make a communication between the cold con- 
 denser and any part of the engine containing steam, this rushes to the con- 
 denser, and becomes water, leaving a vacuum behind. The great merit of 
 Mr. Watt was in the contrivance of this separate condenser, for, until his 
 time, cold water had always been thrown directly into the working cylinder, 
 cooling it so much, that twice or thrice its fill of steam was destroyed at 
 each stroke to warm it again before it could work. This single change 
 saved three-fourths of the quantity of fuel formerly expended. 
 
 Before Watt's day, the only steam engine in use was a rude single- stroke 
 engine as it was called, in which steam, admitted under the piston, allowed 
 the weight of the pump-rods at the far end of the beam to lift the piston, 
 and the steam being then condensed so as to leave a vacuum in the cylinder, 
 the pressure of the atmosphere pushed the piston down to do its work : on 
 this last account the engine was also called au'atmospheric engine. It was 
 used almost solely for pumping water ; but it wasted so much fuel, from 
 causes of which the chief is mentioned in the last paragraph, that the 
 expense was not much less than that of employing horses. 
 
 In the atmospheric engine, the steam which lifted the piston against the 
 atmospheric pressure, required to be at least as strong as that pressure, to the 
 very end of the stroke. Another of Watt's great improvements was, his 
 
 efficient means has not always proved efficacious; since these causes are various and many 
 of them as yet not perfectly understood. 
 
 Those who wish to investigate the subject, we refer to the memoir of M. Arago, and 
 to the report of a committee of the Franklin Institute, who have collected an account of 
 all the explosions in this country, and who have instituted a very interesting series of ex- 
 periments, in order to examine into the causes of the explosion of steam boilers, and devise 
 means for its prevention. This report has been published in the Journal of the Institute. 
 
 We must not omit, however, to mention that when a low pressure boiler does explode, 
 it hns ifeen found to r roduce greater destruction than a high pressure one, in consequence 
 of the great size, and, therefore, larger quantity of water contained in the former. It 
 may, perhaps, be supposed that the steam from a high pressure engine would scald more 
 severely than that from a low pressure one. This, however, is not the fact : on the contrary, 
 whilst the steam issuing from a low pressure engine scalds at all moderate distances from 
 the boiler, that from a high pressure one scalds only at certain distances. Thus the hand 
 may be placed an inch from an aperture in a high pressure engine without any inconve- 
 nience being felt: at a greater distance, however, it will scald most severely. A friend has 
 informed us that he has placed his hand within an inch of the aperture in a boiler from 
 which the steam was issuing at a time when the force of the steam within the boiler was 
 equal to 300 Ibs. on the square inch, without feeling any inconvenience. Some interesting 
 experiments on this subject have been instituted by Peter Ewort, Esq., and an account of 
 which will be found in the fifth volume of the Journal of the Franklin Institute. 
 
 It must not be supposed from any thing that we have said in this note, that explosions 
 of steam boilers cannot be prevented. But we may be allowed to quote on this subject 
 the following remarks of M. Arago. ' No cause of explosion exists which cannot be avoided, 
 by means at once simple and within the reach of every one. As we should not trust fire- 
 arms in the hands of children, so, I think, we should not trust the direction of a steam 
 engine to a man either unskilful, without experience, or wanting in intelligence. It is a 
 mistaken idea, that because steam engines usually move* without attention to them, such 
 attention is not required; Watt contended strongly *against this error." AM. ED. 
 
192 PNEUMATICS. 
 
 excluding altogether the air from his machine, by doing which he not only 
 avoided the cooling effect of the air, but was at liberty to shut off the steam, 
 as it is expressed, or to stop the supply for each stroke before the cylinder 
 was full, and then to make the farther expansion of the quantity admitted 
 impel the piston to the end of the stroke. This principle of causing the mere 
 expansion of steam to do work was afterwards carried to a great extent by 
 Messrs. Hornblower, Woolfe, and others, who constructed engines with two 
 barrels, in the first and smaller of which, the steam was made to act in its 
 dense or strong state, as it issued from the boiler, and when it had finished 
 a stroke there, instead of being at once sent useless to the condense,]", it was 
 admitted to a larger piston, which it moved by its continued expansion alone : 
 the same steam thus doing double work or more. All the advantages of 
 the two cylinders, however, are obtainable from the single cylinder with its 
 condenser, as now used in most of the Cornish mines. Steam of about 60 Ibs. 
 pressure on the inch is admitted to the cylinder, until the piston is driven 
 nearly one-third of its way, and the valve being then shut, the same steam is 
 left to finish the stroke by its expansion. The pressure of the expanding 
 steam gradually diminishes, it is true, in proportion as the volume increases : 
 but in pumping water there is a great saving of time, from having the power 
 more intense at the beginning of the stroke, when the vast mass of water and 
 machinery has first to be put into motion. Steam, while doubling its volume 
 by mere expansion, will do about two-thirds as much work as while origi- 
 nally rising from the boiler, and by every subsequent doubling it might do 
 as much as by the first : the increasing size of the cylinder, however, and 
 increased friction, confine this mode of using it to narrow limits. 
 
 It might be supposed that high pressure engines without condensers would 
 be comparatively wasteful, because in them the steam which has acted must 
 be driven out of the cylinder against the powerful resistance of the atmos- 
 phere, while in the low pressure engine it has instant access to the con- 
 denser, and leaves effective the whole pressure of the fresh steam on the 
 opposite side of the piston. But as in the low pressure engine, nearly half 
 the power of the steam is .expended in overcoming the friction and other 
 impediments of the numerous parts, while in that of high pressure, the parts 
 are so much fewer, and the piston is so much smaller in proportion to the 
 force acting upon it, that the loss from friction is often less than a fourth or 
 even a sixth of the steam power, although the resistance of the air is to be 
 overcome by the high pressure engine, still there is often a saving on the 
 whole. The saving becomes very considerable if the steam be allowed to 
 act by its expansion also, as described in the last paragraph. 
 
 From misapprehension of the law of increase of force by increase of heat 
 in water, explained by the table at page 186, some exceedingly false conclu- 
 sions have been drawn and acted upon at great expense (as lately by Mr. 
 Perkins) in attempts to make engines work with an excessively high pres- 
 sure. Besides making the error now alluded to, and others, Mr. Perkins 
 overlooked the fact, that we possess no material for cylinders and pistons 
 strong enough to bear the contemplated pressure and friction even for a 
 moderate time. Perhaps more striking examples could not be adduced of 
 the absurdities into which even highly ingenious men may fall, when not 
 sufficiently acquainted with the general truths of nature on which the arts 
 which occupy them are founded, than in the history of supposed inventions 
 and improvements connected with the steam-engine. 
 
 The fertile genius of Jame Watt did not stop at the accomplishment of the 
 two or three important particular's described above, but^throughout the whole 
 
EXPLOSION. 193 
 
 detail of the component parts, and of the various applications of the engine, 
 he contrived miracles of simplicity and usefulness. We should exceed the 
 prescribed boun'ds of this work by entering more minutely into the subject ; 
 but we may remark that, in the present perfect state of the engine, it appears 
 a thins almost endowed with intelligence. It regulates with perfect accuracy 
 and uniformity the number of its strokes in a given time, counting or record- 
 ing them, moreover, to tell how much work it has done, as a clock records 
 the beats of its pendulum ; it regulates the quantity of steam admitted to 
 work ; the briskness of the fire ; the supply of water to the boiler ; the 
 supply of coals to the fire ; it opens and shuts its valves with absolute pre- 
 cision as to time and manner ; it oils its joints ; it takes out any air which 
 may accidentally enter into parts which should be vacuous ; and when any 
 thing goes wrong which it cannot of itself rectify, it warns its attendants by 
 ringing a bell : yet with all these talents and qualities, and even when exert- 
 ing the force of hundreds of horses, it is obedient to the hand of a child ; 
 its aliment is coal, wood, charcoal, or other combustible ; it consumes none 
 while idle ; it never tires, and wants no sleep ; it is not subject to malady 
 when originally well made ; and only refuses to work when worn out with 
 age; it is equally active in all climates, and will do work of any kind; it 
 is a water-pumper, a miner, a sailor, a cotton-spinner, a weaver, a blacksmith, 
 a miller, &c., &c. ; and a small engine in the character of a steam pony, may 
 be seen draging after it on a rail-road a hundred tons of merchandize, or a 
 regiment of soldiers, with thrice the speed of our fleetest horse coaches. It 
 is the king of machines, and a permanent realization of the Genii of Eastern 
 fable, submitting supernatural powers to the command of man. 
 
 We need not wonder that the inventor of an engine having such qualities, 
 should be deemed deserving of the highest honours from his fellow-men. In 
 November, 1825, a public meeting was called, to vote a monument to WATT, 
 then not long deceased; and the most distinguished men of the empire, of 
 all parties, philosophers and statesmen, met to vie with each other in speak- 
 ing his praise. Perhaps a series of such eloquent discourses has rarely been 
 pronounced at one time ; but perhaps in the progress of the arts of civiliza- 
 tion there can rarely be offered such motive and occasion. The common 
 voice of that assembly scarcely exaggerated, when attributing to WATT'S 
 genius and perseverance that increase of our national commerce and riches, 
 which had enabled free Britian, single-handed, at an extraordinary crisis of 
 human affairs, to contend with Europe combined against her, and at last to 
 triumph, so as to secure her own happy destinies, and probably much to 
 influence those of the human race. 
 
 As science and the twin sister art are making constant advances, who 
 shall say that even the steam-engine, perfect as we have described it, forms 
 the limit to human discovery of mighty yet obedient force ? It is true that 
 the nature of steam, and the laws of its formation and action, are now so 
 well understood, that the intelligent engineer no more hopes for great 
 improvement in steam-engines, than he hopes for it in the mode of using 
 a waterfall to turn a mill ; but still there are kindred regions of nature left 
 almost unexplored. We shall have occasion to make a remark on this 
 subject in our chapter on the nature of heat. 
 
 The explosion of gunpowder and. of all fulminating mixtures bears so strong 
 an analogy to the phenomenon of the formation of steam, that the mind 
 may advantageously contemplate the subject in this place. 
 
 The ingredients of which gunpowder is formed are chiefly substances 
 
 13 
 
194 PNEUMATICS. 
 
 which, when separate, exist, at any common temperature, in the form of 
 air; and the combustion sets them loose, with a production of intense heat, 
 causing an increase of volume which is instantaneous, and almost irresistible. 
 By experiment and mathematical deduction, it appears that the exploding 
 particles begin to separate from each other with a velocity as if ten thou- 
 sand volumes of air had been condensed into one : and this explains the 
 corresponding force and swiftness with which a bullet is propelled. 
 
 All the fulminating metals are chiefly combinations of the like sub- 
 stances with the metals ; and the ingredients are held together by so slight 
 a tie, that a little friction or elevation of temperature disunites them so as 
 to produce the explosion. 
 
 The escape of condensed air from the chamber of an air-gun, is a species 
 of explosion ; but is very gentle compared with the shock of discharged 
 gunpowder. 
 
 It has lately been shown that a gun-barrel may be connected with a high- 
 pressure steam-boiler, in the same manner as with a chamber of condensed 
 air; and as the steam may be supplied as long as water remains in the 
 boiler, if bullets be allowed to fall into the barrel fast enough, a hundred or 
 more may be thrown out every minute, with the same force and precision 
 as if each issued from a common fire-arm. The rapid succession resembles 
 the issue of water from a jet pipe ; and if such an engine were used in a 
 field of battle, its barrel of death, made to point gradually along a line of 
 men, would mow them down like corn-stalks before the scythe none could 
 escape. The horrible idea and proposal have been excused by saying, that 
 to prove the possibility of such carnage must have the effect of putting an 
 end to war altogether. 
 
 The invention of gunpowder, with the consequent change of military 
 tactics, because it gave to a handful of men possessing it the mastery over 
 thousands who bad it not, was hailed by the philosophers of the day as a 
 certain security against the relapse of civilized mankind into such a state of 
 barbarism as followed the irruption into Europe of the Goths and Vandals : 
 none but well-instructed and disciplined armies could then enter a European 
 kingdom. This consideration, however, has lost its interest, since the in- 
 vention of printing, and other changes in society, have afforded still better 
 and more humane securities. 
 
 Besides the interesting instances above cited of the pressure of the atmos- 
 phere determining whether certain substances shall or shall not have 
 the form of air, there are others that deserve mention, where the effect is 
 modified by the mutual attraction of substances. 
 
 The pressure of the atmosphere at the surface of the earth keeps a cer- 
 tain quantity of air in combination with water, so as to form part of the 
 liquid mass. This air re-appears at once on taking off the pressure. If we 
 place a glass of water under the receiver of an air-pump and then exhaust 
 this, the water is soon crowded with bubbles of air, seen adhering to the 
 glass all round, or rising through the water. This admixture of air in 
 water is necessary to the life of fishes. It is driven off by boiling, and hence 
 the vapid taste of water that has recently been boiled. 
 
 In the making of beer, wine, and other fermented liquors, there is formed, 
 during the fermentation, a large quantity of the substance called carbonic 
 acid. Much of it flies off in its usual form of gas, but, because of the pres- 
 sure of the atmosphere, much still remains in union with the liquid. On 
 
PRESSURE AFFECTING TEMPERATURE. 195 
 
 removing this pressure suddenly, the liquid appears almost to boil, as when 
 a glass of warm beer is placed in the air-pump vacuum, 
 
 A degree of pressure still greater than that of the atmosphere keeps a pro- 
 portionably larger quantity of this carbonic acid in liquid combination; as in 
 bottled porter or sparkling champagne before the cork is drawn ; but as soon 
 as the compression maintained by the cork is removed, the gas escapes, caus- 
 ing the thin champagne to sparkle, and the more viscid beer, which retains 
 the little bubbles as they rise, to be covered with froth. After the spark- 
 ling or frothing has ceased under the atmospheric pressure, the phenomenon 
 may be renewed by placing the glass in the air-pump receiver. 
 
 Carbonic acid so rtadily becomes liquid when its attraction for water assists 
 the compression, that enough of it may be united with water to make a pint 
 become a pint and a half. The soda water, or aerated water, now so gene- 
 rally used as a drink in warm weather, is water with several times its bulk 
 of carbonic acid forced into it by pressure ; and a part of this is seen escaping 
 always at the instant of the confining cork being drawn. 
 
 Carbonic acid forms nearly half of the substance of marble or lime-stone. 
 "When an acid with stronger attraction, as vinegar or sulphuric acid, is 
 poured upon marble, it dispossesses the carbonic acid, and unites itself with 
 the pure lime. The carbonic acid in rising, constitutes the effervescence which 
 then appears. Carbonic acid, for the manufacture of the common soda water 
 and other aerated drinks, is obtained in this way. 
 
 Many mineral waters contain carbonic acid, which remains in tranquil 
 combination, while the water is bearing a certain pressure under ground, but 
 which in part escapes as soon as the water issues to the air and only the 
 atmospheric pressure remains : such waters are called sparkling waters. 
 
 The reason that champagne and aerated waters are so cool when first 
 decanted is, that the carbonic acid, in assuming its gaseous form, absorbs 
 as latent heat, a large proportion of the heat which was previously existing 
 in the liquid. 
 
 The atmospheric pressure, by making the density of the air in any place 
 dependant upon the height of the place above the level of the sea, causes 
 corresponding differences of temperature. 
 
 The explanation of this is simple. If a gallon of air, at the surface of the 
 earth, contain a certain quantity of heat, this must be diffused equally through 
 the space of the gallon; but if the air be then compressed into one-tenth of 
 the bulk, there will be ten times as much heat in that tenth as there was be- 
 fore ; the increase affecting the thermometer to an extent modified by circum- 
 stances explained in a future part of this work. In like manner, if by taking 
 off pressure, the gallon be made to dilate to ten gallons, the heat will be in 
 the same degree diffused, and any one part will be colder than before. It is 
 known that air may be so much compressed under the piston of a syringe, 
 that the heat in it, similarly concentrated, becomes intense enough to inflame 
 tinder attached to the bottom of the piston ; this means, under the name of 
 the match-syringe, being in common use for obtaining an instantaneous light. 
 
 Now, for the reason here explained, the air near the surface of the earth, 
 forming the bottom of the atmosphere, because condensed by the weight of 
 the air above it, is much warmer than if it were suddenly carried higher up, 
 to where, from the pressure being less, it would be more expanded or thin. 
 In many cases the height of mountains may be estimated by the difference of 
 temperature observed at the bottom and at the top. While a thermometer 
 
196 PNEUMATICS. 
 
 stands at 60 at the bottom of St. Paul's Cathedral, in London, another 
 marks only 58 at the top of the dome ; and in the lofty ascent of a balloon, 
 the thermometer soon falls to the freezing point and below it, the cold to 
 the aeronaut becoming almost insupportable. 
 
 In every part of the earth, at a certain elevation in the atmosphere, differ- 
 ent according to the latitude or proximity to the equator, the thermometer 
 never rises above the freezing point, and this limit in the atmosphere is 
 called the line or level of perpetual congelation. In Norway it is at five 
 thousand feet above the level of the sea ; in Switzerland at six thousand five 
 hundred; in Spain and Italy at seven thousand; farther south, at Teneriffe, 
 at nine thousand ; directly under the sun, as in central Africa, and among the 
 Andes in America, it is about fourteen thousand. We see therefore why the 
 snow-capt mountains are not the tenants only of high northern and southern 
 latitudes. In this effect of elevation which renders many of the tropical 
 regions of the earth not only tolerable abodes for man, but as suitable as any 
 others, contrary to the opinion of the ancient philosophers of Europe, who 
 accounted them by reason of the great heat, an everlasting barrier, as regard- 
 ed man, between the northern and southern hemispheres. Much of the 
 tropical land of America is so raised, that it rivals, as to agreeable tempera- 
 ture, even a European climate ; while the lightness and purity of the air, and 
 the brightness of the sun, add delightfully to its charms. The vast expanse 
 of table land forming the empire of Mexico is of this kind, enjoying the 
 immediate proximity of the sun, and yet, by its elevation of seven thousand 
 feet above the level of the ocean, possessing the most healthful freshness. 
 
 The land in many parts has the fertility of a cultivated garden, and can 
 produce naturally most of the riches which vegetation offers over the diver- 
 sified face of the globe. The plains of Columbia in South America, and 
 indeed all along the ridge of the Andes, are similarly circumstanced. The 
 contrast is very striking, after sailing a thousand miles up the level river 
 Magdelena, in a heat scarcely equalled on the plains of India, at once to 
 climb to the table-land above, where Saute Fe de Bogota, the capital of the 
 republic, is seen smiling over interminable plains, that bear the livery of the 
 fairest fields of Europe. 
 
 Persons not understanding the law which we are now illustrating, will 
 express surprise that wind or air blowing down upon them from a snow- 
 clad mountain, should still be warm and temperate. The truth is, that there 
 is just as much heat combined with an ounce of the air on the mountain top 
 ae in the valley; but above, the heat is diffused through a space perhaps 
 twice as great as when below, and, therefore, is less sensible. It may be the 
 same air which sweeps along as a warm gale on a plain at the foot of a moun- 
 tain, which then rises and freezes water on the summit and which in an 
 hour after, or less, is playing among the flowers of another valley, as warm 
 and genial as before. 
 
 As the temperature in different parts of the atmosphere depends thus 
 upon the rarity of the air, and therefore upon the height, the vegetable pro- 
 ductions of each distinct region or elevation are of a distinct character ; and 
 many other peculiarities of place and climate acknowledge the same cause. 
 
 Because the atmospheric pressure determines the temperature of the air in 
 different situations, as BOW explained, it has also a corresponding influence 
 upon the state of aerial humidity, which is modified by the temperature. 
 
 It was explained at page 184. that water and other liquids under a vacuum, 
 
PRESSURE AFFECTING TEMPERATURE. 197 
 
 rise in the form of air or vapour with force, and in quantity having a strict 
 relation to the temperature heat being in fact the cause of their rising; and 
 the table at page 186 exhibits the force, and therefore the density of watery 
 vapour corresponding to some certain temperatures. Now it is a remarkable 
 circumstance, that vapour in the same quantity and of equal tension rises 
 from any liquid, whether placed under the pressure of air, or under a vacuum ; 
 only through a space containing air it diffuses itself more slowly than if the 
 air were not present. As regards the former case, it was for a long time 
 supposed that the air dissolved a liquid as a liquid dissolves a salt: but it 
 now appears that there is merely a mechanical mixture of the two. If the 
 vapour, while rising from a liquid, has not a tension or elastic force equal 
 to the pressure of the atmosphere the process is tranquil, and is called 
 evaporation, and it goes on only as the vapour can diffuse itself among the 
 particles of the air, and therefore slowly in air perfectly quiescent, but quicker 
 as the air is moving more, or as the density of the air is less. But when 
 the vapour, owing to a greater heat, is strong enough to overcome the atmos- 
 pheric pressure of fifteen pounds per inch, and the weight of a certain quantity 
 of liquid over it, the phenomena of boiling arises as already described. 
 
 For the reason now explained, the air of our atmosphere contains diffused 
 through it a large quantity of invisible aeriform water; and if there were no 
 intestine motion, and no changes of temperature in the atmosphere, the 
 quantity of water would soon everywhere reach a maximum, or would be 
 the greatest that the temperature of the place could support ; instead of this, 
 however, from a variety of causes to be explained below, the air is moving 
 about constantly as winds, and the local temperatures are ever fluctuating, 
 and when the temperature is lowered, in situations where a maximum of 
 watery vapour is present, part of this is instantly reduced to the state of 
 water again, and appears, according to circumstance, in the form of mist, 
 rain, snow or hail; while to supply material for these phenomena, evapo- 
 ration is going on wherever, over water, there is not a maximum of vapour 
 in the air. These opposing operations of evaporation and condensation keep 
 up that constant circulation of moisture which is the life of nature. 
 
 When a given quantity of water assumes the aeriform state, it contains the 
 same quantity of latent heat in all cases, whether rising, for instance, from 
 a boiling caldron, or from the surface of the lake. Hence we see why evapo- 
 ration is so cooling a process to any liquid or moistened solid from which it 
 is arising : and as we have already shown that a rapid passing of dry air, or 
 the substance being placed in a vacuum, quickens evaporation, we now see 
 why both of these conditions accelerate the cooling. Wet linen placed in a 
 strong wind, which does not contain a maximum of moisture, becomes dry 
 almost immediately ; a bottle of wine ccTvered with a wet cloth and suspended 
 in a current of air, as is practised in warm climates to prepare wine for the 
 table is quickly cooled; mats hung around the walls of houses in India, and 
 frequently wetted through the day, preserve a delightful freshness in the 
 apartments. Sprinkling water or vinegar over a hot sick room cools and 
 refreshes it ; and watering the streets of a city moderates in them the inten- 
 sity of summer heat. In warm climates water is cooled for drinking by 
 being put into vessels so porous that the external surface is always moist, 
 the vessel being then suspended in a current of air, or during a calm being 
 made to vibrate in the manner of a pendulum. Again, the rapidity of evapo- 
 ration from water under the exhausted receiver of an air-pump, and particu- 
 larly when some other substance which powerfully absorbs watery vapour is 
 included in the receiver, is so rapid, and carries off the heat so quickly, that 
 
198 PNEUMATICS. 
 
 the mass of water freezes before much of it has been carried away. This 
 process is used for making ice in India. 
 
 It is partly because air saturated with moisture, that is to say, having 
 as much water diffused in it as can be supported in the invisible or aeriform 
 state of the existing temperature, lets fall a part on any reduction of the 
 temperature, that air which as a portion of the atmosphere, has been heated 
 by the sun during the day, and has received such moisture, lets it fall again 
 during the night, and exhibits the night fogs of certain seasons, which float 
 upon the surface of the earth, until again acted upon by the beams of the 
 next morning's sun. Fog, when farther condensed, by groups of the minute 
 particles uniting, forms rain; and rain when cooled becomes snow or hail. 
 
 The quantity of de\7 which falls at night is influenced by the quantity of 
 moisture taken up by the atmosphere during the heat of the day; and the 
 immediate cause of the dew is, as was ingeniously proved by Dr. Wells, 
 some years ago, that the temperature of the objects on which it settles has 
 become lower during the night than that of the air around, and than is 
 required to maintain in the invisible state, the moisture in the surrounding 
 atmosphere. There is a tendency in heat to diffuse itself uniformly among 
 bodies, by a constant radiation from one to another, rapid in proportion to 
 the differences of temperature, and which, if continued, would reduce all to 
 the same degree. The earth, therefore, during the day, receives radiated 
 heat from the sun, and becomes comparatively hot, and during the night it 
 gives out heat again by radiation towards the sky, from which there is little 
 or no return. When there are clouds in the atmosphere at night, they 
 receive the heat darted upwards from the bodies on the earth's surface, and 
 they radiate heat back, becoming thus, as it were, a clothing to maintain the 
 warmth of the earth beneath them, and on cloudy nights there is no dew, 
 but with a clear sky, the heat radiated upwards, darts into boundless space, 
 and is lost altogether to the objects which emitted it. These objects, there- 
 fore, which during the day had the same, or even a higher temperature than 
 the atmosphere around, now become colder, and the aeriform water which 
 comes in contact with them is condensed, and forms what we call dew. This 
 beautiful provision of nature supplies the necessary moisture to vegetables 
 during seasons when rain is deficient. Dew on very cold objects freezes as 
 it settles, and is then called hoar frost. A phenomenon which may be 
 classed with dew, is the perspiration, as it is vulgarly called, of massive 
 walls and furniture, occurring on the sudden setting in of warm weather, or 
 on the occasion of a warm moist air of higher temperature than the walls 
 being suddenly introduced, as when a crowd assembles in a cold church : 
 the wall or other object then, from not having yet acquired the temperature 
 of the surrounding air, condenses upon itself a copious deposition of the 
 atmospheric moisture. For a similar reason a bottle of wine brought from a 
 cold cellar or from an ice-pail, into a room with company, is soon covered 
 with thick moisture or dew; as are the glasses also into which the wine is 
 poured. It is another phenomenon of the same kind when we see the 
 moisture of warm breath condensed on any cold polished surface, as on a 
 mirror's face, or on the glasses of a carriage shut up, or on the windows of 
 a room in winter, when the surface is very cold, the moisture being frozen 
 with the appearance of beautiful aboresence. 
 
 Many instruments have been contrived, with the name of hygrometers, 
 for indicating the quantity of water in the atmosphere. A prepared human 
 hair is the essential part of one of the best of those formerly used; the 
 lengthening or shortening of the hair, according to the quantity of moisture 
 
PRESSURE AFFECTING MOISTURE. 199 
 
 around it, being caused to move an index like that of a wheel-barometer, to 
 mark the degrees. This, however, and other common hygrometers, are only 
 philosophical toys; but Mr. Daniel (see his excellent work, entitled Meteoro- 
 logical Essays) has lately given to the philosophical world a correct and 
 simple instrument for .the purpose, depending- on the principle explained 
 above, that whenever the temperature of a body in the atmosphere is 
 reduced below that at which the quantity of watery vapour in the air around 
 it can be maintained in the aeriform or invisible state, dew forms on the 
 body. His apparatus consists of a bulb of glass, which can be cooled to any 
 desired degree from being connected with another bulb enveloped in an 
 evaporating liquid; and when moisture begins visibly to settle upon the first, 
 its temperature is exhibited on a thermometer enclosed within it; and the 
 proportion of water mixed with the air around is then^, as indicated by the 
 table, partially copied here, at page 186. 
 
 A great fall of the barometer marks a diminished pressure in the atmos- 
 phere around, with a consequent dilatation of the air and fall of temperature, 
 as explained a few pages back ; and if the air at such a time hold a maxi- 
 mum of moisture, a part of this must become visible as fog or rain. Thua^ 
 a fall of the barometer, a fall of temperature, and a fall of rain, often occur 
 as associated phenomena. 
 
 Illustrating this by experiment, we find, that on the extraction of air from 
 the receiver of an air-pump, a cloud of mist generally appears in it with the 
 first strokes of the piston : the reason being that the still remaining air, 
 because cooled by the refraction, absorbs heat from the vapor in combina- 
 tion with it, and renders the water visible. The mist is then removed by 
 the subsequent action of the machine, or is re-dissolved when the usual 
 quantity of air is re-admitted. 
 
 We understand from this why rain happens so much more frequently 
 among mountains than on extended plains. When air saturated with mois- 
 ture approaches the mountain ridge to rise over it. for every foot that it rises 
 it escapes from a degree of the pressure which it bore while lower down, 
 and in then dilating, it becomes colder, and lets fall part of its moisture. 
 It is the rain copiously thus produced in mountainous regions which consti- 
 tutes the chief supply of their many rivers, and which, with periodical 
 changes of wind bringing more moisture, causes the extraordinary annual 
 overflowing of such rivers as the Nile, the Ganges, &c. 
 
 Those who have visited the Cape of Grood Hope, will recollect a striking 
 phenomenon illustrative of our present subject, observed there when the wind .. 
 blows from the south-east. Beyond the city, as viewed from the bay, there 
 is a mountain of great elevation, called from its extended flat summit, the 
 Table Mountain. In general its rugged steeps are seen rising in a clear sky ; 
 but when the south-east wind blows, the whole summit becomes enveloped 
 in a cloud of singular density and beauty. The inhabitants call the pheno- 
 menon the spreading of the table-cloth. The cloud does not appear to be at 
 rest on the hill, but to be constantly rolling onward ; yet to the surprise of 
 the beholder, it never descends, for the snowy wreaths seem falling over the 
 precipice towards the town below, vanish completely before they reach it, 
 while others are formed on the other side to replace them. The reason of 
 the phenomena is this : The air constituting the wind from the south-east 
 having passed over the vast southern ocean, comes charged with as much 
 invisible moisture as its temperature can sustain. In rising up the side of 
 the mountain it is rising in the atmosphere, and is therefore gradually escap- 
 ing from a part of the pressure lately borne ; and on attaining the summit it 
 
200 PNEUMATICS. 
 
 has dilated so much, and has consequently become so much colder, that it 
 lets go part of its moisture. This then appears as the cloud just described; 
 but it no sooner falls over the edge of the mountain, and again descends in 
 the atmosphere to where it is pressed, and condensed, and heated as before, 
 than it is re-dissolved and disappears : the magnificent apparition dwelling 
 only on the mountain top. 
 
 When the elevation to which moisture is suddenly carried is very great, 
 the fall of temperature is proportioned, and the separating water becomes 
 snow instead of rain. This phenomenon is remarkably illustrated by a great 
 Hiero's fountain, used in one of the mines of Hungary ; during the play of 
 which, the air in one place is so compressed, that on being suddenly released 
 it expands and cools enough to cause the moisture driven out with it to 
 appear, even in summer, as a shower of snow. . 
 
 The foregoing reasoning explains why, along the sides o/ mountain ridges, 
 clouds are generally seen floating at a certain height only, and therefore in 
 horizontal strata. The water is separated from the air at a certain tempera- 
 ture, which is dependent on the height, and above that height the air is at 
 the time too dry and rare to have clouds. Very lofty summits are always 
 seen much above the clouds, and the admirer of nature who climbs towards 
 them, may often contemplate the grand phenomena of the thunder-storm 
 far beneath his feet. Teneriffe soars so sublimely, that the distant sailor 
 not unfrequently mistakes the line of clouds hanging around its sides for 
 the white streak which elsewhere indicates the cliffs and waves of the 
 sea-shore. 
 
 fluid support or floating in air. (Read the Analysis, page 156.) 
 
 When it was explained under " Hydrostatics," that any body immersed in 
 a fluid has its downward tendency or weight resisted with exactly the force 
 which supported the quantity of the fluid previously occupying the same 
 space, and therefore that the body will sink or swim, according as it is 
 heavier or lighter than its bulk of the fluid, the reasoning was as applicable 
 to the case of a body immersed in an air or gas as in a liquid. 
 
 We hence see why a body weighed in an air appears lighter, by the exact 
 weight of its bulk of the air, than when weighed in an empty space or 
 vacuum ; and why, for the same reason, the jocular question, whether a 
 pound of lead or a pound of cork be the heavier, it is not truly answered by 
 saying that they are of equal weight; the cork being really the heavier, for 
 when balanced in air, bulky cork is more supported than dense lead. A 
 small weighing beam having attached to its opposite ends pieces of cork and 
 lead which equipoise in the air, if placed under the exhausted receiver of an 
 air-pump, quickly exhibits the cork preponderating. 
 
 As any liquid lighter than water, such as oil or spirits, on being set at 
 liberty under the surface of water, will rise, while any heavier liquid, such 
 as brine, syrup, or sulphuric acid, will sink ; and in both cases with force 
 proportioned to the difference of specific gravities : so we find that in com- 
 mon air, a mass of hydrogen, or hotter air, ascends, because specifically 
 lighter ; while oxygen, carbonic acid gas, or colder air, descends, because 
 specifically heavier. This truth is well exemplified in 
 
 The Balloon, - 
 
 which is a thin light bag of varnished silk, generally shaped like a globe or 
 egg, and filled with a fluid lighter than common air. It is made sufficiently 
 
FLOATING. BALLOONS. 201 
 
 large that the difference between its weight when filled and that of an equal 
 bulk of common air, may enable it to carry aloft the material of which it is 
 constructed, with the aeronauts, and their apparatus. It is in principle like 
 a bladder of oil immersed in water. A globe of thirty -five feet diameter 
 has a capacity of nearly twenty-two thousand cubic feet. This quantity of 
 common air weighs cft>out sixteen hundred pounds^ and the same quantity 
 of hydrogen gas, of easily obtained purity, weighs only one-eighth as much 
 as two hundred pounds. Such a globe, therefore, being buoyed up, or 
 supported in common air, with a force of sixteen hundred pounds, while, if 
 filled with hydrogen, it only weighs two hundred, will carry up into the sky 
 founteen hundred pounds of material and load. 
 
 The first balloon was exhibited by a man ignorant of what he was really 
 effecting. Seeing the clouds float high in the atmosphere, he thought thac 
 if he could make a cloud and enclose it in a bag, it might rise and carry 
 him with it. Then, erroneously deeming smoke and a cloud the same, he 
 made a fire of green wood, wool, &c., and placed a great bag over it with the 
 mouth downwards to receive the smoke. He soon had the joy to see the 
 bag full, and, when set free, ascending ; but he understood not that the 
 cause was the hot and dilated air within, which, being lighter than the sur- 
 rounding air was buoyed up; while the visible parts of the smoke, which 
 chiefly engaged his attention, was really heavier than the air, and was an 
 impediment to his wishes. 
 
 This modification called the hot air or fire balloon, was afterwards better 
 understood, and was used by aeronauts, until the more commodious and less 
 dangerous modification, called the inflammable air balloon, or balloon of 
 hydrogen gas, was substituted. 
 
 Since the modern introduction of gas lights, the carburretted hydrogen 
 prepared for them is generally employed for filling balloons. It is con- 
 siderably heavier than pure hydrogen, but is so much more readily obtained, 
 that aeronauts like better to make a larger balloon to suit it, than a smaller 
 one which obliges them to prepare the other. A thin paper bag, filled with 
 the hot air rising from a large lamp, is a miniature hot air or fire balloon; 
 and a common soap bubble, filled with hydrogen is a little inflammable air 
 balloon, which mounts with great rapidity. 
 
 There are, perhaps, few occasions on which a youth is more surprised and 
 delighted than when he first beholds a balloon sailing high in the bosom of 
 the air and bearing a human being to regions far beyond what the soaring 
 eagle has ever reached; while to the intrepid aeronaut himself, the scene of 
 a world displayed beneath him is unquestionably the grandest, except that 
 of the starry heavens, which mortal eye has ever compassed. To him even 
 wide spread London, the queen of the cities of the earth, and a little world 
 within itself, when viewed from a great elevation in the sky, appears but 
 as a dusky patch upon a map, with the far-famed Thames winding there 
 as a silvery line, and the magnificent temples and palaces scattered around 
 appearing but as darker points rising out of the general mist of buildings, 
 in which a million and a half of human beings reside. 
 
 The first aeronautic expeditions astonished the world, and endless reveries 
 passed through mens' minds of important uses to which the new discovery 
 might be applied ; but more mature reflection, and now frequent trials have 
 shown that the balloon, while furnishing philosophers with the opportunity 
 of making some observations in elevated regions of the atmosphere, is still 
 interesting chiefly as a philosophical toy. The French, under the Directory 
 
202 PNEUMATICS. 
 
 in 1796, attempted to use it as a military station, from which the position 
 and motions of an enemy might be descried : but the plan was eventually 
 abandoned. It has since been thought of as a means by which travellers 
 might obtain information while penetrating into unknown countries, like the 
 almost interlineable plain of Australasia. Although aeronauts, while aloft, 
 have the power of making the balloon rise farther by*tb rowing out part of 
 the sand-ballast which they carry with them, or of making it descend by 
 opening a valve at the top, through which the hydrogen may escape, still 
 they have no power of producing a lateral motion. The idea which yet 
 strongly excites the minds of some projectors, that by wings or other means, 
 a balloon may be directed in the sky nearly as a ship is directed on thesea, 
 is not much more reasonable than to suppose that an insect, suspended to a 
 huge block of wood, driven along at the rate of eight or ten miles an hour 
 by river torrent, should have power to stop or sail against the stream. A 
 man in a balloom would generally have to resist or change a motion exceed- 
 ing fifty miles in an hour. 
 
 A balloon which is only half full at the surface of the earth, becomes 
 quite full when it has risen three miles and a half, because, at that altitude, 
 air from below doubles its volume on account of the diminished pressure. 
 A balloon, therefore, if quite distended on first rising, must let air escape 
 as it ascends, or it will burst : this is true also of the drum of the human ear 
 under the same circumstances, and in a contrary way under the opposite 
 circumstances of descending in a diving-bell. 
 
 The downy seeds of plants seen floating about upon the winds of autumn 
 are not lighter than air, but have so much bulk and surface in proportion to 
 their weight, that the friction upon them of the moving air, is greater than 
 their weight, and carries them along. 
 
 A sheet of paper made in some degree to resemble a balloon, by its having 
 a little weight, representing the hanging car, attached by threads from its 
 angles, is often seen rising at a street corner, to the delight of the boy who 
 watches it. Its rise depends upon eddy winds or currents which the corner 
 produces. 
 
 The ascent ofjlame and smoke 
 
 in the atmosphere, affords other examples of a lighter fluid rising in a 
 heavier ; for both these are merely hotter air rising in the midst of colder. 
 
 The phenomenon of flame is produced when a burning substance contains 
 some ingredient capable, on being heated, of assuming the form of air or gas, 
 which ingredient, or ascending, burns or combines with the oxygen of. the 
 atmosphere, with intensity of action sufficient to produce a white heat. It 
 is because charcoal and coke have nothing in them thus volatile, that they 
 burn without flame, appearing like red-hot stones. The flame of a lamp or 
 candle is merely the oil, wax, or tallow converted into gas, and allowed to 
 burn as it is disengaged and rises. The same gas obtained by heating the 
 oil, &c., in vessels which exclude the atmosphere, so as to prevent immediate 
 combustion, and from which tubes lead to suitable receptacles, is the common 
 oil-gas used for illumination. 
 
 Smoke consists of all the dust and visible particles which are separated 
 from the fuel without being burned, and are, moreover, light or minute 
 enough to be carried aloft by the rising current of heated air ; but all that is 
 visible of smoke is really heavier than air, and soon falls again as powdered 
 chalk falls in water. In the receiver of an air-pump, where a candle has 
 been extinguished by exhausting the air, the steam of smoke that continues 
 
FLOATING. FLAME AND SMOKE. 203 
 
 to pour from the wick after the exhaustion, is seen to fallen the pump-plate, 
 because there is no air to support it. 
 
 Chimneys quicken the ascent of hot air by keeping a long column of it 
 together. A column of two feet high rises, or is pressed up with twice as 
 much force as a column of one foot, and so in'proportion for all other lengths j 
 just as two or more corks strung together and immersed in water, tend up- 
 wards with proportionally more force than a single cork ; or as a long spear of 
 light wood, allowed to ascend perpendicularly from a great depth in water, 
 acquires a velocity which makes it dart above the surface, while a short piece 
 under the same circumstances rises very slowly. In a chimney where one 
 foot in height of the column of hot air is one ounce lighter than the same 
 bulk of the external cold air, if the chimney be one hundred feet high, the 
 air or smoke in it is propelled upwards with a force of one hundred ounces. 
 In all cases, therefore, the draught, as it is called of a chimney, is pro- 
 portioned to its length. The following facts are consequences of this truth. 
 
 In. low cottages, and in the upper floors of houses, the annoyance of 
 smoky rooms is much more frequent than were chimneys are longer. 
 
 If there are two fires in the same room, or in any rooms open to each 
 other, which have chimneys of different lengths, and of which the doors and 
 windows are very close, so that the air to supply the draught cannot enter 
 by them, the taller chimney will overpower the shorter, and cause it to 
 smoke into the room ; just as the long leg of a syphon overcomes the short 
 one, or as a long log of wood, held down in water by a cord passing from it 
 round a pulley at the bottom to a shorter log also floating, will rise, and 
 pull down the shorter log. | 
 
 A long chimney, for the reasons above explained, causes a current of 
 air to pass through the fire very rapidly, and it has the advantage also of 
 acting more uniformly than any bellows or blowing machine. On these 
 accounts, of fires of steam engines, and many others, it is the means of 
 blowing generally preferred. The importance of length, in a chimney 
 explains the remarkable appearance of some mining districts and modern 
 English towns, where steam-engines abound. 
 
 When we heap dying embers together, so that the hot air rising among 
 them may become a mass or column of considerable altitude, this column 
 has the effect of blowing them gently, and helps to light them up .again, 
 A piece of burning paper thrown upon the top of a half-extinguished fire, 
 often makes it blaze afresh, by causing a more rapid current of air to pass 
 through it from below. 
 
 The action or draught of a chimney, influenced as we have seen, by its 
 length, depends also on the degree in which the air in it is heated, because 
 this determines the dilitation, or comparative lightness, which makes the 
 air ascend. 
 
 In what are called open fire-places, such as those in the sitting-rooms of 
 Britain, a large quantity of air directly from the apartment enters the chimney 
 above the fire, and mixes with the hot air from the fire itself. This mixture 
 ascends more slowly than if hot air alone entered, and in a proportion 
 dependent on the degree of mixture. The effect of excluding a part of this 
 colder air, is seen when a board or plate of metal is suspended across the 
 opening of the chimney, so as to narrow the entrance : almost instantly a 
 quicker action is produced, and the fire begins to roar as if blown by a bel- 
 lows. This means is often used to blow the fire instead of bellows, or to 
 cure a smoky chimney by increasing the draught. What is call a register 
 
204 PNEUMATICS. 
 
 stove is a kindred contrivance. It has a flap placed in the throat of the chim- 
 ney, which serves to widen or contract the passage at pleasure. Because the 
 flap is generally opened only enough to allow that air to pass which rises 
 directly from the fire, the chimney receives only very hot air, and therefore 
 acts well. The register stove often cures smoky chimneys : and by preven- 
 ing the too ready escape of the moderately warmed air of the room, of which 
 so much is wasted by a common fire-place, it also saves fuel. In what are 
 called close fire-places, as those of steam-engines, or brewers' coppers, when 
 the furnace door is shut, no air can enter the chimney but directly through 
 the fire ; hence the action of such chimneys is very powerful. 
 
 In a room with two fires, or in drawing rooms communicating with each 
 other, although the chimneys be of equal length, that one over the best fire 
 will act the most strongly ; and if the doors and windows of the apartment be 
 so close as to prevent a sufficiency of air from entering by them to supply 
 both fires, cold air will enter by that chimney which has the weakest fire, and 
 the smoke from it will spread into the room. How often is an assembling 
 dinner party annoyed by the smoke of a second drawing-room fire just lighted 
 before their arrival, and which had therefore to contend with the. antagonist 
 fire already in powerful action all the day. "While only one fire was lighted, the 
 cold chimney was admitting the air to feed it, just as an open pane in the 
 window would have done. A room may be so close that no air can find en- 
 trance, and in such a case the smoke of its fire must all spread into the room. 
 When all the windows and doors of a house fit so closely as not to admit 
 air for the acting chimneys, the supply comes down the chimneys that are not 
 in use. Inattention to this fact causes many a good chimney to incur the 
 imputation of being smoky, because on the attempt being made to light a 
 fire at it, the smoke at first is always thrown back. The truth is, that at 
 the time when the servant begins to light the fire, there is a downward cur- 
 rent in the chimney, repelling of course, any heated air and smoke that 
 approaches it, and spreading them over the whole house; but were the 
 room door to be shut for a few minutes, so as to cutoff communication with 
 the other drawing chimneys in the house, while at the same time the windows 
 were opened, the chimney would act at once ; and when sufficiently heated, 
 would continue to act in spite of the others, as well as they. 
 
 There are some cases of smoky rooms not to be so easily corrected as what 
 we have now mentioned. When a low house adjoins a lofty house, the wind 
 blowing towards the latter, is obstructed and becomes a gathering or conden- 
 sation of air against the wall ; and if the top of a low chimney be there, the 
 compressed air enters it and pours downwards. The same happens occasionally 
 from the proximity of trees or rocks. In such cases, to avoid the influence, 
 the chimneys of the low houses are often made very lofty. Again, whenever, 
 from the nature of buildings, eddies of wind occur, or unequal pressures, as 
 at street corners, &c., the chimneys around do not act regularly. It is pro- 
 verbial, that corner houses, or those at the end of a row, are smoky houses ; 
 and we see the uniformity of architecture in a street often destroyed by the 
 necessity of lengthening the chimneys of the houses at the extremities. 
 
 When smoke is found descending into a room where there is no fire, the 
 empty chimney is serving as an inlet for air to the house, while the smoke 
 of a neighboring chimney is passing closely over the top of ijb. 
 
 In summer, when fires are not in use, there is often a strong smell of soot 
 perceived in the apartments during the whole of the day, but which ceases 
 at night. The reason is, that during the day the chimney is colder than the 
 external air, and by condensing the air which enters it, causes a downward 
 
FLOATING. FLAME AND SMOKE. 205 
 
 current through the soot. During*he night, again, when the external air 
 becomes colder, owing to the absence of the sun, the chimney, by retaining 
 the heat absorbed during the day, is hot enough to warm the air in it, and 
 to cause an upward current. These currents, in chimneys left open during 
 the days and nights of summer, are almost as regular as the land and sea 
 breezes of tr6pical countries. 
 
 All these remarks prove how important it is to be able to conceive clearly 
 of the motions going on, according to the simple laws of matter, in the 
 invisible air around us. Were such subjects better and more generally 
 understood, many prevalent errors in the arts of life, influencing much the 
 comforts and health of the community, would soon be corrected. 
 
 If we are filled with admiration on discovering how perfectly the simple 
 law of a lighter fluid rising in a heavier, provides a constantly renewed sup- 
 ply of fresh air to our fires, which supply we should else have to furnish by 
 the unremitted action of some expensive blowing apparatus, still more must 
 we admire that the operation of this law should effect the more important 
 purpose of furnishing the ever renewed supply of the same vital fluid to 
 breathing creatures. The air which a man has once respired becomes poison 
 to him ; but because the temperature of his body is generally higher than 
 that of the atmosphere around him, as soon as he has discharged any air 
 from the lungs, it ascends completely away from him into the great purify- 
 ing laboratory of the atmosphere, and new air takes its place. No art or 
 labor of his, as by the use of fans or punkas, could have done half so well 
 what this simple law unceasingly and invisibly accomplishes, and accom- 
 plishes without effort or even attention on his part, and in his sleeping as in 
 his waking hours. Truly in this, may he be said to be watched over by a 
 kind Providence. 
 
 The warming and ventilating of houses, 
 
 is an important art, founded chiefly on the foregoing considerations, and at 
 present too little understood, not only by the public at large, but even by 
 medical practitioners, whose management of disease, though judicious in 
 other respects, is often rendered vain by error or omission in this. 
 
 Excellent fuel is so cheap in Britain, owing to the profusion which 
 beds of rich coal are scattered in it, that a careless domestic expenditure has 
 arisen ; which, however, instead of securing the comfort and health that 
 might be expected, has led to plans of warming which often prove destruc- 
 tive to both. The mischief lies chiefly in the unsteadiness or fluctuations of 
 our domestic temperature ; for in still colder countries, and where fuel is 
 more expensive, as in the north of continental Europe, the necessity for 
 economy has led to contrivances which give steady temperature and impunity. 
 
 In cold countries, to retain and preserve the heat once obtained, the houses 
 are made with thick walls, double windows, and nice fittings ; and, more- 
 over, with close stoves or fire-places, which draw their supply of air, not 
 from the apartments where they are placed, wasting the temperate air of 
 these, but directly from without. Thus fuel is saved to a great extent, and 
 a uniformity of temperature is produced, both as regards the different parts 
 of the room, so that the occupiers may sit with comfort where they please, 
 and as regards the different times of the day, for the stove being once heated 
 in the morning, often suffices to maintain a steady warmth until night. 
 The temperature can be carried to any required degree, 'and sufficient 
 ventilation is easily effected. 
 
 In England, again, the apartments, with their open chimneys, may be 
 
206 PNEUMATICS. 
 
 compared to great air funnels, constancy pouring out their warm contents 
 through a large opening, and constantly requiring to be replenished. They 
 thus waste fuel exceedingly, because the chimney being large enough to 
 allow a whole room-full of air to pass away in two or three minutes, the air 
 of the room has to be warmed, not once in the course of the day, but very 
 many times. The temperature in them is made to fluctuate by the slightest 
 causes, as the opening a door, the omitting to stir the fire, &c. The heat is 
 very unequal in different parts of the room, rendering it necessary in general 
 for the company to sit near the fire ; where they must often submit to be 
 almost scorched on one side, while they are chilled on the other. There is 
 generally a warm stratum of air above the level of the chimney-piece, sur- 
 rounding, therefore, the upper part of the bodies of persons in the room, while 
 a cold stratum below envelopes the sensitive feet and legs. As a very rapid 
 current is constantly ascending in the chimney, a corresponding supply must 
 be entering somewhere ; and it can only enter by the crevices and defects 
 in the doors, windows, floors, &c. : now there is nothing more dangerous 
 to health than to sit near such inlets, as is proved by the rheumatisms, stiff 
 necks and catarrhs, not to mention more serious diseases, which so frequently 
 follow the exposure. There is an old Spanish proverb, thus translated, 
 
 " If cold wind reach you through a hole, 
 Go make your will and mind your soul," 
 
 which is scarcely an exaggeration. 
 
 Consumption is the disease which carries off a fifth or more of the persons 
 born in Britain; owing in part, no doubt, to the changeableness of the exter- 
 nal climate, but much more to the faulty modes of warming and ventilating 
 the houses. To judge of the influence of temperature in producing this disease 
 we may consider, that miners who live under ground, and are always, 
 therefore, in the same temperature, are strangers to it, while their brothers, 
 and relatives, exposed to the vicissitudes above, fall victims, that butchers 
 and others, who live almost constantly in the open air, so as to be hardened 
 by the exposure, enjoy nearly equal immunity, that consumption is scarcely 
 known in Russia, \ihere close stoves and houses preserve a uniform tempera- 
 ture within doors, while fit clothing gives safety on going out, and that in 
 all countries and situations, whether tropical, temperate or polar, the fre- 
 quency of the disease bears relation to the degree and manner of change. 
 We may here remark, also, that it is not consumption alone which springs 
 from changes of temperature, but a great proportion of acute diseases, and 
 particularly of the common winter diseases of England. There are a few 
 cases of these in which the invalid has not to remark, that if he had avoided 
 cold or wet on some certain occasion, he might yet have been well. 
 
 Wnile temperature is thus so frequently an original cause of disease, it is 
 also a circumstance of the very highest importance in the treatment, as is 
 proved by every fact bearing upon the question. We may, therefore, at first 
 wonder that it should be so negligently and unskilfully controlled as we often 
 see it 'j disease and death being thence allowed to lurk, as it were, undis- 
 turbed in the sanctuaries of our homes : but when we reflect on the subtile 
 and invisible nature of air and heat, and that the science which detects their 
 agencies has been hitherto so little an object of general study, and is, indeed, 
 of modern discovery, the fact is accounted for. 
 
 In England', the open fire-place is so generally in use for common dwell- 
 ings, and the cheerful 'blaze is accounted so essential to the comforts of the 
 winter days and long evenings, that it would be difficult to persuade persons 
 
FLOATING. WARMING AND VENTILATING. 207 
 
 to abandon it : let us hope, then, that when the subject which we are now 
 discussing comes to be better and more generally understood, the open fire, 
 with close flooring, better for double windows, doors that fit well, register 
 stoves, and good general management, may be rendered almost as efficient 
 for warming, and as safe to health, as any other contrivance. 
 
 The following considerations present themselves in this place: Small 
 rooms in winter are more dangerous to health than large ones, because the 
 cold air, entering towards the fire by the doors and windows, reaches the per- 
 sons in the room before it can be tempered by mixing with the warmer air 
 already around them. Stoves in halls and stair-cases are useful, because 
 they warm the air before it enters the rooms ; and they prevent the hurtful 
 chills often felt on passing through a cold stair-case from one warm room to 
 another. It is important to admit no more cold air into the house than is 
 just required for the fires and for ventilation ; hence there .is a great error 
 in the common practice of leaving all the chimneys that are not in use quite 
 open, each admitting air as much as a hole in the wall, or an open pane in 
 the window would do. Perhaps the best mode of admitting air to feed the 
 fires is through tubes, leading directly from the outer air to the fire-place, and 
 provided with what are called throttle-valves, for the regulation of the quan- 
 tity ; the fresh air admitted by them being made to spread in the room 
 either at once, or after having been warmed during its passage inwards, by 
 coming near the fire. In a very close apartment, ventilation must be ex- 
 pressly provided for by an opening near the ceiling, through which the impure 
 air, rising from the respiration of the company, may pass away. With an 
 open fire the purpose is effected, although less perfectly, by the frequent 
 change of the whole air of the room which that construction occasions. 
 
 With a view to have, in rooms intended for invalids, the most perfect 
 security against cold blasts and fluctuations of temperature, and still to retain 
 the so much valued appearance of the open fire, a glazed frame or window 
 may be placed at the entrance to the chimney or stove, so as completely to 
 prevent the passage of air from the room to the fire. The room will then be 
 warmed by the fire through the glass, nearly as a green-house is warmed by 
 the rays of the sun. It is true that the heat of combustion does'not pass 
 through glass so readily as the heat of the sun ; but the difference for the 
 case supposed is not important. The glass of such a window must, of 
 course, be divided into small panes, and supported by a metallic frame-work 
 to resist the heat ' } and there must be a flap or door in the frame- work, for 
 the purpose of admitting the fuel and stirring the fire. Air must be supplied 
 to the fire, as described above, by a tube leading directly from the external 
 atmosphere to the ash-pit. The ventilation of the room may be effected by 
 an opening into the chimney near the ceiling; and the temperature may be 
 regulated with great precision. by a valve placed in this opening, and made 
 to obey the dilatation and contraction of a piece of wire affixed to it, the 
 length of which will always depend on the temperature of the room. The 
 author contrived the arrangements here described, for the winter residence of 
 a person threatened with consumption, and the happy issue of that particu- 
 lar case, and of others treated on similar principles, has led him to doubt 
 whether many of the patients with incipient consumption who are usually 
 sent to warmer climates, and who die there after suffering hardships on the 
 journey, and distress from the banishment sufficient to shake even strong 
 health, might not be saved by judicious treatment in properly warmed and 
 ventilated apartments, under their own roofs, and in the midst of affectionate 
 kindred. And if a boy be almost certainly secured from consumption by 
 
208 PNEUMATICS. 
 
 being made a miner or a butcher, may we not hope that, when all the influ- 
 encing circumstances come to be better understood, something of the same 
 immunity may be obtained for persons in all the professions- and conditions 
 of civilized society ? 
 
 It must not be supposed that the remarks made in this section exhaust 
 even nearly the very important subject of temperature as affecting health. 
 The questions of clothing, of hot and cold bathing, of exercise, and others, 
 equally belong to it, but the consideration of them falls under other depart- 
 ments of study. 
 
 Winds or currents in the atmosphere 
 
 are also phenomena, in a great measure dependent on the law, that lighter 
 fluids rise in heavier. As oil let loose under water is pressed up to the 
 surface and swims, so air near the surface of the earth, when heated by the 
 sun, rises to the top of the atmosphere, and spreads there, forced up by the 
 heavier air around; this heavier air rushing inwards, constitutes the wind 
 felt at the surface of the earth. The cross currents in the atmosphere arising 
 as now described, are often rendered evident by the motion of clouds or 
 balloons. 
 
 If our globe were at rest, and the sun were always beaming over the same 
 part, the earth and air directly under the sun would become exceedingly 
 heated, and the air there would be constantly rising like oil in water, or like 
 the smoke from a great fire ; while currents or winds below would be pour- 
 ing towards the central spot, from all directions. But the earth is constantly 
 turning round under the sun, so that the whole middle region or equatorial 
 belt may be called the sun's place : and therefore, according to the principle 
 just laid down, there should be over it a constant rising of air, and constant 
 currents from the two sides of it, or the north and south, to supply the 
 ascent. Now this phenomenon is really going on, and has been going on 
 ever since Jthe beginning of the world, producing the steady winds of the 
 northern and southern hemispheres, called trade winds, on which in most 
 places within thirty degrees of the equator, mariners reckon almost as con- 
 fidently as on the rising and setting of the sun himself. 
 
 The trade winds, however, although thus moving from the poles to the 
 equator, do not appear on the earth to be directly north and south, for the 
 eastward whirling, or diurnal rotation of the earth, causes a wind from the 
 north to appear as if coming from the north-east, and a wind from the south 
 as if coming from the south-east. This fact is illustrated by the case of a 
 man on a galloping horse, to whom a calm appears to be a strong wind in his 
 face ; and if he be riding eastward, while the wind is directly north or south, 
 such wind will appear to him to come from the north-east, or south-east : 
 or again, is illustrated by the case of a small globe made to turn upon a per- 
 pendicular axis, while a ball or some water is allowed to run from the top of 
 it downwards; the ball or water will not immediately acquire the whirling 
 motion of the globe, but will fall almost directly downwards, in a track 
 which, if marked upon the globe, will appear not as a direct line from the 
 axis to the equator, that is, from north to south, but as a line falling obliquely. 
 Thus, then, the whirling of the earth is the cause of the oblique and west- 
 ward direction of the trade winds ; and not, as has often been said, the sun 
 drawing them after him. 
 
 The reason why the trade winds at their external confines, which are about 
 30 degrees from the sun's place, appear almost directly east } and become 
 
FLOATING. WINDS. 209 
 
 more nearly north and south as they approach the central line, is, that at the 
 confine they are like 'fluid coming from the axis of a turning wheel, and which 
 has approached the circumference, but has not yet acquired the velocity of 
 the circumference ; while, nearer the line, they are like the fluid after it has 
 for a considerable time been turning on the circumference, and has acquired 
 the rotary motion there, consequently appearing at rest as regards that 
 motion, but still leaving sensible any motion in a cross direction. 
 
 While, in the lower regions of the atmosphere, air is thus constantly 
 flowing towards the equator and forming the steady trade winds between 
 the tropics in the upper regions, there must, of course, be a counter-current 
 distributing the heated air again over the globe : accordingly, since reasoning 
 led men to expect this, many striking proofs have been detected. At the 
 summit of the Peak of Teneriffe, observations now show that there is always 
 a strong wind blowing in a direction contrary to that of the trade wind on 
 the face of the ocean below. Again, the trade winds among the West India 
 Islands are constant, yet volcanic dust thrown aloft from the Island of St. 
 Vincent, in the year 1812, was found, to the astonishment of the inhabitants 
 of Barbadoes, hovering over them in thick clouds, and falling, after corning 
 more than 100 miles directly against the strong trade wind, which ships 
 must take a circuitous course to avoid. Persons sailing from the Cape of 
 Grood Hope to St. Helena, have often to remark that the sun is hidden for 
 days together, by a stratum of dense clouds passing southward high in the 
 atmosphere } which clouds consist of the moisture raised near the equator 
 with the heated air, and becoming condensed again as it approaches the 
 colder regions of the south. 
 
 Beyond the tropics, where the heating influence of the sun is less, the 
 winds occasionally obey other causes than those we have now been con- 
 sidering, which causes have not yet been fully investigated. The winds of 
 temperate climates are in consequence much less regular, and* are called 
 variable ; but still, as a general rule, whenever air is moving towards the 
 equator, from the north or south poles where it was at rest, it must have the 
 appearance of an east wind, or a wind moving in the contrary direction of the 
 earth itself, until it has gradually acquired the whirling motion of that part of 
 the surface of the earth on which it is found; and again, when air is moving 
 from the equator, where it had at last acquired nearly the same motion as 
 that part of the earth, on reaching parts nearer the poles, and which have less 
 eastward motion, it continues to run faster than they, and becomes a westerly 
 wind. In many situations beyond the tropics, the westerly winds, which are 
 merely the upper equatorial currents of air falling down, are almost as regular 
 as the easterly winds within the tropics, and might also be called trade winds : 
 witness the usual shortness of the voyages from New York to Liverpool, 
 and the length of those made in the contrary direction. North of the equator, 
 then, on earth, true north winds appear to be north-east, and true south winds 
 appear to be south-west : which are the two winds that blow in England for 
 three hundred days of every year. In southern climates the converse is true. 
 
 While the sun is beaming directly over a tropical island, he warms very 
 much the surface of the soil, and, therefore, also, the air over it j but the rays 
 which fall upon the ocean around penetrate deep into the mass, and produce 
 little increase of superficial temperature. As a consequence of this, there is 
 a rapid ascent of hot air over the island during the day, and a cooler wind 
 blowing towards its centre from all directions. This wind. constitutes the 
 refreshing sea-breeze of tropical islands and coasts. A person must have 
 been among these, to conceive the delight which the sea-breeze brings after 
 
 14 
 
210 PNEUMATICS. 
 
 the sultry stagnation which precedes it. The welcome ripple shorewards is 
 first perceived on the surface of the lately smooth or glassy sea ; and soon 
 the whole face of the sea is white with little curling waves, among which 
 the graceful canoe, lately asleep on the water, now shoots swiftly along. 
 
 During the night a phenomenon of opposite nature takes place. The 
 surface of the earth then no longer receiving the sun's rays, is soon cooled 
 by radiation, while the sea, which absorbed heat during the day, not on the 
 surface only, but through its mass, continues to give out heat all night. 
 The consequence is, that the air over the earth becoming colder than that 
 over the sea, sinks down, and spreads out on all sides, producing the land- 
 freeze of tropical climates. This wind is often charged with unhealthy 
 exhalations from the marshes and forests, while the sea-breeze is all purity 
 and freshness. Many islands and coasts would be absolutely uninhabitable 
 but for the sea-breeze. 
 
 The peculiar distribution of land in the Asiatic part of the globe, produces 
 the curious effect there of a sea-breeze of six months, and a land-breeze of 
 six months. The great continent of Asia lies chiefly north of the line, and 
 during its summer, the air over it is so much heated, that there is a constant 
 steady influx from the south appearing south-west, for the reason given in 
 a preceding page; and during its winter months, while the sun is over the 
 southern ocean, there is a constant land breeze from the' north appearing, 
 for a like reason, north-east. These winds are called monsoons ; and if their 
 utility to commerce were to be a reason for a name, they also deserve the 
 name of trade winds. In early periods of navigation, they served to the 
 mariner the purpose of compass, as well as of moving power ; and one 
 voyage outward, and another homeward with the changing monsoons, filled 
 up his year. On the western shores of Africa and America, also, the trade 
 winds are interfered with by the heating of the land; but much less so than 
 in Asia, and always in accordance with the laws now explained. 
 
 The frightful tornadoes, or whirlwinds, which occasionally devastate 
 certain tropical regions, making victims of every ship or bark caught on 
 the waters, and the shore gusts or squalls met with every where, are 
 owing to some sudden chemical changes in the atmosphere, not yet fully 
 understood. 
 
 The Pneumatic Trough and Gasometer 
 
 of the chemist are contrivances constantly displaying the truth now under 
 consideration, " that a lighter fluid is pushed up and floats on a heavier." 
 They are important parts of the apparatus for operating on substances while 
 in the form of air. 
 
 The trough a may be made of metallic plate, or of wood lined with metal, 
 and of any convenient size. It is nearly filled 
 Fig. 106. with water, and has at one end about an inch 
 
 under the surface of the water, a shelf on which 
 jars or vessels, as b and c, may rest. Any 
 particular air or gas is preserved separate from 
 the atmosphere, by being placed in one of these 
 jars with the mouth downwards. The gas is 
 passed into the jar by the operator first immers- 
 ing the jar in the trough, so as to fill it with 
 water and to expel the common air from it; and 
 then holding its mouth over the gas while rising 
 under the water from another vessel or pipe : 
 
FLOATING. GAS APPARATUS. 
 
 211 
 
 Fig. 107. 
 
 CL 
 
 -s 
 
 a 
 
 ci 
 
 d represents a long-necked vessel, used to contain the ingredients for the pro- 
 duction of gases by chemical action. The gas of course rises to the top of 
 the jar b, and gradually'displaces the water. During the operation of filling, 
 the jar may be supported by the hand or by resting on the shelf; in the 
 latter case the gas is allowed to rise into it through a hole in the shelf, pro- 
 vided with a small funnel gaping downwards to catch the air more readily. 
 The shelf may have room on it for many jars, and it may have more holes 
 than one ; and if the gas under operation be such that water absorbs or 
 changes it, some other liquid, as mercury, may be used instead of water. 
 
 A gasometer or gas-holder, is merely a larger jar or vessel as or, dipping 
 into water, with its mouth downwards, in a 
 trough of its own shape, 6 c, and so sup- 
 . ported or counterpoised by a weight at d, 
 over pullies, that very little force suffices to 
 move it up or down. Air forced into it 
 through a pipe /opening under it, causes it 
 to rise or float higher in proportion to the 
 quantity. The air is made to pass from it 
 again when wanted, either through the same 
 tube or through another as e. 
 
 The huge gasometers, exceeding in size 
 an ordinary house, and containing the supply 
 of gas for the lamps of a town, are vessels 
 suspended as above represented, in great pits 
 or troughs, filled with water. The gas issues 
 with force proportioned to the downward 
 pressure of the containing vessel, which may 
 be nicely regulated in a variety of ways, and 
 is generally made to equal the action of a column of water of two inches in 
 height; that is to say, such, that a pipe issuing from the gas holder, and 
 dipping into water at its other end, shall allow gas to escape, if immersed 
 less than two inches perpendicularly. 
 
 It would be encroaching on the province of the chemist to treat here par- 
 ticularly of the substances which most generally exist in the aeriform state ; 
 but to give an increased interest to the description of the gas apparatus, a 
 few leading facts may be mentioned. 
 
 Of about fifty distinct substances known as the materials of our globe, five, 
 when uncombined, and under common circumstances of heat and pressure, 
 exist as air or gases The water used to fill the apparatus above described 
 is a compound of two of the substances, viz., oxygen and hydrogen. By 
 directing an electrical current through water, it is gradually decomposed, 
 and from one side, a stream of aeriform oxygen may be received, and from 
 the other a stream of hydrogen The two gases may be again united to 
 form water, by mixing them in a proper vessel, and passing an electric spark 
 through them. They combine with explosion. 
 
 This oxygen, so called from its relation to acids, (the name consisting of 
 two Greek words, signifying acid and to form,) has been accounted, for 
 many reasons, the most important substance in nature. It forms eight- 
 ninths, by weight of the ocean ; one-fourth of the atmosphere ; and perhaps 
 one-fourth of the solid matter of the globe : possibly, therefore, although 
 most persons think of it only as an air or gas, there is not a millionth part 
 of the quantity of oxygen in the world existing as air. It unites readily 
 with most other substances, and generally with such intense action as to 
 
212 PNEUMATICS. 
 
 produce the phenomena of fire or combustion;, the word combustible chiefly 
 apply to substances that quickly combine with oxygen. 
 
 Oxygen assumes a singular variety of character in its different combina- 
 tions. Thus with hydrogen, it forms water; with lead, it forms the sub- 
 stance called red-lead ; with nitrogen, in one proportion, it forms atmospheric 
 air, in another proportion, the nitrous oxide, or what is called the laughing 
 gas, in a third, the acid called aqua fortis ; with sulphur it forms the 
 sulphuric acid or oil of vitriol; with iron, and all metals it forms their ores 
 called oxides : and so forth. But the most important character in which we 
 know it, is as that ingredient of our atmosphere, without which animals and 
 vegetables cannot live, and fire cannot burn. Oxygen, from this part of its 
 history, was long named vital or pure air. 
 
 Pure oxygen, in the state of air is a little heavier than common air; but, 
 when holding a quantity of charcoal in solution, it forms aeriform carbonic 
 acid, which is nearly twice as heavy as common air, and may be poured out 
 of one vessel into another like water. Carbonic acid is what issues from 
 soda-water, brisk ale, champagne, &c., while they sparkle. If drawn into 
 the lungs in breathing, it is fatal to life. A charcoal fire left in a close room 
 with sleeping persons, has often been fatal to them, because carbonic acid 
 gas is the product of the combustion. So likewise, houseless wretches in 
 winter lying down in a bricktnaker's field to leeward of a burning-heap of 
 bricks, often fall asleep for ever. The famous Grotto del Cane, in Italy, is 
 a cavern always full of carbonic acid, which springs into it from below, as 
 water springs into a well, and runs over like water from a well : it received 
 its name from the circumstance of dogs dying instantly when thrown into it. 
 Carbonic acid rising in fermentation has often proved fatal to persons leaning 
 over the edge of fermenting vats. It is common to see a rat die instantly, 
 in the attempt to run a plank laid across the mouth of a fermenting tub. 
 
 Hydrogen, the other ingredient of water, so called from its relation to 
 water (the name consists of the Greek words for water and to form,) when 
 in the state of air, is sixteen times as light as oxygen. With it balloons are 
 filled. When it holds in solution a certain quantity of carbon or charcoal it 
 becomes the common gas used for illumination, and is the fire-damp of 
 mines, of which the burning and explosion are so terrible. It forms one- 
 ninth of the ocean, and much of the animal and vegetable bodies. 
 
 Nitrogen, so called from its relation to nitric acid, is the third and last 
 substance which we shall mention. It is what remains of the atmosphere 
 when the oxygen is removed. It forms about four-fifths of the atmosphere, 
 one-fourth of the animal flesh, and is found in small quantities in the other 
 combinations. It will not support life by itself, and therefore formerly was 
 called azote : with a larger portion of oxygen, it forms nitric acid or the 
 aquafortis of old. 
 
 The last few paragraphs may serve to show how many of the manipula- 
 tions of chemistry are directed by the principles of physics or mechanical 
 philosophy; and therefore, how essential to the chemist the preliminary 
 study of physics becomes, 
 
DISCHARGE FROM APERTURES. 213 
 
 PART III. 
 
 OR 
 
 THE PHENOMENA OF FLUIDS. 
 
 
 
 ( CONTINUED. ) 
 
 SECTION III HYDRAULICS PHENOMENA OF FLUIDS IN 
 
 MOTION. 
 
 ANALYSIS OF THE SECTION. 
 
 Whether the particles of matter exist -in the form of solid or fluid, the cir- 
 cumstance does not affect their properties of INERTIA and GRAVITY. 
 Hence liquids and airs, in proportion to their quantity, resist, receive, 
 and impart motion, and have weight and friction, as is true of solids. 
 This is seen in the phenomena of 
 
 1. Fluids issuing from vessels, or moving in pipes and channels. 
 
 2. Waves. 
 
 8. Fluids resisting the motion of bodies immersed in them; or themselves 
 
 moving against other bodies. 
 4. Fluids lifted, or moved in opposition to gravity. 
 
 "Fluids issuing from vessels, or moving in channels. 1 ' 
 
 WATER admitted to a tube ascending from near the bottom of a reservoir 
 will rise in it, as already explained, to the level of the liquid-surface in the 
 reservoir. If such a tube be afterwards cut off, except a small part at the 
 bottom, then prepared as a jet-pipe, the water will spout from this still to 
 the same height, with a certain deduction for the resistance of the air and 
 friction. Now as a body shot upwards to any height has that velocity in 
 departing, which it again acquires by falling back to the same place or level, 
 (with a certain deduction for the resistance of the air,) as explained at page 
 60, it follows that fluid issues from any orifice in a reservoir with velocity 
 equal to what a body acquires in falling as far as from the level of the fluid 
 surface in the reservoir to the orifice. By referring then to the law of fall- 
 ing bodies, as explained at page 50, we may learn the velocity of the issue 
 of water in any case, and therefore t the quantity delivered by an opening of 
 a given magnitude. 
 
214 HYDRAULICS. 
 
 Thus, a body by gravity falls sixteen feet in the first second, with speed 
 gradually increasing, and at the end of the second has a velocity of thirty-two 
 feet per second; therefore a reservoir with an. opening of an inch square at 
 sixteen feet below the water's surface, will deliver, in one second of time, with 
 a certain deduction for resistance of air, friction, &c., thirty-two feet of a jet 
 of water of an inch square ; and according to the same rule, an opening at 
 four times the depth should deliver a double quantity ; at nine times the 
 depth, a triple quantity ; and so on, as really happens. An inquirer is at 
 first surprised that the quantity should not be quadruple, where the height of 
 column or pressure forcing it out is quadruple, ninefold when the pressure 
 is ninefold, &c., but on reflection, he may perceive that the real effects, as 
 stated above, are still exactly proportioned to the causes ; for when only 
 twice as much water is forced out in the same time, there is still an effect, 
 four times as powerful, because each particle of the double quantity issues 
 with twice the force or velocity, and increase of velocity costs just as much 
 force as increase of quantity. Similar reasoning holds with respect to the 
 triple or other quantities. Because a body shot upward with a double velo- 
 city gains a quadruple height, ( see page 60, ) the jet issuing with only double 
 velocity from four times the depth, still reaches the level of the surface of 
 the reservoir. 
 
 The knowledge of this rule for discharging orifices is of the greatest 
 importance in the construction of water-works, because when joined with 
 other rules assigning the effects of friction, bending, unequal width, &c., in 
 pipes, it ascertains the quantity of water which a conduit of any magnitude, 
 length and slope, will deliver. 
 
 It is a curious fact, that more water issues from a vessel through a short 
 pipe, than through a simple aperture of the same diameter as the pipe; and 
 still more if the pipe be funnel-shaped, or wider towards its inner extremity. 
 
 The explanation is, that the issuing particles .coming from all sides to 
 escape, cross and impede each other in rushing through a simple opening, 
 as is proved by the narrow neck which the jet exhibits a little beyond the 
 opening ; but in a tube, this narrowing of the jet cannot happen without 
 leaving a vacuum around the part, and the pressure of the atmosphere, re- 
 sisting the vacuum, causes a quicker flow. The funnel-shape again leads 
 the water by a more gradual inclination to the point of exit, and thus con- 
 siderably prevents the crossing among the particles ; besides that, because 
 its mouth surrounds the narrow neck of the jet, it allows that part to be 
 deemed the commencement of the jet. 
 
 Another remarkable effect of atmospheric pressure on running liquids is, 
 that in a tube of considerable length, descending from the reservoir, it much 
 quickens the discharge. Water naturally falls like any other body with 
 accelerating velocity, but if it so fall in a tube which it fills like a piston, 
 either portions of it below must outstrip portions above, leaving vacuous 
 spaces between, or water from above must be pressed into the tube by some 
 other force than its weight. Now the atmospheric pressure becomes this 
 force, and it prevents a vacuum, partly by impelling water more rapidly into 
 the top of the tube, and partly by resisting the discharge from below. The 
 forcing in of the water at the top of the tube causes that depression of the 
 water surface in the reservoir over it, which becomes more conspicuous as 
 the depth in the reservoir diminishes, and at last is a deep hole in the water 
 extending far into the tube, and sometimes even as in a common funnel 
 extending quite through. 
 
 The friction or resistance which fluids suffer in passing along pipes is 
 
DISCHARGE FROM APERTURES. 215 
 
 i 
 
 much higher than might be expected. It depends on the cohesion of the 
 particles to the surface of the pipe and among one another, and on the par- 
 ticles near the outside being constantly driven from their straight course by 
 the irregularities in the surface of the pipe. An inch tube of two hundred 
 feet in length, placed horizontally, is found to discharge only a fourth part 
 of the water which escapes by a a simple aperture of the same diameter. All 
 passing along tubes is still more retarded. A person who erected a great 
 bellows at a water-fall, to blow a furnace two miles off, found that his 
 apparatus was totally useless. When gas lights were first proposed, 
 some engineers feared that the resistance by friction to the passing air 
 would be fatal to the enterprise. 
 
 Higher temperature in a liquid increases remarkably the quantity dis- 
 charged by an orfice or pipe, apparently by diminishing that cohesion of 
 the particles which exists in certain degrees in all liquids and affects so 
 much their internal movement. The additition of 100 degrees of heat will, 
 in certain cases, nearly double the discharge. 
 
 The flux of water through orifices under uniform circumstances is so 
 steady, that before the invention of clocks and watches, it was employed as 
 a means of measuring time. The vessels were called clepsydrae,. That of 
 Ctesibius is famous, in which the issuing water took the form of tears from 
 the eyes of a figure, deploring the rapid passing of precious time ; and these 
 tears being received into a fit vessel, gradually filled it up and raised 
 another floating figure, who pointed to the hours marked on an upright 
 scale. This vessel was daily emptied by a syphon, when charged to a 
 certain height, and its discharge worked machinery which told the month 
 and the day. The common hour-glass of running sand is another modifica- 
 tion of the same principle, with this remarkable difference, however, that 
 depth of the sand does not quicken the flux. 
 
 The progress of water in an open conduit, such as the channel of a river 
 or acqueduct, is influenced by friction, &c., in the same manner as in close 
 pipes. But for this, a river like the Rhone, drawing its waters from the 
 elevation of 1,000 feet above the level of its mouth, would pour them out, 
 with the velocity of water issuing from the bottom of a reservoir, 1,000 feet 
 deep; that is to say, at the rate of about 170 miles per hour. The ordinary 
 flow of rivers is about three miles per hour, and their channels slope three 
 or four inches per mile. 
 
 The velocity of a water current is easily ascertained by immersing in it 
 an upright tube, of which the bottom bent at right angles becomes an open 
 mouth turned towards the stream. The water in the tube 
 will stand above the surface of the stream, as much as would Fig. 108. 
 be necessary in a reservoir, according to the explanation j ( 
 given above, to cause a velocity of jet equal to the velocity ct 
 of the stream. A modification of this contrivance may be 
 made to measure the velocity of the wind. A common mode 
 of telling the velocity of an open stream, is to observe with 
 a stop-watch the progress of a body floating in some part of 
 it from which its medium speed maybe known; and know- 
 ing that speed and the depth and width of the channel the 
 quantity delivered in a given time becomes a matter of sim- 
 pie calculation. The speed of the wind may be ascertained 
 by observing how long the shadow of a cloud takes to 
 pass across a field of known dimensions. 
 
 The friction of water moving in water is such, that a small stream directed 
 
216 HYDRAULICS. 
 
 through a pool, with speed enough to rise' over the opposite bank, will soon 
 empty the pool. Extensive fens have been drained on this principle. The 
 friction between air and water is also singularly strong, and is proved on a 
 great scale by the magnitude of the ocean-waves, which is a consequence of 
 it ; and on a small scale, by the amusing experiment of making a light 
 round body dance or play upon the summit of a water-jet a chief cause of its 
 remaining there being, that the current of air which rises around the jet by 
 reason of the friction, presses it inward again, whenever it inclines to fall 
 over. Oil thrown upon the surface of water, soon spreads as a film over it. 
 and defends it from farther contact and friction of the air. If oil be thus 
 spread at the windward side of a pond where the waves begin, the whole 
 surface of a pond becomes as smooth as glass ; and even out at sea, where 
 the commencement of the waves cannot be reached, oil thrown upon them 
 smooths their surface to leeward of the place, and prevents their curling 
 over or breaking. It is said that boats having to reach the shore through 
 a raging surf, have been preserved by the crews first spilling a cask of oil 
 in the offing. 
 
 The most magnificent examples that ever existed, or probably ever will ' 
 exist of artificial water-courses, were the acqueducts of ancient Borne, about 
 twenty in number. Several of them exceeded forty miles in length, passing 
 Ihrough hills in their way, and resting on tiers of splendid arches across the 
 valleys. They were constructed of such durable materials, and so skilfully, 
 that the principal of them remain perfect to this day. Considered as one 
 object, they rank, in point of magnitude, with any other work of human 
 labour, not excepting the pyramids of Egypt. 
 
 While the acqueducts are cited as specimens of grandeur, we may mention 
 the fountains in the gardens of France and Italy as specimens of beauty. 
 Those at Versailles are well known. In them the most magical effects are 
 produced by varying the ways in which water is made to spout from orifices. 
 In one place it is seen darting into the air as a single upright pillar : in others 
 many such pillars rise together, like giant stalks of corn ; sometimes an 
 inclination given to the jets makes them bend so as to form beautiful arches, 
 of which a portion appear as the roofs of apartments built of water while 
 others mingle together with endless variety : here and there water-throwing 
 wheels throw out spiral streams, and hollow spheres with a thousand openings 
 are the centres of immense bushes or trees of silvery boughs. Such effects 
 amidst cascades, smooth lakes, and scenes of lovely landscapes, constitute a 
 whole as enchanting, perhaps, as art by moulding nature has ever produced. 
 
 " Waves." 
 
 The form, magnitude and a velocity of waves, are subjects admitting of 
 deep mathematical research ; and are rendered the more interesting, because 
 certain phenomena of sound and light are of kindred nature. Here, how- 
 ever, they must be treated with great brevity. 
 
 A stone thrown into a smooth pond, causes a succession of circular waves 
 to spread from the spot where it falls as a common centre. They become of 
 less elevation as they expand, and each new one is less raised than the pre- 
 ceding, until gradually the liquid mirror becomes again perfect as before. 
 Several stones falling at the same time in different places, cause crossing 
 circles which, however, do not disturb the progress of one another a phe- 
 nomenon seen in beautiful miniature at each leap of the little insects which 
 cover the surface of our pools in the calm hours of summer. The rationale 
 
WAVES. 217 
 
 of the formation of waves in such cases is as follows : When the stone falls 
 into the water, because the liquid is incompressible, a part of it is displaced 
 laterally, and becomes an elevation or circular wave around the stone. This 
 wave then spreads outward in obedience to the laws of fluidity, already 
 explained, and the circle is seen to widen. In the meantime, where the 
 stone descended, a hollow is left for a moment in the water, but owing to 
 the surrounding pressure, is soon filled up, chiefly by a sudden rush from 
 below. The rising water does not stop, however, at the exact level of that 
 around, but like a pendulum sweeping past the centre of its arc, it rises 
 almost as far above the level as the depression was deep. The central eleva- 
 tion now acts as the stone did originally, and causes a second wave, which 
 pursues the first ; and when the centre subsides, like the pendulum still, it 
 sinks again almost as much below the level as it had mounted above : hence 
 it has to rise again, again to fall, and so on for many times, sending forth 
 a new wave at each alternation. Owing to the friction among the particles 
 of the water, each new wave is less raised thten the preceding, and at last 
 the appearance dies away. 
 
 A wave passing through any gap or opening, spreads from it as a new 
 centre ; and a wave coming against a perpendicular surface of wall or rock, 
 is completely reflected from this, and acquires the appearance of coming 
 from a point as far beyond the reflecting surface, as its real origin or centre 
 is distant on the side where it is moving. 
 
 So absolutely level is a liquid surface, and so sensitive or mobile, that the 
 effect of any disturbing cause is perceived at great distances. A boat rowed 
 across a still lake, ruffles its surface to a great extent ; and although the 
 widening waves become at last such gentle risings as not to*be perceptible 
 to the eye, they still produce a rippling noise where they fall among the 
 pebbles on shore. In seas liable to sudden but partial hurricanes, the roar 
 of breakers on distant coasts often tells of the storm which does not other- 
 wise reach them. The author once, in the eastern ocean, had an opportunity 
 of contemplating waves of extraordinary magnitude rolling along during a 
 gloomy calm, and therefore with unbroken surface, appearing like billows of 
 molten lead. At that very time, about a hundred and fifty miles to the 
 north-east, four of the finest ships of the India Company were perishing in a 
 storm. In the polar seas which are comparatively tranquil, because partially 
 defended from the wind by the floating islands of ice, a few sudden waves 
 are occasionally observed, and quickly all is calm again. Such a phenomenon 
 announces, that the occurrence described at page 153 has happened some- 
 where, of an island of ice turning over, when the place of its centre of gravity 
 is changed by partial melting. 
 
 The common cause of waves is the friction of the wind upon the'surface 
 of the water. Little ridges or elevations first appear, which, by continuance 
 of the force, gradually become loftier and broader, until they are the rolling 
 mountains seen where the winds sweep over a great extent of water. The 
 heaving of the Bay of Biscay, or still more remarkably, of the open ocean 
 beyond the southern capes of America and Africa, exhibits one extreme, and 
 the stillness of the tropical seas, which are sheltered by near encircling lands 
 exhibits the other. In the vast archipelago of the east, where Borneo and 
 Java and Sumatra lie, and the Molucca Islands and the Philippines, the sea 
 is often fanned only by the land and sea breezes, and is like a smooth bed in 
 which these islands seem to repose in bliss islands in which the spice and 
 pefume gardens of the world are embowered, and where the bird of paradise 
 has its home, and the golden pheasant, and a hundred other birds of brilliant 
 
218 HYDRAULICS. 
 
 plumage, among thickets so luxuriant, and scenery so picturesque, that 
 European strangers find there the fairy land of their youthful dreams. One 
 who has visited these islands in his early days, may perhaps be pardoned for 
 thus adverting to their beauties. 
 
 In rounding the Cape of Good Hope, waves are met with, or rather a 
 swell, so vast, that a few ridges and a few depressions occupy the extent of a 
 mile. But these are not so dangerous to ships, as what is termed a shorter 
 sea, with more perpendicular waves. The slope in the former is compara- 
 tively gentle, and the rising and falling are much less felt ; while among the 
 latter, the sudden tossing of the vessel is often destructive. When a ship 
 is sailing directly before the wind, over the long swell now described, she 
 advances as if by leaps ; for as each wave passes, she is first descending head- 
 long on its front, acquiring a velocity so wild that she can scarcely be steered ; 
 and soon after, when it has glided under her, she appears climbing on its 
 back, and her motion is slackened almost to rest, before the following wave 
 arrives. To a passenger perched at such a time on the extremity of the 
 bowsprit, and looking back on the enormous body of the ship, with perhaps 
 its thousands of a crew, a hundred feet behind him, heaved by those billows 
 as a cork is on a ruffled lake, the scene is truly sublime. When a coming 
 wave lifts the stern and in the same decree depresses the bow, he is deep in 
 the hollow or valley between the waves, and sees only the ship rushing head- 
 long down towards him as if to be engulphed - r but soon after, when the stern 
 is down, and the bow is raised, he looks from his station in the sky upon an 
 awful scene beneath him and around. 
 
 The velocity of waves has relation to their magnitude. The large waves 
 just spoken of, proceed at the rate of from thirty to forty miles an hour. 
 It is a vulgar belief that the water itself advances with the speed of the wave, 
 but in fact the form only advances, while the substance, except a little spray 
 above, remains rising and falling in the same place, with the regularity of a 
 pendulum. A wave of water, in this respect, is exactly imitated by the wave 
 running along a stretched rope when one end is shaken ; or by the mimic 
 waves of our theatres, which are generally undulations of long pieces of car- 
 pet moved by attendants. But when a wave reaches a shallow bank or 
 beach, the water becomes really progressive, for then, as it cannot sink 
 directly downwards, it falls over and forwards, seeking the level. 
 
 So awful is the spectacle of a storm at sea, that it generally biases the 
 judgment; and, lofty as waves really are, imagination pictures them loftier 
 still. Now no wave rises much more than ten feet above the ordinary sea- 
 level, which, with the ten feet that the surface afterwards descends below 
 this, give twenty feet for the whole height, from the bottom of any water- 
 valley to an adjoining summit. This is easily verified by a person who tries 
 at what heighten a ship's mast the horizon remains always in sight over the 
 top of the waves allowance being made for accidental inclinations of the ves- 
 sel, and for her sinking in the water to considerably below her water line, at 
 the time when she reaches the bottom of the hollow between the two waves. 
 The spray of the sea, driven along by the violence of the wind, is of course 
 much higher than the summit of the liquid wave ; and a wave coming against 
 an obstacle, or entering a narrow inlet, may dash to an elevation much greater 
 still. At Eddystone light house, which is about ninety feet high, placed 
 on a solitary rock ten miles from land, when a surge breaks which has been 
 growing under a storm all the way across the Atlantic, it often dashes to 100 
 feet above the lantern at the summit. 
 
 The magnitude of waves is well judged of when they are seen breaking 
 
WAVES. 219 
 
 on an extended shore or beach. In the deep sea the wave is only an eleva- 
 tion of the water, sloping on either side ; but as it rolls towards the shore, 
 its front becomes more and more perpendicular, until at last it curls over 
 and falls with its whole weight, and when several miles of it break at the 
 same instant, its force and noise may shake- the country abroad. 
 
 Along the east, or Corom-andel Coast of India, at certain season, vast 
 waves are constantly breaking ; and as there are no good harbours there, com- 
 munication between the sea and land is rendered impossible to ordinary boats. 
 The natives of the coast, at Madras, for instance, have hence become almost 
 amphibious. They reach ships beyond the breakers by the help of what are 
 called catamarans, consisting of three small logs of wood tied together. On 
 these they secure themselves, and boldly advance up to the coming wall of 
 water, which they shoot into, and rise to the smooth surface beyond it, like 
 water-fowls after diving. Boats unsuited to the breakers often perish in them. 
 The author of this work had gone on shore with a watering party on the 
 coast of Sumatra, and during the hours spent there, a swell had risen in the 
 sea, which on their return was already bursting along the beach and across 
 the river's mouth in lofty breakers. The boat in which he happened to be, 
 regained the high sea in safety, but a larger boat which followed at a short 
 distance was overwhelmed, and an officer and part of the crew perished. 
 
 There is a phenomenon observed at the mouths 01* many great rivers, called 
 the Boar, which has resemblance to a wave. When the tide returning from 
 sea meets the outward current from the river, and both have the force which 
 in certain situations belongs to them, the stronger mass from the ocean assumes 
 the form of an almost perpendicular wall, moving inland with resistless sweep. 
 This is called the boar. It is in fact the great sea-wave of the tide, produced 
 twice a-day by the attraction of the moon, rolling in upon the land and inlets, 
 where contracting channels concentrate its mass. In the different branches 
 of the Ganges the boar is seen in a remarkable degree. Its roaring is heard 
 long before it arrives. Smaller boats and skiffs cannot live where it comes ; 
 and as it passes the city of Calcutta, even the large ships at anchor there 
 are thrown into such commotion, as sometimes to be torn away from their 
 moorings. The nature and effects of this boar are strikingly illustrated 
 upon certain coasts where extensive tracts of sand are left uncovered at low 
 water. In such situations, of which there are many on the western shores 
 of Britain, the returning tide is seen advancing with steep front, and with 
 such rapidity, that the speed of a galloping horse can scarcely save a person 
 who has incautiously approached too near. Many, every year, are the 
 victims of temerity or ignorance on these treacherous plains. 
 
 In the end of the year 1831, on the low flat coast of the Indian peninsula, 
 north of Madras, one great wave of the kind now described was produced 
 during a very high-spring tide of midnight, by an extraordinary wind, and 
 spread ten miles in upon the inhabited land. It had retired with the ebbing 
 tide before morning, but the next day's sun disclosed a scene of devastation 
 rarely matched. Amidst the total wreck of the villages and fields, there lay 
 the drowned carcases of more than ten thousand human beings, mixed with 
 those of elephants, horses, bullocks, wild tigers and the other inhabitants 
 of the land. 
 
 It has been proposed lately to construct sub-marine boats, or vessels cal- 
 culated to swim so deep in the water as to be below the superficial motion of 
 the waves, and therefore beyond the influence of storms at the surface. Such 
 a boat has been tried with considerable success; and man's increasing fami- 
 liarity with sub-marine matters since the invention of the diving-bell, may 
 
220 HYDRAULICS. 
 
 ultimately lead to improvements rendering the sub-marine vessel, for certain 
 purposes, commodious and safe. 
 
 " Fluids resisting the motion of bodies immersed in them, or themselves 
 moving forcibly against other bodies.'' (See the Analysis.) 
 
 The same force is required to give or to take away, or to bend motion, in 
 a fluid, as in an equal quantity of solid matter. A pound of water enclosed 
 in a bladder is not more easily thrown to a given height than a pound of ice 
 or of lead; nor, if falling into the scale of a weighing beam, does it require 
 less as a counterpoise ; nor, if made to revolve at the end of a sling, does it 
 render the cord less tight. 
 
 A convenient measure of the force of moving water on an obstacle, or of 
 the resistance of still water to a moving body, exists in the facts already 
 explained, that the pressure of a known height of fluid column produces from 
 an orifice a certain velocity of jet, while conversely, that jet, or a current of 
 equal speed, directed against the orifice supports the column. The impulse 
 given or received, therefore, by a flat surface in water, such as the vane of 
 a water-wheel, whether that of a steam-boat pressing against the water, or 
 that of a corn-mill pressed by it, is measured by the weight of the column 
 alluded to, the height of which is, according to the velocity and the breadth 
 or diameter, according to the breadth or extent of the solid surface concerned. 
 This estimate supposes that the pressure of air upon the surface is direct ; if 
 it be oblique, there is a diminution according to the rule given under the 
 head of " resolution of forces." 
 
 Many persons looking carelessly at the subject of fluid resistance, would 
 expect that if a body, as a boat, moving through a fluid at a given rate, meets 
 a given resistance, it should just meet double resistance when moving twice 
 as fast. Now the resistance is four times greater with a double rate. 
 
 This fact is but another example of a principle already explained, and 
 when more closely examined, is easily understood. A boat which moves 
 one mile per hour, displaces or throws aside a certain quantity of water, and 
 with a certain velocity ; if it move twice as fast, it of course displaces twice 
 as many particles at the same time, and requires to be moved by twice the 
 force on that account; but it also displaces every particle with a double 
 velocity, and requires another doubling of the power on this account; the 
 power then being doubled on two accounts becomes a power of four. In the 
 same manner with a speed of three, three times as many particles are moved 
 and each particle with three times the velocity; therefore, to overcome the 
 resistence, a force of nine is wanted ; for a speed of four, a power of sixteen ; 
 for a speed of five, a power of twenty-five, and so forth : the relations being 
 that which mathematicians indicate by saying that the resistance increases as 
 the square of the speed. The corresponding numbers, up to a speed of ten 
 are as here shown. 
 
 Speed ..123456789 10 
 
 Correspondingj j 9 16 25 36 49 64 81 100 
 resistance j 
 
 Thus, if even the resistance at the bow of a vessel was all that had to be 
 considered, the force of one hundred horses would only drag the vessel ten 
 times as fast as the force of one horse. But there is another important 
 element in the calculation, viz ; the lessening, as the vessel's speed quick- 
 
WAVES. 221 
 
 ens, of the usual water pressure on the stern, which pressure while she is 
 at rest is equal to the pressure on the bow, and the force therefore required 
 to produce an increased velocity is still considerably greater than as noted 
 in the table. 
 
 There is not a more important truth in, physics than the law of fluid 
 assistance to moving bodies here treated of; .it explains so many phenomena 
 of nature, and becomes a guide in so many matters of art. We will now 
 set forth some interesting examples. 
 
 It explains at what a heavy expense of coal high velocities are obtained in 
 steam-boats. If an engine of about 50 horse power would drive a boat 7 
 miles an hour, two engines of 50, or one of 100 would be required to drive 
 it 10 miles, and three such to drive it 12 miles, even supposing the increased 
 resistance to the bow, as already stated, to be the measure of the whole work 
 done, which it is not, and that engines worked to the same advantage with a 
 high velocity as with a low, which they do not. For the same reasons, if all 
 the coal which a ship could conveniently carry were just sufficient to drive 
 her 1,000 miles, at a rate of 12 miles per hour, it would drive her more 
 than 3,000 at a rate of 7 miles per hour; and more than6,000 at a rate of 
 5 miles per hour. This is a very important consideration for persons 
 concerned in steam navigation to distant parts. 
 
 The same law shows the folly of putting very large sails on a ship; the 
 trifling advantage in point of speed by no means compensating for the addi- 
 tional expense of making and working the sails, and the risk of accidents in 
 bad weather. The ships of the prudent Chinese have not, for the same 
 tonnage, one-third so much sail as those of Europeans, and yet they move 
 but little slower on that account. A European ship under jury-masts does 
 not lose so much of her usual speed as most people would expect. 
 
 This law explains also why a ship glides through the water one or two 
 miles an hour when there is very little wind, although with a strong breeze 
 she would only sail at the rate of eight or ten miles. Less than the 100th 
 part of that force of wind which drives her ten miles an hour, will drive her 
 one mile per hour, and less than the 400th part will drive her half a mile. 
 Thus, also, during a calm, a few men pulling in a boat can move a large 
 ship at a sensible rate. 
 
 These considerations show strikingly of what importance to navigation it 
 might be to have, as a part of a ship's ordinary equipment, one or two water- 
 wheels, ( or ready means of forming them,) to be affixed upon the ship's side 
 when required, like the paddle-wheels of a steam-boat, and by turning which, 
 the crew might easily deliver themselves from the tedium, or even disastrous 
 consequences of a long calm at sea. This idea occurred to the author while 
 in a ship completely becalmed for weeks on the Line : during which weari- 
 some periods, the breezes were often seen roughening the water a mile or 
 two farther on ; and any means that could have enabled the ship's company 
 to advance her that little distance might have saved the delay. The wheels 
 might be driven by connection with the capstan, at which, under such 
 circumstances, the crew would most willingly work. Delay in a large vessel 
 often costs hundreds of pounds per day, and may retard the execution of 
 important projects. But the propelling of a ship in a calm seems, by no 
 means, the most important purpose which such wheels might serve. If 
 from disease, fatigue, or other cause, the crew were inadequate to existing 
 necessities, two wheels affixed to the extremities of an axis crossing the ship 
 might be equivalent in many cases to additional hands, or to a steam-engine 
 of great power ; for when acted upon by the water as the ship sailed, they 
 
222 HYDRAULICS. 
 
 would turn with the force of water-wheels on shore, and might be made to 
 move the pumps ; to hoist the sails, and to do any work which a steam-engine 
 could perform. Many a gallant vessel has perished because the exhausted 
 crew could not longer labour at the pumps, where such water-wheels as now 
 contemplated, or a wind-mill wheel in the rigging would have performed the 
 duty most perfectly. 
 
 The law that resistance to a body moving in a fluid increases in a greater 
 proportion than in speed of Ihe body, applies where the fluid is aeriform, 
 as well as where it is liquid. 
 
 A bullet shot through the air with a double velocity, for the reason 
 assigned above, experiences four times as much resistance in front, as with 
 a single velocity : the motion is retarded also by the diminution of the usual 
 atmospheric pressure of 15 Ibs. per inch on the posterior surface, which 
 diminution is proportioned to the speed. It is farther true, that when the 
 velocities of bodies moving in the air are very great, the resistance increases 
 in a still quicker ratio than in liquids, probably because the compressibility 
 of air allows it to be much condensed or heaped up before the quick moving 
 body. It is useless to discharge a cannon-ball with a velocity exceeding 
 1,200 feet in a second, because the powerful resistance of the air to any 
 velocity beyond that, soon reduces it to that at least. 
 
 The rule of reciprocal action between a solid and fluid, now explained, holds 
 equally when the fluid is in motion against the solid, as when the solid 
 moves through the fluid. 
 
 If a ship be anchored in a tide's way, where the current is four miles an 
 hour, the strain on her cable is not one fourth part so great as if the current 
 were eight miles. 
 
 A wind moving three miles an hour is scarcely felt : if moving six miles, 
 it is a pleasant breeze ; if twenty or thirty miles, it is a brisk gale ; if sixty, 
 it is a storm; and beyond eighty, it is a frightful hurricane, tearing up 
 trees and destroying every thing. 
 
 Supposing the wind to move one hundred miles per hour, there are one 
 hundred times as many particles of matter striking any body exposed to it, 
 as when it moves only one mile per hour, and each particle strikes, more- 
 over, with one hundred times the velocity or force, so that the whole increase 
 of force is a hundred times a hundred, or ten thousand. This explains how 
 the soft invisible air may by motion acquire force sufficient to unroof houses, 
 to level oaks which have been stretching their roots around for a century, 
 and in some West India hurricanes, absolutely to brush every projecting 
 thing from the surface of the earth. 
 
 The law of rapidly increasing resistance assigns a limit to many velocities, 
 both natural and artificial. 
 
 It limits the velocity of bodies falling through the air. By the law of 
 gravity, a body would fall with a constantly accelerating speed, but as the 
 resistance of the air increases still more quickly than the speed at a certain 
 point, this resistance and the gravity balance each other, and the motion 
 becomes uniform* 
 
ACTION BETWEEN FLUIDS AND SOLIDS. 223 
 
 The parachute, by means of which a person may safely descend to the* 
 earth from a balloon at any elevation, furnishes a good example. The con- 
 trivance resembles a large flat umbrella. The aeronaut attaches himself 
 underneath it, and when it is let loose from the balloon, he is partly sup- 
 ported by the resistance which its broad expanse experiences in falling 
 through the air, tind falls, therefore, in a corresponding degree more slowly. 
 After the first second or two, for the reason stated above, it descends with 
 a uniform motion ; and its breadth is generally made such, as to allow a 
 velocity of about eleven feet in a second, or that which a man acquires in 
 jumping from a chair two feet high. 
 
 No ship sails faster than fifteen miles in an hour. And it is because the 
 resistance to be overcome in steam-carriages on rail-ways, viz., their friction, 
 does not increase with their velocity like the fluid resistance to steamboats, 
 that the speed of the former may so much exceed that of the latter. 
 
 No fish swims with a velocity exceeding twenty miles an hour; not the 
 dolphin, when shooting ahead of our swiftest frigates, nor the salmon, when 
 darting forward with a speed which lifts him over the water-fall. 
 
 And the flight of birds through the thin air has a limited celerity. The 
 crow, when flying homewards against the storm, cannot face the wind in the 
 open sky, but skims along the surface of the earth in the deep valleys, or 
 wherever the swiftness of the wind is retarded by terrestrial obstructions. 
 The great albatross, stemming upon the wing the current of a gale so as to 
 keep company with a driving ship where the air is passing at the rate of a 
 hundred miles an hour, often takes shelter momentarily under the lee-side 
 of the lofty billows. The bird called the stormy petrel abides chiefly in the 
 midst of the Atlantic Ocean, but the irresistible violence of the wind occa- 
 sionally sweeps it from the waves, and causes its appearance on the western 
 shores of Europe. Vessels from the high sea, approaching a coast from 
 which the wind blows, generally become resting places to exhausted land 
 birds driven off the shore by wind which they have not had strength of wing 
 to stem ; sad evidences of the myriads which are constantly perishing where 
 no resting place is found, and where no human eye notes their fate. 
 
 The action or resistance between a meeting fluid and solid, is influenced by 
 the shape of the solid. 
 
 This follows from what has already been said of direct and oblique impulse. 
 If a flat surface directly opposed to the fluid experience a certain resistance, 
 a projecting surface like that of a sphere or short wedge is resisted in a less 
 degree, and a concave surface in a greater. The explanation is, that a flat 
 or plane surface throws the particles of fluid almost directly outwards from 
 its centre to its circumference, and therefore with greater velocity, while the 
 convex or wedge-like surface, although displacing them just as far, still does 
 so more slowly, and therefore with less expenditure of force, in proportion 
 to the obliquity of surface, or as its point is in advance of its shoulder or 
 broadest part ; and a concave surface must give to some of the particles a 
 forward as well as lateral motion. The shape of the hinder part of a solid 
 moving through a fluid is of importance for corresponding reasons. 
 
 The following are instances of projecting or wedge-like surfaces, intended 
 to diminish the resistance. Fishes are wedge-like, both before and behind, 
 their form being modified, however, in relation to other objects than mere 
 speed of motion. Birds are so, also ; and they stretch out their necks while 
 flying, so as to make their form perfect for dividing the air. In the form 
 
224 HYDRAULICS. 
 
 f)f the under part of boats and ships, men have, in a degree, imitated the 
 shape of fishes. The light wherries which shoot about upon the surface of 
 the Thames, appear the very essence of what imagination can picture of form 
 combining utility and grace. There are boats used in China called snake- 
 boats, which are only a foot or two broad, but perhaps a hundred feet in 
 length, and when moved, as they often are, by nearly a 'hundred rowers, 
 their swiftness is extreme. The problem of which it is the object to assign 
 for a ship's hull or. bottom the best possible form that she may have speed 
 of sailing, is not yet completely solved; so that a kind of empiricism prevails 
 in the matter, and very unexpected results often arise. Yet the subject 
 merits much attention, for when vessels have to chase and to flee, speed 
 becomes of the greatest importance; and at all times the sailor's heart swells 
 with delight to find his well-beloved vessel performing well. 
 
 The following instances exhibit the mutual influence of meeting solids and 
 fluids, where the surface of the solid is plane or concave. In a water-wheel, 
 whether the water be moving against the wheel, as is the case where a stream 
 acts to drive machinery, or the. wheel be moving against the still water, as 
 in the case of the paddle-wheels of a steam-boat, the extended faces of the 
 vanes or float-boards give or receive a powerful impulse. When a wheel 
 with float-boards has its lower part merely dipping into a stream of Water, 
 to be driven by the momentum, it is called an undershot-wheel; when the 
 water reaches the wheel near the middle of its height, and turns it by falling 
 on the float-boards of one side as they sweep downwards in a curved trough 
 fitting them, the modification is called a breast-wheel ; and when the float- 
 boards are shut in by flat sides, so as to become the bottoms of a circle of 
 cavities or buckets surrounding the wheel, into which the water is allowed 
 to fall at the top of the wheel, and to act by its weight instead of its momen- 
 tum, the modification is called the overshot-wheel. To have a maximum of 
 effect from wheels moved by the momentum of water, they are generally 
 made to turn with a velocity about one-third as great as that of the water ; 
 and wheels moved by the simple weight of water usually have their circum- 
 ference turning with a velocity of about three feet per second. The subject 
 of water-wheels is one of the most important in practical mechanics ; for 
 moving water performs a great deal of labor for man. 
 
 Oars for boats are made flat, and often a little concave, that the mutual 
 action between them and water may be as great as possible. The webbed 
 feet of water-fowl are oars ; in advancing, they collapse like a shutting um- 
 brella, but open outwards in the thrust backwards, so as to offer a broad 
 concave surface to the water. The expanded wings of birds are in like 
 manner a little concave towards the air which they strike. The sails of 
 ships, when they are receiving a fair wind, are left slack so as to swell and 
 become hollow. 
 
 The resistance between a meeting solid and fluid being nearly proportioned 
 to the breadth of the solid, it follows that large bodies, because containing 
 more matter in proportion to their breadth or surface than smaller bodies 
 of similar form, are less resisted in proportion to their weights, than 
 smaller bodies. 
 
 The science of measures tells us that a bullet or other solid of two inches 
 diameter, has eight times as much matter in it as a similar solid of one inch 
 diameter, while it has only four time the breadth or surface. Thus, by 
 putting eight dice or little cubes together, as here represented, we have a 
 
ACTION BETWEEN FLUIDS AND SOLIDS. 225 
 
 larger cube, of which compared with a single dice, the edge is evidently 
 twice as long, the surface four times as great ; and the quantity of matter 
 eight times as great; again, twenty -seven dice simi- 
 larly put together form a cube with sides three times Fig. 109. 
 as long, and the surface nine times as great ; and sixty- 
 four dice form a cube with sides four times as long, 
 and a surface sixteen times as great. All solids similar M^TL- 
 have to each other this kind of relation, which, in the 
 language of the science of quantity, is called the rela- 
 tion of cubes : they are said to be to each other as the 
 cubes of any of their corresponding lines. Hence if a 
 bullet of eight pounds, and a bullet of one pound be shot off with equal 
 velocity, because that of eight pounds has only half as much surface in pro- 
 portion to its weight, and therefore to its moral inertia or force, as the 
 other, it will go much farther than the other. 
 
 This important rule explains why shells and large shot may be thrown 
 four or five miles, while smaller cannon-balls, musket-bullets, pistol and 
 swan shot, and the common small shot of the sportsman, all of which are 
 generally discharged from their respective pieces with the same commencing 
 velocity, have a shorter range, as the size of the projectile is less. Even 
 water is sometimes thrown from a gun or powerful syringe to stun birds, 
 that they may be obtained with uninjured plumage ; but it soon divides in 
 the air so minutely that it reaches only a short distance. 
 
 Water falling through the air from a great height, goes on suffering a 
 gradual division into smaller and smaller portions, which at last may be said 
 to be nearly all surface : and then the resistance of the air lets them fall 
 very slowly indeed. The relation of the size and resistance is well shown 
 by the difference of celerity in the descent of a minute fog, a drizzling mist, 
 and common rain. The toy called the iuater-hammcr, is merely a little 
 water enclosed in a tube exhausted or empty of air ; and when, by turning 
 the tube, the water is made to fall from one end to the other, as there is no 
 air to impede or divide it in its descent, it falls as one mass, and makes a 
 sharp noise like the blow of a hammer. 
 
 The same law explains why a spider's thread or a single filament of silk 
 floats so long in the air before it falls; why there is almost constantly sus- 
 pended in the air, wherever active man resides, that immense quantity of 
 very minute solid particles, which when rendered visible by the sun's light 
 passing directly through them, are called motes in the sunbeam particles of 
 which are constantly settling on household furniture, and rendering necessary 
 the daily operation of dusting or cleaning; why the fine dust sent aloft 
 during the eruption of volcanoes is often carried by the wind to a distance 
 of hundreds of miles; why in the deserts of Africa the strong winds often 
 transport fine sand from place to place, overwhelming caravans, and forming 
 new mountains, which succeeding blasts are again to lift; why in the 
 bottom of a river, or in a tides-way, fine mud is found where the current is 
 slow ; sand where it is quicker ; pebbles, or large stones, where it is quicker 
 still j while in rapids and water-falls, only massy rocks can resist the fluid 
 force. Now .rocks, pebble, sand and mud, may all be the same material in 
 portions of different magnitude. 
 
 This law explains the operation of levigating, by which substances inso- 
 luble in water are obtained in the state of a very fine powder. Any such 
 substance is first ground or powdered in the ordinary way, and mixed with 
 water. The grosser parts then soon fall to the bottom, while the fine dust 
 
 15 
 
226 HYDRAULICS. 
 
 t. 
 
 remains longer suspended. This is afterwards obtained separately by pour- 
 ing the fluid which bears it into another vessel, and allowing more time for 
 the slow subsidence. The fine powder of flint used in the manufacture of 
 porcelain is obtained by levigation as is also that of calamine stone ; and 
 other powders used in medicine and various arts. 
 
 This law farther explains how, by means of air or water, bodies of differ- 
 ent specific gravities, although mixed ever so intimately, may be easily 
 separated. If pieces of cork, and lead be let fall together through the air, 
 the lead will reach the ground first, and may be swept away before the cork 
 arrives ; but in a vacuum the whole would reach the ground at the same 
 time as is proved by the common experiment of the guinea and feather 
 falling in the exhausted receiver of an air-pump. Again, when a mixture of 
 corn and chaff, as it comes from any threshing machine, is showered down 
 from a sieve in a current of air, the chaff being longer in falling, is carried 
 farther by the wind, while the heavier corn falls almost perpendicularly. 
 The farmer, therefore, by winnoioing in either a natural or artificial current 
 of air, readily separates the grain from the chaff; and, if he desire it, may 
 even divide the grain itself into portions of different quality. Similar to the 
 operation of separating chaff from corn by wind, is that of separating sand 
 or mud from gold-dust by water : the soil containing gold-dust is first 
 spread on a flat surface, over which a current of water is then made to pass \ 
 which current carries away the lighter rubbish, and leaves the gold. If a 
 mass of metal be affixed on the end of a rod of wood, the rod then, whether 
 simply falling through the air, or advancing as an arrow, will follow the 
 heavier metal as its points. The cork of a shuttlecock is always foremost 
 for the same reason. 
 
 The instances enumerated under this head serve to show how many and 
 varied the results may be which flow from a single principle. 
 
 When a fluid and a solid meet each other obliquely, the impulse or effect is 
 still perpendicular to the surface of the solid, as if they met directly, but 
 is less forcible as the obliquity of the approach is greater. 
 
 Suppose a b to represent the upper edge of a smooth board or of any flat 
 polished surface, standing in a current, the fluid approaching this surface 
 in whatever direction, must act upon it as if approaching perpendicularly, 
 because on account of its smoothness, the fluid can take 
 Fig. 110. no hold of it to push it endways, either towards a or b. 
 
 But the impulse of a stream acting on the surface will 
 be less forcible if the surface be oblique to the stream, 
 both because less fluid will touch, and because the velo- 
 city of the effective approach will be less. The line c d 
 marks the breadth, and therefore force, of the part of 
 a stream reaching the board directly ; and the shorter 
 line/c marks the smaller breadth that can touch it, of 
 a stream coming obliquely in the direction c I : in the 
 oblique stream, moreover, if the line c b mark the whole 
 velocity, the shorter line c a marks the slower rate of 
 the direct approach of any one particle to the board. 
 (This subject was treated of at page 57, under the head 
 of Resolution of Forces.) 
 
 Hence the wind blowing upon the sail of a ship, however obliquely 
 
OBLIQUE FLUID ACTION. 
 
 227 
 
 Fig. 112. 
 
 CL 
 
 always presses it directly forward or perpendicularly Fig. 111. 
 
 to its surface, but acts less forcibly as the obliquity is 
 greater. If the wind be represented, as to direction 
 and strength, by the line ed approaching the sail a b, 
 it will act on the sail as if it came from/*, but with 
 the smaller force f d, instead of the whole force e d. 
 The effect, therefore, is the same as if the sail were 
 pulled by the rope d c. We see in this how a ship 
 can be made to sail in a certain degree against the 
 wind > for all the sails being.adjusted so as to receive 
 the wind in the direction here shown, they all act to 
 produce the same result as if ropes were pulling from 
 
 each in the direction of dc; and a force like/c?, or a rope like<ic, urging side- 
 ways as well as forwards as instanced in the tow-rope of a canal boat 
 makes the vessel advance rapidly forward, but scarcely at all sideways, be- 
 cause the form of vessels causes them to pass forward at least twenty times 
 more easily with their sharp bow than sideways with the long keel; and 
 therefore a force urging equally sideways and forwards makes a ship advance 
 twenty miles in the direction of her keel, that is forwards, for one mile which 
 she deviates sideways. The deviation sideways, which, in sailing vessels, 
 must take place to a certain extent whenever the wind is 
 at all oblique, is called the lee-way. 
 
 A vessel having to sail from b to a, while the wind 
 blows directly against her course, or from a to b, is 
 obliged to sail dose to the wind, as represented in fig. 112, 
 first perhaps to e, as represented by this figure, with the 
 left or larboard side to the wind, then to tack as it is 
 called, or turn around, at e, and to sail to d } with the 
 right or starboard side in the wind ; then to go on the lar- 
 board tack again to c, and thence to the port at a. 
 
 In making way against a contrary wind, the sails of a 
 ship are pointed so nearly edgewise to the wind, that 
 unless very flat, a great portion of their surface becomes useless. The 
 Chinese manner of rigging is, in this respect at least, superior to the Euro- 
 pean ; for in it bamboo reeds attached across the sails, render them as flat 
 as boards. When a Chinese ship has her sails pointed edgeways to a spec- 
 tator, he only sees the masts which support them. 
 
 The reason why a ship of several masts generally sails faster when the 
 wind is more or less from a side, than when directly astern, is, that in the 
 former case all the sails are acting, although individually not to the best 
 advantage, while in the latter the sails in front are becalmed by those behind 
 them. A ship with a side-wind may move faster than the wind itself, as is 
 often true of the outer extremities of the wind-mill's vanes. A correspond- 
 ing relation of motions is observed when a slippery wedge is forced out two 
 or three inches laterally from its place, by a weight which descends only one 
 inch perpendicularly. 
 
 The law now under consideration explains the action of the rudder of 
 ships, that contrivance, by which a single steersman can direct the course 
 of an enormous vessel through rocks and shoals more steadily and safely 
 than an adroit charioteer can guide his tiny vehicle on a common road. The 
 helm or rudder is a flat projection from the stern-post of the ship, turning on 
 strong hinges, in the manner of a door or gate, and moved by a beam or lever 
 
 .-''. 
 
 b 
 
228 HYDRAULICS 
 
 called the tiller, which proceeds from it forwards to where the 
 steersman stands. In small vessels the tiller is above the de'ck 
 and the steersman applies his hand directly to it ; but in large 
 ships it is below, and is moved by ropes, rising from it to the 
 wheel on the deck, where the steersman stands, with the com- 
 pass before him. While the rudder points directly astern, as 
 to a, like a continuation of the keel and stern post, it does not 
 affect the vessel's course ; but if it be inclined ever so little to 
 one side, as to b on the left or larboard side, the water imme- 
 diately acts on it in the direction of c b. perpendicular* to its 
 surface, and pushes the stern to the right or starboard side, 
 an action equivalent to pulling to the left or larboard. 
 It is possible to make a ship or boat steer itself, by placing a powerful 
 vane on the mast-head, and connecting it with the tiller-ropes by two pro- 
 jecting arms from its axis. If it were desired to make the ship sail directly 
 before the wind, the tiller-ropes would be fixed to the arms of the vane so 
 that the helm should be in the middle position, when the vane was pointing 
 directly forward : should the vessel then from any cause deviate from her 
 course, the vane, by its changed position with respect to her, would have 
 produced a corresponding change in the position of her helm, just .such as 
 to bring her back to her course. Again, it is evident that, by adjusting such 
 a vane and rudder to each other in different ways, any other desired course 
 might be obtained, and which would alter only with the wind. The vane, 
 to have the necessary power, would require to be of large size ; it would be 
 a wide hoop, for instance, with canvas stretched upon ifc ; and the rudder, to 
 turn with little force, might be hung on an axis passed nearly through its 
 middle, instead of, as usual, by hinges at one edge. Cases have occurred 
 where shipwrecked persons might have sent intelligence of their disaster to 
 a distant coast, by a small vessel, or even a block of wood fitted up in this 
 way. The method admits also of other applications, particularly in war. 
 
 As fluids act on surfaces, in a direction perpendicular to them, the water 
 on the right side of a ship's bow is always pressing it towards the left side, 
 but owing to the equivalent and contrary pressure there, the ship holds her 
 course evenly between the two, or straight forward. When a ship, however, 
 owing to the side wind, lies over, or heels, as it is called, that side of the bow 
 which sinks most is more pressed than the other ; and were there not a 
 counteracting inclination of the rudder then made, constituting what is 
 called weather-helm, the ship's head would come round to the wind. Now 
 ships so rarely have the wind exactly astern, that to diminish the almost 
 constant necessity for weather-helm, the mast or masts, and consequently 
 the mass of the sails are placed more towards the bow than the stern. 
 
 Again, because the bow of a ship is oblique downwards as well as side- 
 ways, the water, when she moves, is constantly tending to lift the bow ; 
 hence when the vessel is dragged by a low horizontal rope, as in the case of 
 a boat attached to a sailing ship's stern, or is moved by paddle-wheels like 
 steam-boats, the bow rises much out of the water, and the stern sinks in the 
 hollow or furrow of the track : but when she is driven by sails, as these are 
 high on the mast, and are acting therefore on a long lever to depress the 
 bow, the two opposite tendencies just balance each other, and the vessel sails 
 evenly along. 
 
 The form of the fore part of a ship has less influence upon her speed of 
 sailing than the form of the hind part, called the run, from the middle to the 
 stern. When a ship is at rest ; there is of course as much forward pressure 
 
OBLIQUE FLUID ACTION. 229 
 
 of water about the stern as of backward pressure on the bow ; but when she 
 sails, she is running away from the propelling pressure, and is increasing 
 the resisting pressure. A gradual tapering of the hind part, therefore, or a 
 fine run, as it is called, which allows the water to apply itself readily to it, 
 as it passes along, must influence much the rate of sailing. The fore part 
 of any mass drawn through the water, however blunt or square, becomes in 
 effect sharp or rounded by a quantity of water which it pushes on before it. 
 A tree, or the tapering mast of a ship, can be drawn through the water 
 more easily with the large end foremost than in a contrary way. 
 
 The common windmill furnishes another illustration of the action of fluids 
 on oblique surfaces. The face of the windmill 
 is turned directly to the wind, but the four flat Fig. 114. 
 
 vanes or sails, of which the great wheel consists, 
 are individually oblique. Thus the edge a of the 
 vane a e, is more forward as regards the coming 
 wind or a spectator in front, than the edge e, and m 
 the action of the wind, therefore, being perpen- 
 dicular to the oblique surface a e, pushes it in a 
 degree towards a. The same remark applies to 
 each of the other vanes where the edges b c and 
 d are in 'front and those marked by the fainter 
 lines are behind ; so that each vane produces an 
 equal effect in turning the wheel. The law of the " decomposition offerees," 
 explained in page 57, tells in what proportion the force of the wind is exerted 
 to push the wheel backwards against its supports, and to turn it round. 
 
 Windmills were first used in Europe in the fourteenth century, and they 
 are still of great importance in countries where there are no waterfalls, and 
 little fuel for steam-engines. In some of the richest European landscapes, 
 every height is crowned by its busy windmill, grinding corn, or sawing 
 wood, or pressing oil-seeds ; and over the plains, similar wheels are pump- 
 ing water for domestic use, or incessantly draining the land. 
 
 The smoke-jack of our chimneys is a small windmill, driven by t h 
 ascending current of air in the chimney. 
 
 The feathering of an arrow acts in part on the principle of the windmill. 
 The feathery projection from the shaft is not quite straight, but winds round 
 it a little, like the thread of a screw ; and the arrow, therefore, constantly 
 turns as it flies, and goes straight to its object although the shaft itself be 
 bent, because any deviation is constantly correcting itself. 
 
 The rifle-barrel in fire-arms has spiral furrows or threads along its interior 
 surface, so that the bullet in passing out receives a turning motion corres- 
 ponding to that of an arrow, and producing similar results. A bullet which 
 receives any other turning motion than round the line of its course and 
 most bullets from a common barrel do acquire such, owing to the irregularity 
 of their form, or unequal friction at the mouth of the piece is sure to 
 deviate from its course, because unequally pressed or resisted by the atmo- 
 sphere. A good rifle fixed to its place will send a succession of shots through 
 the hole made in the target by the first shot, at the distance of 200 yards. 
 Duels have been fought with rifles, and the parties having fired at the same 
 moment, have been corpses the moment after. 
 
 It might be supposed that a wheel which the wind turned by direct action 
 on the rim, as water turns common water-wheels, would be preferable to the 
 windmill-wheel above described, which is turned by oblique action on the 
 face : accordingly, a wheel like a water-wheel, only with broader vanes, has 
 
230 HYDRAULICS. 
 
 been placed in a house or cover, so that only one side at a time was exposed 
 to the wind ; but it is a powerless machine. The oblique vane wheel may 
 apply to use only half or less of the force of the air which reaches it, but 
 its wide expanse receives a stream of air of thirty feet in diameter, while an 
 ordinary window would admit that required for a wheel of equal size of the 
 other construction. 
 
 There are some situations where it would be an advantage to have water- 
 wheels like the common windmill-wheel, viz , where the stream is sluggish, 
 and is deep enough to allow a large wheel to be wholly immersed. 
 
 A small wheel of this sort, with broad oblique vanes, has been used as a 
 means of ascertaining the rate of a ship's sailing. It is allowed to drag 
 astern in the water ; and the number of revolutions made in a given time 
 marks the ship's speed. 
 
 A windmill-wheel made to turn during a calm by force applied to its axle, 
 would be pressed endways, or in the direction of its axle, just as if wind 
 were blowing upon it, owing to the reaction of the still air, through which 
 its oblique vanes were made to sweep. Such a form of wheel fitted to work 
 in water, and called a water-screw, has been applied at the bow or stern of 
 steamboats to propel them in canals where there was no room for side 
 wheels. But as from the obliquity of the surfaces only apart of the applied 
 power becomes propulsive the remainder being wasted in the lateral strain 
 or twisting of the water the method is not applicable to general purposes. 
 Two. small windmill-wheels placed horizontally one above the other, on 
 the same axis, and made to turn in opposite ways by springs or otherwise, 
 would rise in the air, carrying a certain load with them, and would consti- 
 tute, therefore, a flying-machine. 
 
 The effect of a single oar projecting from the stern, used to propel a boat 
 or vessel, in the manner called sculling, is referable to the law now under 
 consideration. The oar or scull rests on a round-headed prop or nail at the 
 stern, and is made to vibrate from side to side. In all positions it has the 
 surface which pressess the water turned obliquely backwards; hence the 
 reaction of the water drives the boat forward. In China, large vessels are 
 moved by a single sculling oar, which half the ship's company may be urging 
 at the same time. A sculling oar may be regarded as a single vane of such 
 a propelling-wheel or water-screw as above described, made to sweep across, 
 behind the vessel, alternately to the right and to the left. 
 
 The action of a fish's tale and of the bending of an eel or snake in water, 
 partly resembles that of the sculling oar. Many people believe that the tail 
 of the fish is only the rudder of the body, and that the fins give it forward 
 motion as is true of a bird's tail and wings but the fish's tail is in fact 
 the great instrument of motion, while the fins serve 
 chiefly to steady and direct the motion. 
 
 A paper kite rising in the air is another example 
 belonging to this place. Its cord d is attached to it 
 above the middle of its loop, and therefore so as to 
 make it present always an oblique surface to the wind ; 
 and by the action of the wind, perpendicular to its 
 surface, it rises as if pushed up in the direction c a, 
 or as if drawn up in the direction of a b. A kite 
 might be made large enough to lift a man. Cats 
 have been sent up at kites' tails, and have fallen 
 down safely under parachutes from the greatest eleva- 
 tions. It might be safer for a man to rise at a kite's 
 
LIFTING OF WATER. 231 
 
 tail to reconnoitre an enemy's position, or to survey an unknown country, 
 than under a balloon, as was practiced by the French during the revolutionary 
 wars. He might have the security of a parachute, and the power of regulat- 
 ing the obliquity of attachment of the rope, so as to command his ascent or 
 descent at pleasure. An exhibition was made in October, 1827, between 
 Bath and London, of a car drawn along the highway by kites. That they 
 might ascend to a great elevation, where the wind is generally stronger than 
 below, they were attached to each other in a row, so that the second kite 
 mounted as if its cord were held by a hand at the first, the third as if rising 
 from the second, and so forth. The projector of this novelty hoped that he 
 had pointed out a most valuable means of travelling across extensive plains, 
 sandy deserts, tracks of snow, &c. ? and in all cases, nearly with the speed of 
 the wind. 
 
 " Fluids lifted in opposition to gravity." (See the Analysis.) 
 
 F Water, as we -have seen in former parts of this work is to the living uni- 
 verse, in some degree what the blood is to the animal body, and a constant 
 supply and circulation are required. This supply has been provided for to 
 an extraordinary extent, by the operation of natural causes ; but for many 
 purposes of human society, water is still required where none naturally 
 exists. A great variety of means have been employed in raising it, some 
 of which, sufficient to illustrate the whole, are now to be considered. 
 
 Water may be raised in a bucket which is attached to a rope to be pulled 
 up by the hand. The rope carrying the bucket may be drawn up more 
 easily by being wound round a barrel or axle turned by a winch. There 
 may be a succession of buckets on a rope, rising one after the other, and 
 when emptied, descending again on the opposite side of the wheel or axle 
 which lifts them : the rope to which they are attached being a circle or end- 
 less rope, and constituting with them what is called the bucket-machine. 
 Instead of buckets on such an endless rope or chain there may be a succes- 
 sion of flat pieces of wood, which, on being drawn up through a large tube 
 or barrel, like loose-fitting pistons, will raise a copious stream of water ; this 
 is the contrivance called the chain-pump. Or simply an endless rope of hair, 
 very rough, passing round one wheel above, another below, may be whirled 
 quickly by turning the upper wheel, so that a mass of water adhering by 
 friction to its rising half, shall be thrown into a reservoir at the top where it 
 passes over the upper wheel : several such ropes may be joined side by side 
 to increase the effect. But the most important of all water-raising engines 
 are the lifting and forcing pumps, already described at pages 171 and 172. 
 They are used to draw from wells, to drain mines, to send a supply over 
 cities, to pump ships, to throw water for extin- 
 guishing fires and for many other purposes. Fig. 116. 
 
 A stream of water passing through a garden, 
 or in the midst of fields, may have beauty with 
 little utility, unless it can be employed to irrigate 
 the vegetable creation around. In the fields and 
 gardens of Persia, where the heat of the sun is 
 very intense, the streams are caused by their own 
 action, to lift a part of their water into elevated 
 reservoirs, from which it again flows in sloping 
 channels to wherever it is required, A large 
 water-wheel is placed so that the stream may turn 
 
232 
 
 HYDRAULICS. 
 
 it, and around its circumference buckets are attached to be filled as they 
 sweep along below, and to be emptied into a reservoir as they pass above- 
 or instead of buckets, the spokes of the wheels themselves are made hollow, 
 and curved as here represented, so that their extremities dip into the water 
 at each evolution, they receive a quantity of it, which runs along them as 
 they rise, and is discharged into a reservoir at the centre. These are usually 
 called Persian wheels, but they are as commonly employed on the banks of 
 the Nile and elsewhere as in Persia. 
 
 A pipe wound like a screw upon a sloping barrel, and made to dip its lower 
 mouth into water at each revolution of the barrel, will also raise water ; the 
 lower portions of the turning pipe will always be full of it, and it will be 
 
 rising in them to the top, as if on 
 
 Fig. 117. an inclined plane. Archimedes 
 
 was the inventor of this beautiful 
 water-screw, and his name has re- 
 mained to it. It may be turned by 
 the hand or by a passing stream 
 wftich acts on the vanes of a water- 
 wheel fixed upon it. 
 
 Water may be raised by produc- 
 ing centrifugal force at the upper end of a bent pipe which dips into a res- 
 ervoir. Supposing the pipe to be bent as here represented, 
 Fig. 118. and the horizontal arm a to turn like the spoke of a wheel, 
 round the upright portion as the axis, if the pipe be once 
 filled with water, and be turned with sufficient speed, it will 
 continue to throw out a constant stream from the end a. To 
 increase the discharge there may be several horizontal arms 
 from one large upright pipe, and emptying themselves into a 
 circular trough or reservoir; and to prevent the necessity of 
 refilling the apparatus after every interruption of its motion, 
 a valve opening upwards must be placed at the bottom. This 
 contrivance has been called the centrifugal pump, because the 
 water is raised at b as in a pump, by the pressure of the atmosphere, to sup- 
 ply the place of that which is thrown out from a by the centrifugal force. 
 The velocity of rotation must bear proportion to the height of the discharging 
 aperture a, above the surface of the water in the reservoir. 
 
 It had long been observed in household experience and elsewhere, that 
 while water is running through a pipe, if a cock at the extremity be suddenly 
 shut, a shock and noise are produced there. The reason is, that the forward 
 motion of the whole water contained in the pipe being instantly arrested, and 
 the momentum of a liquid being as great as if a solid, the water strikes the 
 cock with as much force as if it were a long bar of metal or a rod of wood 
 having the same weight and velocity as the water. Then as a fluid presses 
 equally in all directions, a leaden pipe of great length may be widened, or 
 even burst in this experiment. Lately this forward pressure of an arrested 
 stream has been used as a force for raising water, and the arrangement of 
 parts contrived to render it available has been called, on account of the shocks 
 produced, the water-ram. The ram may be described as a sloping pipe in 
 which the stream runs, having a valve at its lower end, to be shut at inter- 
 vals to arrest the stream, and having a small tube rising from near that end 
 towards a reservoir above, to receive a portion of the water at each interrup- 
 tion. Now water allowed to run for one second, in a pipe ten yards long, 
 
LIFTING OF WATER. 
 
 233 
 
 Fig. 119. 
 
 O, 
 
 two inches wide and sloping six feet, acquires 
 momentum enough to drive about half a pint, 
 on the shutting of the cock, into a tube lead- 
 ing to a reservoir forty feet high. Such an 
 apparatus, therefore, with the valve shutting 
 every second, raises about sixty half-pints or 
 four gallons in a minute. The* valve is so 
 contrived that the steam works it as desired 
 In this figure, which represents the lower end 
 
 of the water-ram, a is the opening by which _ 
 
 the stream escapes from it, and the valve or 
 
 flap seen below the opening is that which by suddenly shutting arrests the 
 stream. The valve is made so heavy, that the stream must run for a certain 
 time to acquire force enough to shut it : and in the instant of its shutting, 
 a little of the advancing water passes upwards through the valve b towards 
 the reservoir. The water in the main pipe then becoming stagnant again, 
 no longer has power, by its weight alone, to keep the valve a shut : this, 
 therefore, falls open, and the stream begins again, again to be arrested as 
 before ; and as long as the supply of water lasts, the action of the apparatus 
 continues. The action of a water-ram has been compared to the beating of 
 an animal's pulse, The upright tube has usually a reservoir at the bottom, 
 where it first receives the water, constituting there an air-vessel 5, (described 
 at page 161) which, by the air's elasticity, converts the interrupted jets first 
 received, into a nearly uniform current towards the reservoir. The supply 
 of air to this vessel is manifested by the contrivance called a sJiiftmg-valve. 
 In the preceding examination of the doctrines of fluidity, we have had to 
 touch on many of those phenomena of nature and art which are the most 
 important to man ; yet we have seen how beautifully simple and intelligible 
 they are all rendered when referred, by a methodical arrangement, to a few 
 heads dependent on the '" fundamental truths." Each one of the many 
 particulars belonging to this department, and which, when now explained, 
 appears so simple and obvious, has yet been a distinct step in the slow 
 progress of discovery or invention, and probably, when first made, has filled 
 some ingenious mind with intense and purest delight. 
 
234 ACOUSTICS. 
 
 PART III. 
 
 (CONTINUED.) 
 
 SECTION IV. ACOUSTICS, 
 OR PHENOMENA OF SOUND AND HEARING. 
 
 ANALYSIS OP THE SECTION. 
 
 1. SOUND is heard when any sudden shock or impulse is given to the air, 
 or to any other body which is in contact directly or indirectly with the ear. 
 
 2. If such impulses be repeated at very short intervals, the ear cannot attend 
 to them individually, but hears them as a CONTINUED SOUND, which is 
 UNIFORM, or what is called a TONE, if the impulse be similar and at equal 
 intervals, and is GRAVE or SHARP, according as they are few or many in 
 a given time / and all continue^ sound is but a repetition of impulses. 
 
 3. When the number of impulses in a givn time producing some uniform 
 continued sound has a simple relation, as of half, third, fourth, &c., to 
 the number producing some other such sound, which is heard either simul- 
 taneously with it, or a little before or after, the ear is generally much and 
 pleasingly affected by the circumstance ; and the sounds are said to have 
 MUSICAL RELATION to each other, or to be ACCORDANT, while all others 
 are termed DISCORDANT. 
 
 4. The shock which causes the sensation of sound SPREADS or is propagated 
 in all bodies, somewhat as a wave spreads in water, uith decreasing 
 strength as the distance increases, but with a velocity nearly uniform, and 
 which in air is about 1,1.^2 feet per second. 
 
 5. Sound is REFLECTED from smooth surfaces, and hence arise many curious 
 and pleasing effects called ECHOES, &c. 
 
 6. The structure of the ear illustrates the law of sound. 
 
 EARLY inquiries into nature had remarked that in most instances of noise 
 or sound there was present a shock or trembling of the sounding body, often 
 visible, but sometimes only sensible to the touch, or discoverable by other 
 means ; it was noted for instance, in the string of a harp, in the reed of a 
 hautboy, in the prongs of a tuning-fork, in the lip of a bell; but it was 
 reserved for the moderns to understand fully, that the animal organ called 
 the ear, is merely a structure of parts admirably adapted to be affected by 
 the concussions or tremblings of things around ; and that sounds in all their 
 varieties are merely such motions, affecting the ear through the medium of 
 
CONTINUED SOUND. 235 
 
 the air which surrounds it, or of some other body, or series of bodies reach- 
 ing from the trembling thing to the ear. 
 
 The delicacy and complexity of an organ destined to feel and to distin- 
 guish such slight and varying influences, and the vast importance of it to 
 man, as that which makes him capable of using language, besides being his 
 ever-watchful monitor of surrounding occurrences, and the channel by which 
 the fascination of music enters, render this subject, to all who love to read 
 in nature the attributes of its author, a most favourite study. 
 
 Because all the bodies around us are immersed, in common with ourselves, 
 in the ocean of air which covers the earth, we are much more frequently 
 warned of the shocks and tremblings of which we have been speaking, by 
 their effect on the air, than in any other way; hence the early prejudice that 
 air was necessary to sound, and hence, in part, the reason why the doctrines 
 of sound have generally been accounted a part of pneumatics. We shall 
 now find, however, that all bodies are more or less fitted to convey these 
 tremblings, and that air in many cases is neither the quickest nor the best 
 conductor. Although our notions on the subject are thus corrected, it is 
 still convenient to study the theory of sound as a part of Pneumatics. 
 
 1. " Sound is heard wTien any sudden shock or impulse occurs in a body 
 having communication through the air or otherwise, with the ear." 
 (Read the Analysis.) 
 
 Common instances of a single impulse are the blow of a hammer the 
 clap of hands the crack of a whip a pistol-shot any explosion the 
 thunder-clap. 
 
 The loudness of sound conveyed by the air depends on the air's density. A 
 bell enclosed in the receiver of an air-pump is heard less and less distinctly 
 as the air is exhausted, and in a vacuum is not heard at all. Even the blow 
 of a hammer, in a vacuum, is not heajrd if care is taken to prevent the shock 
 from being communicated through neighbouring solid bodies^ In the thin 
 air surrounding a lofty mountain-top the report of a pistol is much less loud, 
 and human voices are weaker. In the condensed atmosphere of a diving- 
 bell a whisper is loud. When volcanoes and various other resemblances 
 to the constitution of our earth were first discovered in the moon, some 
 persons fancied that during the stillness of night we should hear the thunder 
 there: but supposing the thunder to happen, and to be ever so loud, it 
 could not be heard on earth, because there is no medium to bear thither the 
 pulses of sound there is a vacuum between. 
 
 2. Impulses quickly repeated cannot be individually attended to ~by the ear, 
 and hence they appear as one continued sound, of which the pitch or tone 
 depends upon the number occurring in a given time ; and all continued 
 sound is but a repetition of impulses. (Read the Analysis.) 
 
 If a wheel with teeth be made to turn and to strike any elastic plate, as 
 a piece of quill, with every tooth, it will, when moved slowly, allow every 
 tooth to be seen and every blow to be separately heard ; but with increasing 
 velocity the eye will lose sight of the individual teeth, and the ear ceasing 
 to perceive the separate blows, will at last hear only a smooth continued 
 sound called a tone, of which the^character will change with the velocity of 
 the wheel. 
 
 In like manner the vibrations of a long harp-string, while it is very slack, 
 are separately visible, and the pulses produced by it in the air are separately 
 
236 ACOUSTICS. 
 
 audible; but as it is gradually tightened, its vibrations quicken, so that, 
 where it is moving, the eye soon sees only a broad shadowy bellying line ; 
 and the distinct sound which the ear lately perceived, seeming now to run 
 together on account o'f the shortness of the intervals, are felt as one uniform 
 continued tone, which constitues the note or sound then belonging to the 
 string. 
 
 Again, if a current of air passing through a tube or opening, be in any 
 way interrupted at regular and very short intervals, as by a little stop-cock 
 placed in the opening, of which cock the plug, instead of being only partially 
 turned by a cross handle, as in a common beer-cock, has a wheel fixed upon 
 it, so that any desired rapidity of rotation may be given to it, then at every 
 time when the passage for air becomes open, there will be a certain shock 
 given to the air around, and the repetition of such shocks will constitute a 
 musical tone. This apparatus can produce all tones, and it enables us with 
 great precision to ascertain the number of pulses required to constitute any 
 given tone. 
 
 It is the elasticity of any string used to produce a tone which causes the 
 repetition of the percussion, and therefore the continuance of the sound, 
 thus : the string having been pulled at its middle to one side, and then let 
 go, is, owing to its elasticity, carried back quickly to the straight position ; 
 but by the time that it has reached this, it has acquired a momentum which, 
 like the momentum of a vibrating pendulum, carries it nearly as far beyond 
 the middle station as the distance whence it came : it has to return, 
 therefore, by its elasticity, from this second deviation, in the same way ; but 
 still passing the middle as before, it has again to return ; and thus continues 
 vibrating uniformly as a pendulum does, until the resistance of the air and 
 friction gradually bring it to rest. A large vibration of any string, like a 
 large oscillation of a pendulum, occupies very nearly the same time as a 
 smaller, because the farther that the string is displaced or bent, the more 
 forcibly, and therefore quickly, is it pulled back again by its elasticity ; 
 hence the uniformity of the tone produced by a musical string is not injured 
 by the different force with which the finger of the player may touch the 
 string. According, however, as the vibrations of a string are more extensive 
 or quicker, the impulses given to the air are more sharp or forcible, and 
 hence the sound becomes louder. And this explains why sharp sounds are 
 generally also loud. Vibrations which are comparatively few and slow, 
 strike the ear very gently, as in the flapping of a pigeon's wing, or in the 
 play of a switch. 
 
 The most familiar instance of sounding -vibration is that of an elastic cord 
 extended between two fixed points, as in stringed instruments of music ; but 
 because elastic bodies generally, when by any force their natural form is for 
 a time altered, recover it when allowed, not by a first effort, but like the 
 string of a pendulum, after a series of oscillations, almost all such bodies 
 repeat many times an impulse once given to them, and thus may become the 
 means of producing a continued sound. If a solid rod of steel, glass, or any 
 other elastic substance, be firmly fixed at one end and left free at the other, 
 and if that other be then pulled a little to one side of its station or rest, and 
 suddenly let go, it will immediately seek its station again, but by the momen- 
 tum acquired in the approach, will go beyond it : it will then return as be- 
 fore, but again to pass, and so will continue to vibrate with diminishing force 
 for considerable time. A boy at school, thus, sticks the point of his pen- 
 knife into the bench, and by one touch makes it produce a continued uniform 
 sound of considerable duration. The prongs of a tuning-fork, or of the com- 
 
CONTINUED SOUND. 237 
 
 mon sugar tongs, vibrate and sound in the same way. In. the musical snuff- 
 boxes and chimney-clocks, the sounds are produced by the vibration of little 
 rods of steel, fixed by one end, in a row, like the teeth of a comb, and 
 touched by small pins or points projecting from a turning barrel. Any 
 elastic flap, as of metal or of tough wood, placed over an opening, so as to 
 stand away from it a little when not pressed by passing air, but to close the 
 opening if so pressed, becomes a sounding reed when air is gently forced 
 through the opening : thus, the air pressing on the flap to close them causes 
 a momentary interruption of the current, but the flap immediately recoiling 
 from the blow, as well as by reason of its own elasticity, again opens the 
 passage, and the continued rapid alteration of the shutting and opening pro- 
 duces the tone. The reed of a clarionet is a thin plate of elastic wood, made 
 to vibrate in this way. The drone of the bag-pipe and the common straw- 
 pipe, are reeds of nearly the same kind. The Chinese organ, and the sweet 
 instrument lately introduced under the name of .^Eolian, have reeds which 
 differ from these, by beating through the opening instead of merely on its 
 face. Elastic rods simply resting on supports at both ends, or suspended by 
 their middle, will also vibrate; a musical instrument is thus made of pieces 
 of glass laid upon two strings, and struck by a cork hammer : in the island 
 of Java, a rude instrument of the same kind is made of blocks of hard 
 elastic woodF. A portion of a hollow sphere of elastic metal very readily 
 takes on a vibration, during which its form is constantly changing from the 
 perfect round to- the oval, and conversely ; there are consequently repeated 
 percussion of the air, and a continued sound, and the thing is called a bell. 
 A bell admits a great variety of shapes, and may be made of any elastic sub- 
 stance, as metal, glass, earthenware, (buyers ring earthenware to ascertain 
 its soundness,) and even of hard wood. The Chinese gong is a metallic vessel 
 shaped like a common sieve, having a manner of vibration very peculiar, 
 and producing sounds that are rousing and sublime. The drum has a tense 
 elastic membrane on which the blows of the drum-stick are received : its 
 tone ceases quickly, because the motion of so broad a surface is much' 
 resisted by the air. In the flute, flageolet, common organ-pipes, &c., the 
 air is forced through narrow passages, and is divided by sharp edges, in such 
 a way as to suffer repeated but perfectly regular condensations or interrup- 
 tions surncient to affect the ear*; and hence the endless variety of sweet 
 continued sounds which these contrivances are known to produce. 
 
 To the production of a tone, it is of no consequence in what way the 
 pulses of the air are caused, provided they follow with sufficient regularity ; 
 witness, in addition to some of the instances given above, the pure sound 
 produced by the motion of a fly's wing supposed by many to be the voice 
 of an insect. The clacking of a corn-mill, and the noise of a stick pulled 
 along a grating, are not tones, because the pulses follow too slowly. 
 
 Where a continued sound is produced by impulses which do not, like 
 those of an elastic body, follow in regular succession, the effect ceases to be 
 a clear uniform sound or tone, and is called a noise. Such is the sound of 
 a saw or grind-stone the roar of the waves breaking on a rocky shore, or 
 of a violent wind in a forest the roar and crackling of houses or of a wood 
 in flames the mixed voices of a talking multitude the diversified sounds 
 of a great city, including -the rattling of wheels, the clanking of hammers, 
 the voices of street-criers, the. noises of manufactories, &c. ; which rough 
 elements, however, at last mingle so completely that the combined result 
 has often been called " the hum of men/' from analogy to the smooth 
 mingling miniature sounds which constitute the hum of a bee-hive. 
 
238 ACOUSTICS. 
 
 " Grave and sharp sounds." (Read the Analysis.) 
 
 The difference of sounds, which depends on the different number of vibra- 
 tions of the sounding body in a given time, divides them into those called 
 bass, low, or grave notes, for comparatively few and slow vibrations ; and 
 those called high, shrill, or sharp, for vibrations more numerous and quick. 
 
 The frequency of vibrations in strings increases with their shortness, light- 
 ness and tension for if a string be long or heavy, there is a greater mass of 
 matter to be moved, and hence a slower motion; and if a string be slack, 
 the force of elasticity which pulls it from any deviation back to the straight 
 line is so much the less. It is found that a string taken of half the length 
 or of one-fourth the weight, or of quadruple the tension of another string, 
 vibrates twice as fast on any one of these accounts. 
 
 These truths are familiarly illustrated in the violin. The low or bass 
 string is thick and very heavy from being covered with metallic wire, and the 
 others gradually diminish in magnitude and weight, up to the smallest or 
 treble. The strings are tuned to each other by being attached by one end 
 to moveable pins, which, when tuned, increase or diminish their tension ; and 
 the sound produced by each may be afterwards varied to a certain extent, 
 by the performer pressing different parts of it with the finger against the 
 board, so as to shorten the vibrating portion. 
 
 An analogous law, as to the influence upon tone, of weight and dimen- 
 sions, holds with respect to bells, glasses, reeds, &c., and enables us to use 
 these also in the construction of musical instruments. 
 
 3. " When the number of impulses producing some continued sound has a 
 simple rotation, as of half, third, fourth, &c., to the number producing 
 some other sound, which is heard either simultaneously, or a little before 
 or after it, the ear is much and pleasingly affected ; and the sounds are 
 said to have musical relation to each other, or to be accordant, while all 
 others are termed discordant." (Read the Analysis.) 
 
 Understanding now that all continued uniform sounds are produced by a 
 repetition of similar beats or vibrations, we perceive that in the series from 
 grave to sharp, there^must be such as, with respect to the number of beats 
 in a given time, are related to each other, as the numbers 1, 2, 3, 4, &c., or, 
 which is the same thing, as 10, 20, 30, &c. Now as between two sounds, 
 one of which has 20 beats while another has 10, there will be a coincidence 
 by every second beat of the- quicker, and between sounds whose beats are to 
 each other as 30 to 20, there must be a coincidence at every third beat of 
 the quicker, and so forth, we should naturally expect the ear to be differently 
 affected by such correspondence than when the coincidence is either less 
 frequent, or is irregular. Accordingly we find that all sounds which have 
 such simple relations to each other, are remarkably agreeable to the ear, 
 either when heard together, or in close succession ; while those in which the 
 coincident beats are farther apart, are heard with indifference, or are felt to 
 be positively harsh or disagreeable. It is in fact offering itself to be noticed 
 here, that the coincident or double pulses of any two concordant sounds be- 
 come the cause of elements of a third sound, perfectly distinct from them, 
 but which is always heard with them, and is called their grave harmonic or 
 resultant: it is the same as a simple sound having as many vibrations in a 
 given time as there are coinciding beats between the two other sounds. 
 
 If a long musical string be made to sound, and the number of its vibrations 
 in a given time be ascertained, we find that if only half of it be allowed to 
 
MUSIC. 239 
 
 vibrate at a time, as when a finger presses its middle against a board, that 
 half will vibrate twice as fast ; and similarly, a third part three times as 
 fast; a fourth part four times as fast; and so on, producing the sounds or 
 tones most nearly related to each other. A fine illustration of this is afforded 
 by the string of a violoncello, when made to vibrate by a bow moved very 
 gently across it, near the bridge ; for it often divides itself spontaneously 
 into two, three or four, &c., equally vibrating parts or bellies, with points 
 of rest between them called knots : when this happens, there are heard not 
 only the sound or note belonging to the whole length of the string, but, 
 also, more feebly, the subordinate notes belonging to its half, third, or 
 fourth, &c., according to circumstances, beautifully mingling with the first 
 sound, and forming with it a rich harmony. Often in such a case the sub- 
 ordinate sounds swell with such force as to overpower for a time the funda- 
 mental note : but any one such sound is rarely of long duration. The same 
 harmonic sounds may be produced still more certainly, while drawing the 
 the bow across the string, by touching the string lightly with the finger, at 
 one of the points where we wish it to divide. Even a tune may be so 
 played. 
 
 The sounds thus belonging to a single cord or string, and produced by its 
 spontaneous division into different numbers of equal parts, constitute, when 
 heard together or in succession, what may be called the simple music of 
 nature herself. It is produced pleasingly, as just described, by the single 
 string of a violoncello ; but in the most perfect manner by the instrument 
 called the ^Eolian harp. 
 
 The JEolian harp is a long box or case of light wood, with harp or violin 
 strings extended on its face. These are generally tuned in perfect unison 
 with each other, or to the same pitch, as it is expressed, except one serving 
 as a bass, which is thicker than the others, and vibrates only half as fast; 
 but when the harp is suspended among trees, or in any situation where the 
 fluctuating breeze may reach it, each string, according to the manner in 
 which it receives the blast, sounds either entire, or breaks into some of the 
 simple divisions above described j the result of which is the production of 
 the most pleasing combination and succession of sounds that ear has ever 
 listened to, or fancy perhaps conceived. After a pause this fairy harp may 
 be heard beginning with a low and solemn note, like the bass of distant 
 music in the sky : the sound then swells as if approaching, and other tones 
 break forth, mingling with the first, and with each other ; i-n the combined 
 and varying strain, sometimes one clear note predominates and sometimes 
 another, as if single musicians alternately led the band : and the concert 
 often seems to approach and again to recede, until with the unequal breeze 
 it dies away, and all is hushed again. It is no wonder that the ancients, 
 who understood not the nature of air, nor consequently even of simple 
 sound, should have deemed the music of the ^Eolian harp supernatural, and, 
 in their warm imaginations, should have supposed that it was the strain of 
 invisible beings from above, come down in the stillness of evening or night 
 to commune with men in a heavenly language of soul intelligible to both. 
 But even now that we understand it well, there are few persons so insensible 
 to what is delicate and beautiful in nature, as to listen to this wild music 
 without emotion ; while the informed ear finds it additionally delightful, as 
 affording an admirable illustration of those laws of sound which human 
 ingenuity at last has traced. 
 
 And the simple scale of sound, called a chord, which nature thus gives 
 by the spontaneous dividing of a single string, has considerable vacancies in 
 
240 
 
 ACOUSTICS. 
 
 it, human taste or feeling, long before there was any theory of music, had 
 joined to it the notes of two additional strings, one sharper or more acute 
 than it, and the other more grave ; of which additional notes, while part 
 agreed, or were in unison with certain notes of the principal chord, the 
 remainder just served to fill up its larger intervals, and to complete a scale 
 of nearly uniform interval as three ladders having unequal intervals 
 between their steps, might still, if placed together, complete a stair of easy 
 ascent. The relation between these strings or chords is such, that the prin- 
 cipal beats thrice or twice of the low chord, and the high chord beats thrice 
 for twice of the principal : and in the complete scale of notes, the principal 
 is five notes above the lower and five notes below the higher. So truly 
 natural is the scale thus formed, that it has arisen in all nations, however 
 remote and unconnected ; and an untutored individual in attempting to raise 
 his voice by regular steps, falls into it almost as readily as the learned pro- 
 fessor. The scale has eight steps or notes between any tone, and the tone 
 above it vibrating twice as fast, or the tone below it vibrating half as fast j 
 these two tones or notes being hence called the octaves above and below the 
 note with which they are compared, and the intermediate notes which fill up 
 either octave from the fundamental note are distinguished by the names of 
 second, third, fourth, &c., in ascending or descending. The numbers which 
 express the relations of beats among the notes of an octave are easily found, 
 from our knowing the relative number of beats in the notes of any one 
 simple chord, and the relation as above described of the -three chords form- 
 ing the compound scale. The following table exhibits these numbers or 
 the arithmetical expression for the notes of an octave, as well as the 
 corresponding lengths of a given string required to produce them, and the 
 English designation of the notes by letters, and the continental designation 
 by names, these names being the first syllables of certain verses sung by 
 learners. 
 
 Number of vibra- ) 
 
 1 
 
 I 
 
 
 
 I 
 
 1 
 
 
 
 
 
 2 
 
 tions* . . j 
 
 
 
 
 
 
 
 
 
 Length of string 
 
 1 
 
 ! 
 
 4 
 
 5 
 
 | 
 
 i 
 
 - 
 
 T 8 5 
 
 a 
 
 English characters 
 
 C 
 
 1) 
 
 E 
 
 F 
 
 G 
 
 A 
 
 B 
 
 c 
 
 Continental names 
 
 ut 
 
 re 
 
 mi 
 
 fa 
 
 sol 
 
 la 
 
 SI 
 
 ut 
 
 The musical scale, however far extended, is a repetition of similar octaves, 
 so that any note in it vibrates just twice as often as the corresponding note 
 in the octave below, and half as often as that in the octave above. The 
 lowest note which is perceptible to the human ear has about thirty beats in 
 a second, and the highest about thirty thousand : and there is included be- 
 tween these two, a range of nearly ten octaves. To certain ears the extremes 
 of this range are totally inaudible, as if their power did not reach so far. 
 Some persons do not hear at all the sharp note of the grasshopper, while 
 some are equally insensible to the lowest tones of an organ or piano ; and 
 yet to all the perception of intermediate sounds may be equally perfect. 
 Few musical instruments comprehend more than six octaves, and the human 
 voice in general has only from one to three, the female voice being in pitch 
 an octave higher than the male. 
 
 If the intervals in the musical scale were all equal, a performer might 
 choose indifferently any note as a fundamental or key-note, and would only 
 have to attend to the number of intervals above and below it; but, in fact, 
 the relation of the three constituent chords is such that the third and seventh 
 intervals, in ascending from a key-note, are only about half as large as the 
 others. It is owing to this circumstance that in changwy the key on any 
 
MUSIC. 241 
 
 instrument, certain notes belonging to other keys are half a note too low or 
 too high, that is, too flat or too sharp, and must he changed accordingly. 
 And hence, when an instrument is to be used to play on all keys, its larger 
 intervals must be divided into two parts. The fact of these unequal inter- 
 vals, ill understood, is what gives an appearance of great complexity and 
 difficulty to musical science. 
 
 Melody, in music, is when notes, having the simple numerical relations 
 of beat which we have been describing, are played in succession ', harmony 
 is when two or more such notes are sounded together. The effect of both 
 is delightfully increased by what is called measure, viz., making the dura- 
 tion of the notes or strains correspond with certain regular divisions of time. 
 This gives to the ear a prescience, to a certain degree, of what is coming, 
 with the pleasure of having expectation realized, as happens similarly from 
 the metre and rhyme of poetry ', it moreover enables the memory to retain 
 musical combinations of sound for the airs of the ^olian harp, which 
 observe no time, cannot be learned or repeated. The music of a single 
 drum is that of time only. 
 
 Melody, harmony, time and varying intensity of sound, are the four con- 
 stituents of music, and it seems that almost every state of mind has, in some 
 combination of these, an appropriate expression, intelligible to the general 
 feelings of the human race. The exact relation between the movements of 
 the animal spirits, as it has been expressed, or the fluctuating stream of 
 feeling, and the varying flow of sound in a musical composition is not clearly 
 understood, but the fact of their correspondence and its consequences are 
 most remarkable. Under many circumstances, the association between the 
 feeling and expression is so strong, that the latter is often spontaneously be- 
 traying itself; witness the almost constant humming or low song of some 
 contented beings the singing and whistling of careless childhood, or of the 
 light-hearted rustic living among the beauties of nature the heart-rousing 
 strain of the hunter or warrior and the tender expression of many of the 
 modifications of anxiety and sorrow. The musical sensibilities are by no 
 means limited to the human race, for there is no expression more exquisite 
 than in the song of the nightingale during the evenings of spring, or of the 
 thrush and blackbird, in the same season, amid the quiet retreats of our 
 woodlands, the music of which untutored songsters is made up of the same 
 elements as our own. 
 
 The accompaniment of an air afforded to a singer by one or more instru- 
 ments, and which is so pleasing, is chiefly the sounding, simultaneously, in 
 a subdued manner, some other notes of the chords to which the several vocal 
 notes belong. Duetts and more complicated concert-pieces have their origin 
 from the same source : and highly cultivated musical sense can even follow 
 and enjoy several melodies played together. 
 
 Musical notes, by whatever instrument produced, have to each other the 
 same numerical relations, in the beats or vibrations which constitute them. 
 The different qualities of tone, therefore, from different instruments, can 
 only depend on the peculiarities of the single beat, as to whether they are 
 sharp or soft, strong or weak, &c. Such is the extraordinary nicety of per- 
 ception which the human ear possesses in this respect, that it cannot only 
 distinguish different kinds of instruments, as a flute and clarionet, playing the 
 same note, but different instruments of the same kind, even to the extent 
 for instance, of recognizing each one of a hundred voices singing the same 
 air. One of the greatest charms of concert music is, that the voice and the 
 different instruments may take up separately, parts of the strain suited to 
 
 16 
 
242 ACOUSTICS. 
 
 their individual expression the flute and clarionet, for instance, breathe soft- 
 ness; the trumpet and drum arouse ; the harp rolls forth its brilliant chord; 
 the violin leads the flowing sounds through rapid and endless variety; and 
 so of the rest. 
 
 That there might be correspondence in instruments when played together 
 and a k*nown pitch when played apart, it became necessary to fix on some 
 tone or number of vibrations as a point of comparison. Hence, tuning-forks 
 have been made of steel, with length of prongs calculated to produce a certain 
 note. The note is usually the fourth, A or la from the bass of the piano- 
 forte, and vibrates about 430 times in a second ; and when the note of the 
 game name on any instrument is tuned in unison with this, the other notes 
 can be easily adjusted according to the harmonic relations above explained. 
 
 Almost every substance or contrivance that can produce a uniform con- 
 tinued sound may enter into the composition of a musical instrument; hence 
 the almost endless variety which the world has seen, The chief classes of 
 instruments are stringed instruments, wind instruments and bells or rods. 
 
 Of the stringed instruments we may mention the harp, the lyre or lute, 
 the guitar, the viol of all sizes, and piano-forte. The harp, lyre and lute 
 were the inventions of antiquity, and have brought down with them to the 
 present times a thousand delightful associations. They awakened to inspi- 
 ration the bards and poets of the young world, and they were the beloved 
 companions of many of the noblest spirits of succeeding times. Their 
 great charm appears to have been in their power to heighten the emotions 
 produced by music's twin sister, poetry; and the combined effects seem to 
 have been magical. The other instruments mentioned are of comparatively 
 modern invention, particularly the piano-forte; and their perfection has 
 assisted in carrying the combination of musical sound to degrees of com- 
 plexity and difficulty of which antiquity dreamt not. It is a question, how- 
 ever, whether the style of much of the music now in vogue does not prove 
 rather a degeneracy, than a desirable refinement of musical taste. Music is 
 a language of nature, intelligible at once to all susceptible minds, and, in a 
 degree even to inferior animals; but modern art is attempting to make of it 
 an artificial and conventional language, in which there may be fashion and 
 change. The ornaments and accompaniments are now often so overwhelm- 
 ing, that the melody, in which the idea and sentiment really reside, is 
 masked and almost lost ; and an unpractised ear, particularly if listening to 
 an organ, often discovers only an unmeaning succession of chords. And 
 when a singer, abandoning the natural simplicity of melody, strains to exe- 
 cute with the voice the complicated movements which belong properly to 
 instrumental accompaniments, the attempt destroys the poetry, by either 
 rendering the words inaudible, or by sacrificing their natural expression to 
 some supposed appropriate expression of the ornamental music. These 
 considerations may account in part for the insensibility of so many highly- 
 endowed persons to wliat is now called excellent music. Some of the 
 tricks on the voice and on instruments, at present so common, are, to 
 natural or graceful music, what tumbling or rope dancing are to natural 
 or graceful gesture. And when we hear noted professors avow their in- 
 ability to sing a simple ballad, or to play an unadorned melody, must we 
 not conclude that the natural sense of music has left them, as the relish 
 for simple but the most invigorating fare has left the morbid epicure? 
 
 The guitar, as affording an accompaniment to vocal music, has many 
 advantages. It is not too loud, yet the strains are very distinct ; it admits of 
 most touching expression ; it is very easily learned by any one who should 
 
MUSIC. 213 
 
 attempt to learn ^nusic ; it is portable and cheap. The great facility of ac- 
 companiment on it depends on this, that the player is able by one position 
 of the hand to touch the strings so that the sound of all the six shall belong 
 to the same chord : three positions of the hand, therefore, for one key, pro- 
 duce all the notes and chords which a simple accompaniment requires; and 
 the hand soon falls into these so readily, that the player is hardly sensible 
 of exerting volition. 
 
 Among wind instruments are ihe flute, the flageolet, the organ, the clario- 
 net, the hautboy, the horn, the trumpet, &c. The pitch or tone of a tubular 
 wind instrument, just as of a musical string, has relation to its length ; and 
 the vibrations causing the sound seem to be waves or condensations of air 
 passing from the mouth to the extremity of the tube ; being more frequent, 
 therefore, as the tube is shorter; when the bottom of the tube is closed, 
 the wave has to come back again, and thus renders the note twice as grave. 
 It appears, also, that on blowing more strongly, the air in the tube divides 
 into separate vibrating portions, as a string may divide to produce its har- 
 monic sounds, and produces thus all the harmonic sounds belonging to the 
 fundamental note at the tube. By blowing into a common G-erman flute, 
 for instance, it is possible to produce five ascending harmonics without mov- 
 ing the fingers at all. The music of a trumpet is limited to these five notes 
 of the same chord ; but in the flute and other instruments with holes, the 
 effective length of the tube is calculated from the upper end to the nearest 
 hole left open ; and each length has its harmonics. If a tuning fork, Jew's- 
 harp, or any such sounding body, be held at the open end of a tube or other 
 empty space of dimensions calculated to produce a frequency of undulation, 
 in its contained air, according with the pulses of the sounding body, then 
 the tube or space will immediately give out its own beautiful tone ; and if 
 the space be enlarged or diminished in a double, treble or any other simple 
 proportion as a tone may be, by a piston moved up or down in it then 
 will its note become the fifth, octave, twelfth, &c., above or below the origi- 
 nal tone, although that tone continues unchanged. The tones of the Jew's- 
 harp are well known to depend altogether on the varying dimensions of the 
 player's mouth ; but to obtain perfect music from it, three harps at least, to 
 be substituted one for the other during the performance, are required to 
 produce the notes of the three constituent chords of the common musical 
 scale. In wind-instruments with reeds, the tone depends on the stiffness, 
 weight, length, &c., of the vibrating plate or tongue of the reed, as well 
 as on the dimensions of the tube or space with which it may be connected. 
 This truth is well illustrated in that instrument, the .ZEolian, already men- 
 tioned, which, in improved and varied forms, promises to become common, 
 and one of the most expressive wind-instruments. The sounds of the human 
 voice are the sweetest of all, and are produced by the vibrations of two de- 
 licate membranes situated at the top of the windpipe, with a slit or opening, 
 called the glottis, left between them, for the passage of the air. The tones 
 of the voice are grave or acute, according to the varying tension of these 
 membranes, and to the size of the opening. In the organ there is a pipe 
 for each note, and wind is admitted from the bellows to the pipes, by the 
 action of the keys, like the keys of a piano-forte. The organ may be 
 played also very perfectly by a barrel, made to turn slowly under the keys, 
 and to lift them in passing, by pins projecting from it at the required situa- 
 tions. Very complicated pieces of music are thus set on barrels, but by a great 
 cost of study and labour, and, therefore, of money ; now a plain barrel, made 
 to turn near the keys of an organ during performance on it by the hands, 
 
244 
 
 ACOUSTICS. 
 
 Fig. 120. 
 
 might be made to record, with mathematical accuracy, every touch of the 
 most finished player, by receiving marks of some kind from the keys as they 
 were lifted ; and to repeat with absolute accuracy, therefore any performance, 
 however delicate and exquisite, it would only be farther necessary to drive 
 pins into the barrel where the marks remained, and afterwards make these 
 pins lift the keys. 
 
 Bells are often conjoined in sets, having the musical relation, and to some 
 persons their music is very agreeable. There are, in the tolling of a single 
 bell, a loudness and a solemnity rendering it a fit accompaniment of funeral 
 rites. 
 
 The Chinese gong partakes of the nature both of the bell and of a great 
 drum, and has something in its sound which is singularly affecting. In its 
 own country it bears a part in one of the most imposing ceremonies which 
 man has ever imagined. On certain festivals as the sun is sinking in the 
 west, the whole population of China, a host of more than a hundred millions, 
 issues forth under the single canopy of heaven, to testify, amid the thunder 
 of gongs and the continued discharge of fire works, that adoration and grati- 
 tude towards the Deity which human nature, in all ages and climes, has felt 
 to be due, and has eagerly sought to express, however blind as to the sublime 
 simplicity of religious truth. 
 
 Bells or goblets of glass sound still more perfectly than those of metal, 
 and by gentle friction on their edges with a bow or the wetted finger, their 
 tones may be continued for any length of time, and may be made to swell 
 
 and diminish like a human voice or the 
 notes of a violin. A set of glasses, there- 
 fore, attuned to each other, according to 
 the harmonic scale, becomes, for certain 
 species of music, the most perfect of all 
 instruments. It is in fact an ^Eolian harp 
 at command. Dr. Franklin, who first 
 constructed a set, doubled the long line of 
 glasses upon itself, and placed the half- 
 notes as outside rows. The author of this 
 work, however, during some experiments 
 on sound, found the zig-zag arrangement 
 here represented to possess certain advan- 
 tages. The small open circles represent 
 the mouths of the glasses standing in a box 
 a b c, and the relation of the glasses to the 
 written musical notes is shown by the 
 
 common music lines and spaces which connect them. The learner discovers 
 immediately that one row of the glasses produces the notes written upon the 
 lines, and the other row the notes written between the lines ; and he is men- 
 tally master of the instrument by simple inspection. This arrangement also 
 renders the performance easy, for the notes most commonly sounded in suc- 
 cession are contiguous : and the relations of the notes forming a tune are so 
 obvious to the eye, that the theory of musical combination and accompani- 
 ment is learned at the same time. The set of glasses here represented has 
 two octaves, and with the additional flat seventh and fourteenth, seen at a and 
 c, which, when required, may be substituted for the corresponding glasses in 
 the rows, it is capable of playing the greater part of our simple melodies. 
 All the half-notes, if desired, may be placed in outside rows. The player 
 stands at the side of the box between a and b } and has the notes ascending 
 towards the right hand, as in a piano-forte. 
 
MUSICAL EAR. 245 
 
 Musical ear. 
 
 Philosophers 'have not yet been able to account for a remarkable difference 
 among individuals, as regards their perception of the musical relations of 
 sounds. Many persons, without understanding any thing of acoustics, or 
 having studied music as a science can tell instantly whether various notes 
 heard together or in succession, have the mutual relations which we call 
 musical and which we now know to depend on the comparative numbers 
 of beats in a given time ; and they quickly recognize and learn to repeat 
 tunes, and to sing a fit second or bass to the performance of another ; while 
 there are persons, again, with an equally perfect sense of hearing, who can 
 neither know if an air be played in tune, nor what air it is, nor can they ever 
 sing alone or accompany. The former class of persons are said to have a 
 musical ear, and the latter to want it; and although cultivation will raise 
 mediocrity to considerable expertness, it cannot bestow the faculty where 
 originally deficient. On this subject there is a very common misconception, 
 which becomes a source of great mortification on one side, and of arrogance 
 on the other, viz , that the possession of a musical ear, or the power of dis- 
 tinguishing notes, is the indication of all the finer qualities of the mind 
 while the want of it proves an opposite deficiency; and Shakspeare's opinion 
 of him " that hath no music in himself," is often triumphantly cited as ap- 
 plicable to all who want the distinguishing ear. The truth, however, is, that 
 many who possess this characteristic in a remarkable degree, are deficient in 
 almost all else that humanity reveres, witness the weak minds and disor- 
 derly lives of so many professed musicians, while many, again, who have 
 it not, are otherwise examples of excellence, and exquisitely sensible to other 
 beauties and harmonies of nature. They may not be deaf, for instance, to 
 the general music of spring, when all nature bursts forth in voice of rejoicing, 
 nor to the awful music of the storm they may feel as touching music the 
 silence of a lone wood, contrasted with the unceasing din of multitudes or 
 even the stillness of night in a great city, where the astronomer, contempla- 
 ting the wondrous spheres above, hears only the tongues of passing time in 
 the church towers, or the call of watchmen, faintly sounding in the distance. 
 In fine, many distinguished poets and philosophers have had no musical ear. 
 That the charm of music is often as much from early associations as from 
 peculiar aptitude in the individuals, is proved by the effects so well known 
 of the Swiss airs, when heard by native Swiss in foreign lands ; and, indeed, 
 of the national melodies of all countries, whose people are happy, and mix 
 song with their usual occupations, it not being in nature, that at any period 
 of life, or in any clime, a man should cease to deem those modulations lovely, 
 which recall the ecstatic emotions of his infancy and childhood ; modula- 
 tions learned in general from a parent's voice, perhaps an excellant mother's, 
 whose affection was so long around him as a shield ; whose tears fell to 
 chide his errors, and to reward where there was promise of virtue ; whose 
 steady judgment was his guide, whose faultless life was his example, 
 and who in all things to him was a personification of God's goodness on 
 earth. 
 
 It is the prejudice of which we are now speaking with respect to musical 
 ear and musical taste, that in the present day, condemns many young women, 
 possessed of every species of loveliness and talent, except that of note-dis- 
 tinguishing, to waste years of precious time in an attempt to aquire this 
 talent in spite of nature ; but when they have succeeded as far as they can, 
 
246 ACOUSTICS. 
 
 they have only the merit of being machines, upon which tunes are set as 
 upon a barrel-organ, and of which the performance is often far from being 
 pleasing to good judges. Such persons when liberty comes to them with age 
 or marriage, generally abandon the offensive occupation ; but tyrant fashion 
 will force their daughters to run the same course. The waste of time now 
 spoken of, is only one of many evil consequences which arise from the pre- 
 vailing false notions with respect to music : a subject which, however inter- 
 esting, cannot be farther pursued in this place. 
 
 " The trembling which causes the sensation of sounds spreads in all bodies, 
 solid or fluid." (Read the Analysis.) 
 
 As air consists of material particles held far apart from each other by the 
 repulsion of heat among them, we can conceive how an impulse given to a 
 certain portion of the particles is transmitted to those beyond, by the increase 
 of repulsion as they approximate ; and from the second layer in the same 
 manner to the third, and so on. And as in fluids the particles all mutually 
 rest against, or repel each other, we can conceive why a motion produced in 
 any part of a mass should be felt in every direction. The explosion of gun- 
 powder, in which there is a sudden formation of a quantity of air, gives a 
 shock all round which spreads a spherical wave to a great distance. 
 
 Although material particles in the form of liquid or solid are much nearer 
 to each other than in the form of air, we still have many proofs, as stated at 
 page 30, that they are not in absolute contact, and we therefore see the rea- 
 son why the impulses producing sound should be transmitted through a 
 liquid or solid in the same manner as through air, and even, by reason of 
 the greater proximity "of the particles, more quickly and forcibly than in air. 
 
 Instances of air carrying sound were given at page 2.35. As farther ex- 
 amples we may cite the cases of what are called sympathetic sounds. Every 
 elastic body being sonorous, that is to say, being fitted to tremble when struck, 
 with a certain frequency of oscillation, depending on its weight and shape, 
 &c., if the air around it be made to tremble by any cause, with the velocity 
 which it is fitted to take on or produce, it immediately begins to tremble in 
 unison with the air; and its motion or sound may continue after the original 
 cause has ceased. Thus almost any sound produced near a piano-forte whose 
 dampers are raised, finds a responsive string, and if bits of paper are strewed 
 upon the strings generally, those falling on the strings which return unisons 
 or octaves to the sounding body are soon shaken off, while the others remain. 
 A harp or guitar in a room with talking company, is often mingling a note 
 with their conversation. A wine-glass or goblet may be made to tremble, 
 and if on a table at all inclined, even to fall, by a person sounding on a vio- 
 loncello near it, the note accordant to its own. 
 
 Sounding bodies vibrate much more quickly, or have sharper tones, if 
 placed in light hydrogen, than in common air, and more quickly in common 
 air than in any of the heavier gases : because the lighter the surrounding 
 fluid, the less is the resistance to a body moving in it. Thus also a bell will 
 ring under water, but with a much graver sound than in the air. 
 
 That water is a vehicle of sound, is proved by the fact last mentioned, 
 by the distinctness with which the blows of workers around a diving-bell are 
 heard above, by the fact that fishes hear very acutely, &c. 
 
 And the following are instances of sound conveyed by solids. A scratch 
 of a pin at one end of a wooden log is distinctly heard by a person applying 
 his ear at the other end, although through the air it is not at all audible even 
 to the person who makes it. Savages often discover the proximity of ene- 
 
SPREADING OF SOUND. 247 
 
 mies or of prey, by applying an ear to the ground and hearing the tread. 
 The approach of horsemen at night is easily discovered in the same way. . 
 The report of a cannon placed on the ice is carried much farther by the ice, 
 than by the air around. In the military operation of mining, or cutting away 
 under ground for the purpose of entering a citadel, or blowing up fortifications, 
 the approach of the enemy is often discovered by the subterranean sound of 
 the pioneer's tools. The awful muttering of earthquakes is merely the sound 
 of subterranean explosions, conveyed from amazing distances, by the solid 
 earth 
 
 A superstitious man sleeping in the upper story of a lofty house had for 
 some time heard, during the stillness of the night, a singular beating noise 
 near the head of his bed. There was no adjoining house beyond the wall, 
 nor was there any thing going on near him in his own house to account for 
 it, and he at last deemed it supernatural. Accident at last discovered that 
 in a hovel built at the bottom and outside of the wall against which his bed 
 stood, there was a wooden clock hanging, of which the sound, while all else 
 was still, became audible aloft. 
 
 It is easy to ascertain whether a kettle boils, by putting one end of a stick 
 or poker on the lid, and the other end to the ear ; the bubbling of the water 
 then appears as loud as the rattling of a carriage in the street. A slight 
 blow given to a steel poker or common triangle, of which an end is held to 
 the ear, produces a sound which is even painfully strong. 
 
 The readiness with which solids receive and transmit sound is farther per- 
 ceived in the fact, that a small musical box, while held in the hand, is 
 scarcely audible, but when pressed against a table, or a door, will rival a 
 little harp. The vibration communicated from the box pervades the whole 
 of the wood, and the extended surface then acting on the air increases the 
 effect. The construction of violins, harps, guitars, &c., and of sounding 
 boards generally, is governed by the same law. In the dancing-master's 
 kit or small fiddle, which he carries in his pocket, there may be the same 
 strings and the same bow as for a violin, but it has very little sound, because 
 the extent of its surface is so small. 'A heavy piece of metal called a 
 sourdine, when fixed upon the bridge of a violin, damps the sound, because 
 it is a dead mass resisting the motion of the elastic wood. 
 
 The fact of solids conveying sounds so much more perfectly than air has 
 lately been applied to useful purposes in medicine. Dr. Laennec, of Paris, 
 proposed some years ago to listen to what was going on in the interior of the 
 body, and of the chest particularly, by applying one end of a wooden 
 cylinder, which he called a stethoscope or chest inspector, to the surface, and 
 resting the ear against the other end. The results of this happy thought 
 have been important. 
 
 The actions going on in the chest are, the entrance and exit of the air in 
 respiration, the voice, and the motion of the blood in the heart and blood- 
 vessels; and so perfectly do all these declare themselves to a person listen- 
 ing through the stethoscope, that an ear once familiar with the natural and 
 healthy sounds, instantly detects certain deviations from them. Hence this 
 instrument becomes a means of ascertaining certain diseases in the chest 
 almost as effectually as if there were convenient windows for visual inspec- 
 tion ; and when it is considered that a fourth or fifth part of the inhabitants 
 of Europe die of diseases of the chest, such as inflammations, abscesses, 
 consumption, dropsical collections, aneurism, and various affections of the 
 heart and blood-vessels, each of which requires an appropriate treatment, 
 the importance of such a means may be judged of. By many medical men 
 
248 ACOUSTICS. 
 
 this instrument was at first ridiculed as quackery and nonsense, and many 
 have yet to learn the use of it. May not both of these facts be attributed 
 to the error which has existed in medical education, of leaving so many 
 practitioners without that knowledge of the general laws of nature, which 
 should enable them to appreciate at once any means likely to be useful in 
 their art, from whatever quarter offered ? 
 
 " Velocity of sound." (See the Analysis.) 
 
 The velocity of light is such, that for any distance on earth its passage 
 may be regarded as instantaneous. The velocity of sound is very much less. 
 -If a woodman be observed at his occupation on the hill, his axe is seen 
 to fall a considerable time before the sound of his blow reaches the specta- 
 tor's ear. The flash of a gun fired at a distance is seen long before the 
 report is heard. 
 
 Most accurate experiments have been made to ascertain the velocity with 
 which sound travels in the atmosphere; and it is found to be 1,142 feet per 
 second, or a mile in about four seconds and-a-half; varying little either with 
 the density or temperature of the air. 
 
 By noting then how long the flash of a gun is seen before the report 
 reaches the ear, we learn the distance of the ship or battery from which the 
 gun is fired. A chasing ship may thus often discover whether she be nearing 
 or not the object of her pursuit. In the same manner the distance of 
 thunder may be ascertained : and the reason of the long-continued roll of 
 thunder is, that although the lightning darts instantly through the chains of 
 clouds, perhaps of miles in length, the claps or explosion at each interruption 
 of the chain are only heard successively, as the sound arrives at the ear. 
 The pulse at the wrist of a healthy man is a convenient measure of time 
 for ascertaining distances by the motion of sound, each beat making 
 nearly a second, and therefore indicating a distance of nearly a quarter of 
 a mile. 
 
 A line of muskets fired at the same instant cannot appear a single report 
 to any person who is not in the centre of a circle, of which the line forms 
 a part. 
 
 An extended orchestra of musicians cannot be heard equally well from all 
 situations near them. 
 
 Wind affects the velocity of sound just as a current in water affects the 
 motion of a sailing ship. 
 
 Sound decreases in intensity from the centre where it originates, according 
 to the same law as gravitation or light ; that is to say, at double distance it 
 is only one-fourth part as strong, at triple, a ninth, and so on. 
 
 By confining it, however, in tubes, which prevent it spreading, its force 
 diminishes much less rapidly, and it will, therefore, extend to much greater 
 distances. In many manufactories, and even private dwellings now, there 
 are pipes for the conveyance of sound leading to all parts ; so that on ring- 
 ing a bell to attract attention, verbal orders may be given through them to 
 great distances. 
 
 Sound travels in water four times quicker, and in solids fr-om ten to 
 twenty times quicker, than in air. The blow of a hammer given to a wall 
 by a person at one end, may be heard twice by a person at the other, viz., 
 almost immediately by an eur applied to the wall, and a little after through 
 the air. 
 
 " Reflection of sound." (Read the Analysis.) 
 As a wave of water turns back at a smooth wall or obstacle, so that at any 
 
REFLECTION OF SOUND. 249 
 
 distance after the reflection, it appears what it would have been at the same 
 distance beyond the wall, only moving in an opposite direction ; so the 
 pulses or waves of sound are regularly reflected from flat surfaces, and 
 produce what is called an echo. Such flat surfaces of nature's works are 
 found only among the rocks and hills; and hence arose the beautiful fiction 
 of the ancient poets, that echo was a nymph who dwelt concealed among 
 the rocks. Science has now disclosed the secret of the viewless echo ; but 
 who does not vividly recollect the wonder and delight with which he has 
 listened in the morning of his days, to his shrill call returned to him from 
 some bold precipice, across the plain or river, or, perhaps, sent down to him 
 again from the vaulted roof of ocean's caves ! 
 
 The quickness with which an echo is returned to the spot where the 
 sound originates, depends, of course, upon the distance of the reflecting sur- 
 face ; and, as sound travels 1,142 feet in a second, a rock at half that distance 
 returns a sound exactly in one second. The number of syllables that can 
 be pronounced in a second, will, in such a case, be repeated distinctly, while 
 the end of a longer phrase would mix with the commencement of the echo. 
 The breadth of a river may easily be ascertained where there is an echoing 
 rock on the farther shore. A perpendicular mountain's side, or sublime 
 cliffs, such as in many parts skirt the British coasts, return an audible echo 
 of artillery, or of thunder, to a distance of many miles. 
 
 If two bold faces of rock or wall be parallel to each other, a sound pro- 
 duced between them is repeated often, playing like a shuttlecock between 
 them, but becoming more faint each time until it is heard no more. In 
 some situations, particularly when the sound plays thus above the smooth 
 surface of water, a pistol-shot may be counted forty times. 
 
 The resonance of enclosed spaces depends on this continued reverberation. 
 It often increases the effect of music by converting a simple melody, which 
 is a succession of notes, into a harmonized piece, where each note is accom- 
 panied by some accordant tones ; and a young flute-player is often first 
 charmed with his own music when he finds himself performing a duet with 
 echo in a cave or under a spacious arch : but resonance injures the distinct- 
 ness of speech, so as even in some ill-contrived halls of assembly or theatres, 
 to render the articulation unintelligible. Small rooms or near surfaces give 
 no perceptible echo, because the interval of time between the original sound 
 and its repetition is too short for the ear to appreciate. 
 
 It is worthy of remark, that every apartment or confined space, has a 
 certain musical note proper to it, the pitch of which depends upon the 
 number of pulses or repetitions of a sound produced there in a given time 
 by the returns from its walls. The velocity of sound being uniform, this 
 number must depend on the size of the apartment. 
 
 There is a curious effect of echo which both illustrates the nature of the 
 phenomenon, and proves that a tone or musical sound is merely a repetition 
 of pulses following each other very quickly. Iron railings are generally 
 formed of square bars, of which any side is a plane surface, and may produce 
 an echo. Now a sound such as the sharp blow of a hammer, occurring near 
 the end of such a railing, is echoed to a corresponding place on the other side 
 by every bar in it ; and as the echoes do not return all at once, but in regular 
 succession, according to the increasing distances of the bars, the consequent 
 regular succession of slight pulses, with uniform and small intervals, affects 
 the ear, not as the echo of a single blow, but as a continued musical tone, 
 the pitch of which depends on the distance of the bars from each other. 
 The writer of this had observed, in passing on horseback along a particular 
 
250 
 
 ACOUSTICS. 
 
 Fig. 121. 
 
 a I 
 
 portion of road, where there was first a piece of wall and then two pieces of 
 paling with rails or bars of different width', that there was from the wall a 
 clear echo of the horse's cantering feet, and afterwards opposite the palings 
 a ringing sound for every step of the horse. He had first concluded that 
 the road there was singularly hard, although it did not appear so, and he 
 slackened the horse's pace to save his feet, until, observing one day that the 
 ringing sound was of different pitch opposite the two pieces of pailing, and 
 so as to correspond with the different width of the bars, the true explanation 
 occurred to him that the sound was an echo of the nature above described. 
 That an echo may be perfect, the surface producing it must be smooth, 
 and of some regular form ; for the wave of sound rebounds according to the 
 same law as a wave of water, or a ray of light, or an elastic ball, &c., as 
 explained at page 65, viz., perpendicularly to the surface, if it fall perpen- 
 dicular, but if it fall obliquely on one side, departing with an equal degree 
 of obliquity on the 'other. To express this very important law shortly, we 
 
 say that " the angle of reflection is equal to 
 the angle of incidence." According to this 
 law, any irregular surface must break an echo ; 
 and if the regularity be very considerable, 
 there can be no distinct or audible reflection 
 at all. A regular concave surface, on the 
 contrary, as e g, may concentrate sound, and 
 bring all which falls upon it, as from a I c d t 
 to the same centre or focus, as at/", so as to 
 produce there a very powerful effect. 
 
 We thus see the reason why echo is much 
 
 less perfect from the front of a house which has windows and doors, than 
 from the plane end, or any plane wall of the same magnitude, and why 
 the resonance of a room is so irregular and indistinct when the room contains 
 curtains, carpets and other furniture, or a crowded assembly. Halls for 
 music have generally plane bare walls. Theatres for the drama, again, have 
 boundaries broken in all ways by rows of boxes, and various ornaments. 
 
 The concentration of sound by concave surfaces produces many curious 
 effects both in nature and in art. 
 
 There are remarkable situations where the sound from a cascade is con- 
 centrated by the surface of a neighbouring cave so completely, that a person 
 accidentally bringing his ear into the focus, is suddenly astounded, as if the 
 universe were crushing around him. A chair placed in the cave, so that a 
 person sitting down in it must bring his ear into the focus, insures the success 
 of the sometimes amusing experiment. 
 
 The centre of a circle is the focus in which sound issuing 
 from it is again collected after reflection ; hence the powerful 
 echo near the centre of a round apartment. An oval has two 
 centres or foci one towards each end, as a and b and the 
 nature of the curve is such, that sound, or light, or heat, 
 issuing around from either of the foci, as a, by obeying the 
 law of reflection above stated, is all directed from the various 
 points, as at c d e, &c., to the other focus, as at b. Hence 
 a person uttering a whisper in one focus of an oval room is 
 very audible to the other, although he may not be heard by 
 persons placed between. Such a room may be called a whispering gallery. 
 Concave surfaces facing each other, as two alcoves in a garden, or covered 
 recesses on opposite sides of a street or bridge, will enable persons seated in 
 
 Fig 122. 
 
KEFLECTION. 251 
 
 i 
 
 their foci to converse by whispers across louder noises in the space between, 
 and without themselves being overheard in that space. 
 
 The reason why a tube conveys sound so far, is, that its sides confine or 
 repress by a continued reflection, the advancing sound which, in the open 
 air, would quickly spread laterally and be dissipated. And the reason that 
 the plane surface of a smooth wall, or of water, &c., also conveys sound so 
 far, is, thatit similarly prevents the lateral spreading and dissipation, although 
 only on one side. Persons far apart may converse along a smooth wall. 
 The barking of dogs and the clear voice o a street-crier, in a town situated 
 on the border of a lake, may be heard across the water in a calm evening, at 
 a distance of more than five miles the sound of bells, of course, is audible 
 much farther. And in the stillness of night, even the splashing oars of a 
 boat will announce its approach to persons waiting at a great distance. 
 
 If a sound-reflecting surface be curved inwards, that is, be concave, it not 
 only prevents the spreading of any sound which passes along it, but is con- 
 stantly condensing the sound by driving the external part inwards. Hence, 
 in a circular space, such as a gallery under a dome, persons close to the 
 wall 'may whisper to each other at all distances. 
 
 An ear-trumpet is a tube wide at one end where the sound enters, and 
 narrow at the other where the ear is applied : its sides are so curved that, 
 according to the law of reflection, all the sound which enters is brought to a 
 focus in the narrow end. It thus increases many fold the intensity of a 
 sound which reaches the ear through it, and enables a person who has be- 
 come deaf to common conversation, to mix again with pleasure in society, 
 The concave hand held behind the ear answers in some degree the purpose 
 of an ear- trumpet, and in a very large theatre is sometimes useful even to 
 persons of quick hearing. A notorious instance of a sound-collecting surface 
 was the ear of Dionysimj in the dungeons of Syracuse : the roof of the prison 
 was so formed as to collect the words and even whispers of the unhappy 
 prisoners, and to direct them along a hidden conduit to where the tyrant sat 
 listening. The wide-spread sail of a ship, rendered concave by a gentle 
 breeze, is also a good collector of sound. It happened one day onboard a ship 
 sailing along the coast of Brazil, far out of sight of land, that the persons 
 walking on deck, when passing a particular spot, heard very distinctly, during 
 an hour or two, the sound of bells, varying as in human rejoicings. All on 
 board came to listen, and were convinced, but the phenomenon was most 
 mysterious. Months afterwards it was ascertained, that at the time of obser- 
 vation the bells of the city of St. Salvador, on the Brazilian coast, had been 
 ringing on the occasion of a festival : their sound, therefore, favoured by a 
 gentle wind, had travelled over perhaps 100 miles of smooth water, and had 
 been brought to a focus by the concave sail in the particular situation on 
 the deck where it was listened to. It appears from this that a machine 
 might be constructed having the same relation to sound that a telescope 
 has to light. A friend of the author, on the 18th of June, 1814, while 
 sitting near the wall of his garden, situated near Dover, heard distinctly the 
 firing of the cannon at the battle of Waterloo. 
 
 The speaking-trumpet is made according to the same law of reflected 
 sound, with the view of directing the strength of the voice to a particular 
 point. The sea captain uses it to hail ships at a distance, or to send his 
 orders aloft, where the unaided voice would be lost in the noise of the wind 
 and waves. A similar form of mouth is used for the bugle horn and 
 common trumpet, and fits them to sound the note of command amid the 
 uproar of contending armies. 
 
252 A C U-S T I C S . 
 
 Some amusing effects have been produced by operating on sounds with 
 tubes and concave surfaces. What was termed the invisible girl, was a 
 contrivance where the questions of vistors were caught by a concealed con- 
 cave, and carried to the director who sat at a distance ; and his replies, as 
 in the whispering gallery, become audible to the inquirers alone. 
 
 The concave, undulating, and perfectly polished surface of many sea- 
 shells, fits them to catch, mix, and return the pulses of sounds that happen 
 to be trembling about them, so as to produce that curious resonance from 
 within which closely resembles the sound of the distant ocean so closely, 
 that the spirited boy, after studying the interesting stories of voyagers which 
 paint dangers to be nobly braved, and charm of nature to be seen in distant 
 lands, often feeds his imagination with this voice of a shell, and fancies 
 himself already riding among the billows. 
 
 The animal ear, 
 
 so admirably adapted to perceive the evanescent tremblings of the air, has 
 of course a structure in nice relation to their nature as now explained. The 
 parts of the ear, and the progress of the sound to the sentient nerve, may 
 be simply described as follows : 
 
 1st. There is external to the head, a wide-mouthed tube or ear-trumpet 
 a, for catching and concentrating the waves of 
 Fig. 123. sound. It is moveable in many animals, so 
 
 that they can direct it to the place from which 
 the sound comes. 
 
 2d. The sound concentrated at the bottom of 
 the ear-tube, falls upon a membrane stretched 
 across the channel, like the parchment of an 
 ordinary drum, over the space called the tym- 
 panum or drum of the ear b, and causes the 
 membrane to vibrate. That its motion may 
 be free, the air contained within the drum has 
 
 free communication with the external air, by the open passage /, called the 
 Eustachian tube, leading to the back of the mouth. A degree of deafness 
 ensues when this tube is obstructed, as by wax ; and a crack or sudden 
 noise, with immediate return of natural hearing, is generally experienced 
 when, in the effort of sneezing or otherwise, the obstruction is removed. 
 
 3d. The vibrations of the membrane of the drum are conveyed farther in. 
 wards, through the cavity of the drum, by chain of four bones (not here 
 represented on account of their minuteness,) reaching from the centre of the 
 membrane to the oval door or window leading into the labyrinth e. 
 
 4th. The labyrinth, or complex inner compartment of the ear, over which 
 the nerve of hearing is spread as a lining, is full of water; and therefore by 
 the law of fluid pressure (see page 128,) when the force of the moving mem- 
 brane of the drum, acting through the chain of bones, is made to compress 
 the water, the pressure is felt instantly over the whole cavity, as in a hydro- 
 static press. The labyrinth consists of the vestibule e, the three semi-circular 
 canals c, imbedded in the hard bone, and a winding cavity, called the 
 cochlea d, like that of a snail-shell, in which fibres, stretched across like 
 harp-strings, constitute the lyra. The separate uses of these various parts 
 are not yet perfectly known. The membrane of the tympanum may be 
 pierced, and the chain of bones may be broken without entire loss of hearing. 
 Considerable diversity of form and dimension is found in different animals. 
 
THE EAR. 253 
 
 The bone containing the cavities of the ear is the hardest in the body, 
 and is the first formed. 
 
 The ear has the power of judging of the direction in which sound comes. 
 A person in a thicket, listening to the song of various birds, although they 
 be concealed from his eye by the luxuriance of the vernal foliage, still judges 
 correctly by the ear in what tree every little songster is concealed. The 
 same truth is strikingly exemplified in the fact that, when horses or mules 
 march in company at night, those in front direct their ears forward ; those 
 in the rear, backwards ; and those in the centre, laterally or across ; the 
 whole troop seeming to be actuated by one feeling, which watches the 
 common safety. 
 
 The intensity of sound is to the ear a measure of distance. In a windy 
 night, the sound of a distant bell may be brought so quickly, that it has not 
 yet had time to spread and be weakened ; and a person is often roused from 
 a reverie by its unusual loudness and apparent nearness. When a stormy 
 wind blows directly upon a coast, and rolls the great waves in upon the 
 sandy beach or among the rocks, the countryman living far inland hears the 
 uproar, as if the ocean had burst its barriers, and were pouring in upon the 
 land. The scene-contrivers at our theatres heighten the illusion of an 
 approaching procession, by letting the accompanying music be first heard 
 from a closed chamber or in a feeble tone, and afterwards with gradually 
 increasing loudness. To the imagination, already excited perhaps to the 
 highest pitch by the drama of some divine mind, the advancing host is thus 
 most vividly portrayed ; and when at last, with the thunder of drums and 
 trumpets from the front of the stage, the troop also appears, the effect is com- 
 plete. It is the varying loudness of the JEolian harp which produces the 
 feeling that the heavenly choir is sometimes approaching and sometimes 
 
 [For an account of the Doctrines of Fluidity in relation to animals, see 
 Part V. Sec. II.] 
 
254 IMPONDERABLE SUBSTANCE. 
 
 PART IT. 
 
 DOCTRINES OF IMPONDERABLE SUBSTANCE. 
 
 To minds beginning this study, it may facilitate the conception of a sub- 
 stance which is without weight, or at least is imponderable by human art, 
 to consider the nature of air. Until lately men were so imperfectly 
 acquainted with the constitution of the universe around them, that a person 
 placed in an apartment which offered to view nothing but the naked walls, 
 would have said that it was empty, meaning literally what he said ; and 
 even when advertised that there was air in the room, he would still have been 
 far from possessing a clear notion that it was full of aerial fluid just as an 
 open vessel immersed in the sea is full of water, and that if air were not 
 allowed to escape from it, even so small a body as an apple could not be 
 pressed into it additionally by less force than fifty or sixty pounds. This truth, 
 however, is now clearly understood, and daily exemplified in easy pneumatic 
 experiments, and in no way more strikingly than by the recent adoption of 
 the substance of air in place of feathers, as stuffing for beds and pillows. 
 An air-tight bag or sack suspended by its lip in the air, and held quite open 
 by a hoop near its^niouth, would appear empty, but if then firmly closed above 
 the hoop, it would have imprisoned its fill of air, just as a bag similarly man- 
 aged underwater would imprison its fill of water ; and while in some respects 
 the air would be softer and locally more yielding than feathers, its entire mass 
 would be much less compressible. Now this air, when weighed by means which 
 modern science has furnished, is found in a cubic foot to contain somewhat 
 more than an ounce, and by strongly pressing it, or by causing it to combine 
 chemically with some other substance, we can reduce it to a very small bulk, 
 either with the form of a liquid or of a solid j proving how small a quantity 
 of ponderable matter, under certain circumstances, will occupy great space. 
 And common air is by no means the lightest known substance, which as 
 powerfully resists the intrusion of other bodies where it exists. Hydrogen 
 gas, for instance, of the same space-occupying force, weighs only a fourteenth 
 part as much, and therefore a few drachms of it confined in a bag or bed as 
 broad as the foundation of a house, would support a house or cask as large 
 as a house filled with water to a height of thirty feet, the gas itself being 
 then eighty thousand times lighter than its bulk of gold ; and if the pressure 
 on it were diminished, it would readily expand to a volume a thousand times 
 as great, and would still be exerting a considerable outward elasticity. Again, 
 a mixture of oxygen and hydrogen gases, while uniting with explosive force 
 to form water, dilates for the time, even under the great pressure of the 
 atmosphere, to a bulk about twenty times greater than the gases have while 
 separate. 
 
 The mind, pursuing the idea of such expansion or occupancy of space by 
 a small quantity of matter, and reflecting on the wonderful divisibility of 
 matter or minuteness of the ultimate atoms, as explained in Part I. of this 
 work, might almost admit as a possible reality Newton's hypothetical illustra- 
 
IMPONDERABLE SUBSTANCE. 255 
 
 tion of that divisibility, viz., that even one ounce of substance uniformly dis- 
 tributed over the vast space in which our solar system exists, might leave 
 no quarter of an inch without its particle. Now a fluid in any degree 
 approaching in rarity to this, although it might press, resist, communicate 
 motion, and have other influences in common with more ponderable matter, 
 would have neither weight nor inertia discoverable by means at present 
 known to man. While we are contemplating, then, or modifying the agen- 
 cies of what causes the phenomena of heat and cold, of light and darkness, 
 of electricity in its forms of thunder and lightning, of galvanism, or of mag- 
 netism, in a word, the most striking phenomena of nature, we may be dealing 
 with matter of the subtle constitution now spoken of. And as in the terres- 
 trial atmosphere there are at least two fluids present, $&., oxygen and nitrogen, 
 of distinct nature, so in a more subtle ether, filling all space, there may be 
 various ingredients. 
 
 A majority of philosophers now incline to the opinion here, sketched, that 
 there is at least one such subtle fluid or ether occupying completely the 
 space of the universe, and tending to uniform diffusion by reason of a strong 
 mutual repulsion of its particles, which fluid pervades denser material sub- 
 stances somewhat as water pervades a sponge or a mass of sand, being 
 attracted in a peculiar way by each substance, and which fluid may or may 
 not have weight and inertia. They believe farther that the phenomena 
 above alluded to, and which human art can exhibit with highest beauty, or 
 with awful intensity, are produced by the motion of other affections of that 
 fluid, as the sensation of sound in all its varieties is produced in the delicate 
 structure of the ear by a certain motion in the air, or in any other body 
 having communication with the ear j or as the sensation of jar is perceived 
 by a hand held to one end of a log of wood when a blow is given to the other 
 end. Some philosophers again suppose that the causes of the phenomena 
 are material particles projected through space, somewhat as sand might be 
 scattered by an explosion, and which particles are present only when the 
 effects are. apparent. Some combine these two hypotheses. And some hold 
 all the phenomena of heat to be mere motions in the common matter of the 
 bodies in which the heat exists. 
 
 We mention these hypotheses, not with the view of entering upon a minute 
 examination of their respective merits, or even of asserting that any one of 
 them is true, but merely to make the reader aware of the directions which 
 inquirers' minds have taken in pursuing the investigation. . To understand 
 the subjects as far as men yet usefully understand them, and sufficiently for 
 a vast number of most useful purposes, it is only necessary, as in other de- 
 partments of science, to classify important phenomena, so that their nature 
 and resemblances may be clearly perceived. When, in treating of the human 
 mind, we speak of its retaining an idea, or being depressed, or being heated 
 with passion, &c., we speak of subjects sufficiently definite, although we may 
 have no hypothesis as to the intimate nature of the phenomena : and in the 
 same manner may we speak of the accumulation, radiation, or other affec- 
 tions of heat and light. We know nothing of the cause even of gravity, the 
 grandest influence in nature, but we can calculate its effects with admirable 
 precision. 
 
256 HEAT. 
 
 PART IV. 
 
 / 
 
 (CONTINUED.) 
 SECTION I. ON HEAT. 
 
 ANALYSIS OP THE SECTION. 
 
 Heat ( by some called Caloric ) may be strikingly referred to as that which 
 causes the difference between winter and summer, between tropical gardens 
 and polar wastes. Its inferior degrees are denoted by the term COLD. It 
 cannot be exhibited apart, nor prove to have weight or inertia, and the 
 change of its quantity in bodies is most convenient // estimated by the con- 
 comitant change of their bulk', any substance so circumstanced as to allow 
 this to be accurately measured constituting a THERMOMETER. 
 
 Heat diffuses itself among neighbouring bodies until all have the same tem- 
 perature, that is, until all similarly affect a thermometer. It spreads 
 partly through their structure, or by conduction, as it is called, with a 
 slow progress, different for each substance, and in jl.uids modified by the 
 motion of their particles ; and it spreads partly also by being shot or 
 radiated like light from one body to another, through transparent media 
 or space, with readiness affected by the material and state of the giving 
 and receiving surfaces. 
 
 Heat, by entering bodies, expands them, and through a range which in- 
 cludes, as three successive stages, the forms of SOLID, LIQUID and AIR or 
 GAS ', becoming thus in nature the grand antagonist and modifier of that 
 attraction which holds corporeal particles together, and which, if acting 
 alone, would reduce the whole material universe to one solid lifeless mass. 
 Each particular substance, according to the nature, proximity , &c., of 
 its ultimate particles, takes a certain quantity of heat ( said to mark its 
 capacity ), to produce in it a given change of temperature or calorific ten- 
 sion ', undergoing expansion then in a degree proper to itself, and 
 changing its form to liquid and air at points of temperature proper to 
 itself ; the expansion in bodies generally increasing more rapidly than 
 the temperature, because the cohesion of their particles lessens with in- 
 crease of distance ; being remarkably gre-tter therefore in liquids than in 
 solids; and in air than in liquids; and the rate of expansion, moreover, 
 being much quickened as the bodies approach their points of changing 
 foim to liquid or air, to produce which changes, a large quantity of heat 
 enters them, but in the new arrangement of particles and increased 
 volume of the mass, it becomes hidden from the thermometer, and is 
 therefore called LATENT HEAT. For any given substance the changes of 
 form happens so constantly at the same temperature, that they mark fixed 
 points in the general scale of temperature, and enable us to regulate and 
 
CAUSE OF SEASONS AND CLIMATES. 25 
 
 compare thermometers. Heat, by expanding different substances une- 
 qually, influences much their chemical combination. 
 
 Heat influences also the functions of vegetable and animal life. The great 
 source of heat is the fun ; but electricity, combustion and other chemical 
 action, condensation, friction, and the actions of life, are also excitants* 
 
 K Heat may be strikingly referred to as that which causes the difference 
 between winter and summer? between the gardens of the equator and 
 polar wastes. 7 ' (See the Analysis, page 256.) 
 
 In the winter of climates, where the temperature is for a time below the 
 freezing point of water, the earth with its waters is bound up in snow and 
 ice, the trees and shrubs are leafless, appearing everywhere like withered 
 skeletons, countless multitudes of living creatures, owing either to the bitter 
 cold or deficiency of food, are perishing in the snows nature seems dying 
 or dead ; but what a change when spring returns, that is, when heat returns ! 
 The earth is again uncovered and soft, the rivers flow, the lakes are again 
 liquid mirrors, the warm showers come to foster vegetation, which soon 
 covers the ground with beauty and plenty. Man, lately inactive, is recalled 
 to many duties; his water-wheels are everywhere at work, his boats are 
 again on the canals and streams, his busy fleets of industry are along the 
 shores ', winged life in .new multitudes fill the sky, finny life similarly fills 
 the waters, and every spot of earth teems with vitality and joy. . Many 
 persons regard these changes of season as if they came like the successive 
 position of a turning-wheel, of which one necessarily brings the next; not 
 adverting that it is the single circumstance of change of temperature which 
 does all. But if the colds of winter arrive too early, they unfailingly pro- 
 duce the wintery scene, and if warmth come before its time in spring, it 
 expands the bud and the blossom, which a return of frost will surely destroy. 
 A seed sown in an ice-house never awakens to life. 
 
 Again, as regards climates, the earthly matters forming the exterior of 
 our globe, and, therefore, entering into the composition of soils, are not 
 different for different latitudes, at the equator, for instance, and near the 
 poles. That the aspect of nature then, in the two situations, exhibits a con- 
 trast more striking still than between summer and winter, is- merely to an 
 inequality of temperature, which is permanent. Were it not for this, in both 
 situations the same vegetables might grow, and the same animals might find 
 their befitting support. But now, in the one, namely, where the heat abounds, 
 we see the magnificent scene of tropical fertility ; the earth covered with 
 luxuriant vegetation in endless, lovely variety, and even the hard rocks fes- 
 tooned with green, perhaps with the vine, rich in its purple clusters. In the 
 midst of this scene, animal existence is equally abundant, and many of the 
 species are of surpassing beauty the plumage of the birds is as brilliant as 
 the gayest flowers. The warm air is perfume from the spice-beds, the sky 
 and clouds are often dyed in tints as bright as freshest rainbow, and happy 
 
 * It i? to be remarked here, that many phenomena in which heat plays an important 
 part, have been already described in preceding chapters of this work ; for instance, the 
 action of the steam-engine, the phenomena of winds, many facts in {meteorology, Ac., 
 under the head of Pneumatics. In a separate treatise on heat, these could not with pro- 
 priety have been omitted ; but in a comprehensive system of science like the present, they 
 find their fit place, where, being surrounded by subjects resembling them in more intri- 
 cate particulars, they can be more concisely and clearly explained. 
 
258 HEAT. 
 
 human inhabitants call the scene a paradise. Again, where heat is absent, 
 we have the dreary spectacle of polar barrenness, namely, bare rock or 
 mountain, instead of fertile field ; water everywhere hardened to solidity ; 
 no rain, nor cloud, nor dew; few motions but drifting snow; vegetable life 
 scarcely existing, and then only in sheltered places turned to the sun and 
 instead of the palms and other trees of India, whose single leaf is almost 
 broad enough to cover a hut, there are bushes and trees, as the furze and fir, 
 having what may be called hairs or bristles, in the room of leaves. In the 
 winter time, during which the sun is not seen for nearly six months, new 
 horrors are added, viz., the darkness and dreadful silence, the cold benumb- 
 ing all life, and even freezing mercury a scene into which man may pene- 
 trate from happier climes, but where he can only leave his protecting ship 
 and fires for short periods, as he might issue from a diving-bell at the bottom 
 of the ocean. That in these now desolate regions, heat only is wanted to 
 make them like the most favoured countries of the earth, is proved by the 
 recent discoveries under ground of the remnant of animals and vegetables 
 formerly inhabiting them, which now can live only near the equator. While 
 winter, then, or the temporary absence of heat, may be called the sleep of 
 nature, the more permanent torpor about the poles appears like its death ; 
 and when we farther reflect, that heat is the great agent in numberless im- 
 portant processes of chemistry and domestic economy, and is the actuating 
 principle of the mighty steam-engine which now performs half the work of 
 society, how truly may heat, the subject of our present chapter, be con- 
 sidered as the life or soul of the univer&e. 
 
 " Heat cannot be exhibited in a separate state, nor proved to have weight or 
 inertia." (Read the Analysis, page 256.) 
 
 Although heat is known to be abundant in the sunbeam, and to radiate 
 around from a blazing fire, we cannot otherwise arrest or detect it in its pro- 
 gress than by allowing it to enter, and remain in some ponderable substance. 
 We know hot iron, or hot water, or hot air, but nature no where presents to 
 us, nor has art succeeded in showing us, heat alone. 
 
 If we balance a quantity of ice in a delicate weigh-beam, and then leave 
 it to melt, the equilibrium will not be in the slightest degree disturbed. Or 
 if we substitute for the ice, boiling water or red- hot iron, and leave this to 
 cool, there will be no difference in the result. If we place a pound of mer- 
 cury in one scale of the weigh-beam and a pound of water in the other, and 
 then either heat or cool both through the same number of therinometric 
 degrees, although about thirty times more heat (as will be explained below) 
 enters or leaves the bulky- water than the dense mercury, they will still 
 remain equivalent weights. 
 
 Again, a sunbeam, with its intense light and heat, after being concen- 
 trated by a powerful lens or mirror, may be made to fall upon the scale of 
 a most delicate balance, but will produce no depressing effect on the scale, 
 as would follow if what constitutes the beam had the least forward motal 
 inertia or momentum. 
 
 Such are the facts which have led certain inquirers to deny the material 
 or separate existence of heat, and to hold that it is merely motion of one 
 kind among the material particles of bodies generally, as sound is motion 
 of another kind among the same particles. The following facts they consider 
 to have the same bearing in the argument : Heat can be produced without 
 limit by friction, as when savages light their fires by rubbing together two 
 
CAUSE OF SEASONS AND CLIMATES. 259 
 
 pieces of wood when Count Rumford made great quantities of water boil, 
 by causing a blunt borer to rub against a mass of metal immersed in water 
 when Sir Humphrey Davy quickly melted pieces of ice by rubbing them 
 against each other in a room cooled below the freezing point, &c. Intense 
 heat is produced by the explosion of gunpowder or other fulminating mix- 
 ture, yet it cannot be conceived to have existed in the small bulk of the 
 powder before the explosion. Other inquirers, on the contrary, have deemed 
 to be proofs of the separate materiality of heat such facts as now follow ; 
 that it is radiated through the most perfect vacuum which we can produce, 
 and even more readily than through air; that it radiates in the same place 
 in all directions, without impediment from the crossing rays ; that it be- 
 comes instantly sensible on the condensation of any material mass, as if then 
 squeezed out from the mass ; as when, by compressing air suddenly, we in- 
 flame a match immersed in it ; or when, on reducing the bulk of iron by 
 hammering, we render it very hot, the warming being greater at the first 
 blow (which most changes the bulk) than afterwards, that when, on mix- 
 ing bodies which combine so intimately as to occupy less space than when 
 separate, there is a disengagement of heat proportioned to the diminution 'of 
 the volume : that the laws of the spreading of heat in bodies do not 
 resemble those of the spreading of sound, or of any other motion known 
 to us; and that, as to the great and sudden extrication of heat by friction 
 or explosion, it may be as truly a rush of the fluid to the part, as in the 
 case of an electrical accumulation or discharge These facts, moreover, 
 they think, square well with their assumption that the phenomena of heat 
 are produced by an exceedingly subtle fluid, or ether, pervading the whole 
 universe, and softening, or melting, or gasifying bodies, according to the 
 quantity present in each; its own parts being strongly repulsive of each 
 other, and seeking, therefore, widest and most equable diffusion. 
 
 " The change of its quantity in bodies is most conveniently estimated by the 
 concomitant change, of their bulk, any substance so circumstanced, as to 
 allow this to be accurately measured, constituting a thermometer ." (Read 
 the Analysis, page 256.) 
 
 If we heat a wire, it is lengthened ; if we heat water in a full vessel, a 
 part runs over ; if we heat air in a bladder, the bladder is distended ; in a 
 word, if we heat any substance, its volume increases in some proportion to 
 the increase of temperature, and we may measure the increase of volume. 
 The reasons why, in such investigations, a contrivance in which the expan- 
 sion of .mercury may be observed, viz., the mercurial thermometer, is com- 
 monly preferred to others, can only be fully understood by the mind which 
 has considered the whole subject of heat ; and we touch upon the matter 
 here, only for the purpose of stating that a mercurial thermometer is a 
 small bulb, or bottle of glass filled with mercury, and having a long very 
 narrow stalk or neck, in which the mercury rises when expanded by heat, 
 or falls when heat is withdrawn ; the stalk between the points at which 
 mercury standings in freezing and boiling water, being divided into an 
 arbitrary number of degrees, which division appearing on a scale applied to 
 the stalk, is continued similarly above and below these points. 
 
 " Heat diffuses itself among neighboring bodies until all have acquired the 
 same temperature ; that is to say, until all will similarly affect a thermo- 
 meter." (See the Analysis.) 
 
 An iron bolt thrust in among burning coals soon becomes red hot like 
 
260 HEAT. 
 
 them. If it be the heater of a tea-urn, it will, when afterwards placed 
 amidst the water, part with its lately acquired heat to the water, until both 
 are of the same temperature. Boiling water, again soon imparts heat to an egg 
 placed in it, and a feverish head yields its heat to a bladder of cold water 
 or ice. A. hundred objects enclosed in the same apartment, if tested, after 
 a time, by the thermometer, will all indicate the same temperature. 
 
 " TJie inferior degrees of heat are denoted ~by the term COLD." 
 
 When the hand touches a body of a higher temperature than itself, it re- 
 ceives heat according to the law now explained, and it experiences a pecu- 
 liar sensation ; when it touches a body of lower temperature than itself, it 
 gives out heat for a like reason, and experiences another and very different 
 sensation. The two are called the sensations of heat and of cold. Now 
 heat and cold, considered as existing in the bodies themselves, although thus 
 appearing opposities, are really degrees of the same object, temperature , 
 contrasted by name, for convenience sake, in reference to the particular 
 temperature of the individuals speaking of them just as any two nearest 
 mile-stones on a road, although merely marking degrees of the same object, 
 distance, might receive from persons living between them the opposite names 
 of east and west, or of north and south. It is to be remarked, moreover, 
 that the sensation of heat is producible also by a body colder than the 
 hand, provided it be less cold than a body touched immediately before, or 
 than the usual temperature ; and the sensation of cold is produced under 
 the opposite circumstances of touching a comparatively warm body, but 
 which is less warm than something touched just before. This explains the 
 remarkable fact that the same body may appear at the same time, and to 
 the same person, both hot and cold. If a person transfer one hand to com- 
 mon spring-water from touching ice, that hand will deem the water very 
 warm : while the other hand, transferred to it from a warm bath, would 
 deem it very cold. For a like reason ; a person from India, arriving in 
 England in the spring, deems the air cold, while the inhabitants of the 
 country are diminishing their clothing, because the heat to them is becom- 
 ing oppressive. Such facts show how necessary it was for men to discover 
 more correct thermometers than their bodily sensations. 
 
 " Spreading partly through their structure, or by conduction, as it is called, 
 with a progress proper to each substance." (Read the Analysis, page 
 256.) 
 
 If one end of a rod of iron be held in the fire, a hand grasping the other 
 end soon feels the heat coming through it. Through a similar rod of glass 
 the transmission is much slower, and through one of wood it is slower still. 
 The hand would be burned by the iron, before it felt warmth in the wood, 
 although the inner end were blazing. 
 
 On the fact that different substances are permeable to heat, or have the 
 property of conducting it, in different degrees, depend many interesting 
 phenomena in nature and the arts ; hence it was important to ascertain the 
 degrees exactly, and to classify the substances. Various methods for this 
 purpose have been adopted. For solids similar rods of the different sub- 
 stances, after being thinly coated with wax, have been placed with their in- 
 ferior extremities in hot oil, and then the comparative distances to which, in 
 a given time, the wax was melted, furnished one set of indications of the 
 comparative conducting powers : or, equal lengths of the different bare rods 
 
SPREADING BY CONDUCTION. 261 
 
 being left above the oil and a small quantity of explosive powder being placed 
 on the top of each, the comparative intervals of time elapsing before the 
 explosions gave another kind of measure : or equal balls of different sub- 
 stances, with a central cavity in each to receive a thermometer, being heated 
 to the same degree and then suspended in the air to cool, until the thermo- 
 meter fell to a given point, gave still another list. A modification of the last 
 method was adopted by Count Runiford to ascertain the relative degrees in 
 which furs, feathers, and other materials used for clothing, conduct heat, or, 
 which is the same thing, resist its passage. He covered the ball or stem of 
 a thermometer with a certain thickness of the substance to be tried, by placing 
 the thermometer in a larger bulb and stem of glass, and then filling the 
 interval between them with tfie substance ; and after heating this apparatus 
 to a certain degree, by dipping it in liquid of the desired temperature, he 
 surrounded it by ice, and marked the comparative time required to cool the 
 thermometer a certain number of degrees. The figures following the names 
 of some of the substances in the subjoined list, mark the number of seconds 
 required respectively for cooling it 60. 
 
 These experiments have shown, as a general rule, that density in a body 
 favours the passage of heat through it. The best conductors are the metals, 
 and then follow in succession diamond, glass, stones, earths, woods, &c., as 
 here noted : 
 
 Metals silver, copper, gold, iron, lead. 
 
 Diamond. 
 
 Glass. 
 
 Hard stones. 
 
 Porous earths. 
 
 Woods. 
 
 Fats or thick oils. 
 
 Snow. 
 
 Air 576 
 
 Sewing silk - - - 917 
 
 Wood fishes 927 
 
 Charcoal .... 937 / 
 
 Fine lint - ... 1,032 
 
 Cotton .... 1,046 
 
 Lamp black ... 1,117 
 
 Wool - - - - 1,118- 
 
 Raw silk .... 1,284 
 
 Beavers' fur 1,296 
 
 Eiderdown - - - 1,305 
 
 Hares' fur - - 1,315 
 
 Air appears near the middle of the preceding list, but if its particles are 
 not allowed to move about among themselves, so as to carry heat from one 
 part to another, it conducts (in the manner of solids) so slowly that Count 
 Runiford doubted whether it conducted at all. It is probably the worst con- 
 ductor known, that is, the substance which when at rest impedes the passage 
 of heat the most. To this fact seem to be owing, in a considerable degree, 
 the remarkable non-conducting quality of porous or spongy substances, as 
 feathers, loose filamentuous matter, powders, &c., which have much .air in 
 their structure, often adherent with the force of attraction which immersion 
 in water, or even being placed in the vacuum of an air-pump, is insufficient 
 to overcome. 
 
 While contemplating the facts recorded in the above table, one cannot but 
 
262 * HEAT. 
 
 reflect how admirably adapted to their purposes the substances are which 
 nature has provided as clothing for the inferior animals ; and which man 
 afterwards accommodates with such curious art to his peculiar wants. Ani- 
 mals required to be protected against the chills of night and the biting blasts 
 of winter, and some of them which dwell among eternal ice, could not have 
 lived at all, but for a garment which might shut up within it nearly all the 
 heat which their vital functions produced. Now any covering of a metallic, 
 or earthy, or woody nature, would have been far from sufficing ; but out of a 
 wondrous chemical union of carbon with the soft ingredients of the atmo- 
 sphere, those beautiful textures are produced called fur and feather, so greatly 
 adorning while they completely protect the wearers ; textures, moreover, 
 which grow from the bodies of the animals, in* the exact quantity that suits 
 the climate and season, and which are reproduced when by any accident they 
 are partially destroyed. In warm climates the hairy coat of quadrupeds is 
 comparatively short and thin ; as in the elephant, the monkey, the tropical 
 sheep, &c. It is seen to thicken with increasing latitude, furnishing the soft 
 and abundant fleeces of the temperate zones ; and towards the poles it is 
 externally shaggy and coarse, as in the arctic bear. In amphibious animals, 
 which have to resist the cold of water as well as of air, the furs grow particu- 
 larly defensive, as in the otter and beaver. Birds, from having very warm 
 blood, require plenteous clothing, but require also to have a smooth sur- 
 face, that they may pass easily through the air; both objects are secured 
 by the beautiful structure of feathers, so beautiful and wonderful that writers 
 on natural theology have often particularized it as one of the most striking 
 exemplifications' of creative wisdom. Feathers, like fur, appear in kind and 
 quantity suited to particular climates and seasons. The birds of cold regions 
 have covering almost as bulky as their bodies, and if it be warm in those of 
 them which live only in air, in the water-fowl it is warmer still. These 
 last have the interstices of the ordinary plumage filled up by the still more 
 delicate structure called down, particularly on the breast, which in swim- 
 ming first meets and divides the cold wave. There are animals with warm 
 blood which yet live very constantly immersed in water, as the whale, seal, 
 walrus, &c. Now neither hair nor feathers, however oiled, would have 
 been a fit covering for them; but kind nature has prepared an equal protec- 
 tion in the vast mass of fat or thick oil which surrounds their bodies sub- 
 stances which are scarcely less useful to man than the furs and feathers of 
 land animals. 
 
 While speaking of clothing we may remark, that the bark of trees is also 
 a structure very slowly premeable to heat, and securing, therefore, the tem- 
 perature to vegetable life. 
 
 And while we admire what nature has thus done for animals and vegetables, 
 let us not overlook her scarcely less remarkable provision of ice and snow, 
 as winter clothing for the lakes and rivers, for our fields and gardens. Ice, 
 as a protection to water and its inhabitants, was considered in Sec. I., in the 
 explanation of why, although solid, it swims on water. We have now to 
 remark that snow, which becomes as a pure white fleece to the earth, is a 
 structure which resists the passage of heat nearly as much as feathers. It, 
 of course, can defend only from clouds below 32 or the freezing point; but 
 it does so most effectually, preserving the roots and seeds and tender plants 
 during the severity of winter. When the green blade of wheat and the beau- 
 tiful snow-drop flower appear in spring rising through the melting snow, they 
 have recently owed an important shelter to their wintry mantle. Under deep 
 snow while the thermometer in the air may be far below zero, the tempera- 
 
SPREADING BY CONDUCTION. 263 
 
 ture of the ground is rarely below the freezing point. Now this temperature, 
 to persons some time accustomed to it, is mild and even agreeable. It is 
 much higher than what often prevails for long periods in the atmosphere of. 
 the centre and north of Europe. The Laplander, who during his long winter 
 lives under ground is glad to have additionally over head a thick covering of 
 snow. Ainorijfr the hills of the west and norm of Britain, during the storms 
 of winter, a house or covering of snow frequently preserves the lives of tra- 
 vellers, and even of whole flocks of sheep, when the keen north wind catch- 
 ing them unprotected, would soon stretch them lifeless along the earth. 
 
 It is because earth conducts heat slowly, that the most intense frost pene- 
 trates but a few inches into it, and that the temperature of the ground a few 
 feet below its surface is nearly the same all the world over. In many mines, 
 even although open to the air, the thermometer does not vary one degree in 
 a twelvemonth. Thus also water in pipes two or three feet under ground 
 does not freeze, although it may be frozen in all the smaller branches 
 exposed above. Hence, again, springs never freeze, and therefore become 
 remarkable features in a snow covered country. The living water is seen 
 issuing from the bowels of the earth, and running often a considerable way 
 through fringes of green, before the gripe of the frosts arrest it; while 
 around it, as is well known to the sportsman, the snipes and wild duck and 
 other birds are wont to congregate. A spring in a frozen pond or lake may 
 cause the ice to be so thin over the part where it issues, that a skater arriving 
 there will break through and be destroyed. The same spring water which 
 appears warm in winter, is deemed cold in summer, because although always 
 of the same heat, it is in summer surrounded by warm atmosphere and 
 objects. In proportion as buildings are massive, they acquire more of those 
 qualities which have now been noticed of our mother earth. Many of the 
 G-othic halls and cathedrals are cool in summer and warm in winter as are 
 also old-fashioned houses or castles with thick walls and deep cellars. 
 Natural caves in the mountains or sea-shores furnish other 'examples of a 
 similar kind. 
 
 When in the arts it is desired to prevent the passage of heat out of, or. into 
 any body or situation, a screen or covering of a slow conducting substance 
 is employed. Thus to prevent the heat of a smelting or other furnace from, 
 being wasted, it is lined with fire bricks, or is covered with clay and sand, or 
 sometimes with powdered charcoal. A furnace so guarded may be touched 
 by the hand, even while containing within the melted gold. . To prevent the 
 freezing of water in pipes during the winter, by which occurrence the pipes 
 would be burst, it is common to cover them, with straw ropes, or coarse flan- 
 nel, or enclose them in a larger outer pipe with dry charcoal, or saw dust, 
 or chaff, filling up the interval between. If a pipe on the contrary, be for 
 the conveyance of steam or other warm fluid, the heat is retained, and, there- 
 fore, saved by the very same means. Ice-houses are generally made with 
 double walls, between which, dry straw placed, or saw dust, or air, prevents 
 the passage of heat. Pails for carrying ice in summer, or intended to serve 
 as wine coolers, are made on the same principle viz., double vessels, with 
 air or charcoal, filling the interval between them. A flannel covering keeps 
 a man warm in winter it is also the best means of keeping ice from melting 
 in summer. Urns for hot water, tea pots, coffee pots, &c., are made with 
 wooden or ivory handles, because if metal were used, it would conduct the 
 heat so readily that the hand could not bear to touch them. 
 
 It is because glass and earthenware are brittle, and do not allow ready pas- 
 sage to heat, that vessels made of them are so frequently broken by sudden 
 
261 HEAT. 
 
 change of temperature. On pouring boiling water into such a vessel, the 
 internal part is much heated and expanded (as will be explained more fully 
 .in a subsequent page ) before the external part fcas f e it the influence, and 
 fiis is hence riven or cracked by its connection with the internal. A chimney 
 mirror is often broken by a lamp or candle placed on the marble shelf too 
 near it. The glass cylinder of an electrical machine will sometimes be 
 broken by placing it near the fire, so that one side is heated while the othor 
 side receives a cold current of aii approaching the fire from a door or window. 
 A red hot rod of iron drawn along the pane of glass will divide it almost 
 like a diamond knife. Even cast iron, or the backs of grates, iron pots, &c., 
 although conducting readily, is often, owing to its brittleness, cracked by 
 unequal heating or cooling, as from pouring water on it when hot. Pouring 
 cold water into a heated glass will produce a similar effect. Hence glass 
 vessels intended to be exposed to strong heats and sudden changes, as retorts 
 for distillation, flasks for boiling liquids, &c., are made very thin ; that the 
 heat may pervade them almost instantly and with impunity. 
 
 There is a toy called a Prince Rupert's Drop, which well illustrates our 
 present subject. It is a lump of glass let fall while fused into water, and 
 thereby suddenly cooled and solidified on the outside before the internal part 
 is changed ; then as this at last hardens and would contract, it is kept 
 extended by the arch of external crust, to which it coheres. Now if a por- 
 tion of the neck of the lump be broken off, or if other violence be done, 
 which jars its substance, the cohesion is destroyed, and the whole crumbles 
 to dust with a kind of explosion. Any glass cooled suddenly when first 
 made, remains very brittle, for the reason now stated. What is called the 
 Bologna jar is a very thick small bottle, thus prepared, which bursts by a 
 grain of sand falling into it. The process of annealing, to render glass ware 
 more tough and durable, is merely the allowing it to cool very slowly by 
 placing it in an oven, where the temperature is caused to fall gradually. 
 The tempering of metals by sudden cooling seems to be a process having 
 some relation to that of rendering glass hard and brittle. 
 
 It is the difference of conducting power in bodies which is the cause of a 
 very common error made by persons in estimating the temperature of bodies 
 by the touch. In a room without a fire all the articles of furniture soon 
 acquired the same temperature; but if in winter, a person with bare feet were 
 to step from the carpet to the wooden floor, from this to the hearth-stone, and 
 from the stone to the steel fender, his sensation would deem each of these 
 in succession colder than the preceding. Now the truth being that all had 
 the same temperature, only a temperature inferior to that of the living body, 
 the best conductor, when in contact with a body, would carry off heat the 
 fastest, and would, therefore, be deemed the coldest. Were a similar expe- 
 riment made in a hot-house, or in India, while the temperature of every thing 
 around were 98, viz., that of the living body, then not the slightest difference 
 would be left in any of the substances : or lastly, were the experiment made 
 in a room where by any means the general temperature were raised consider- 
 ably above blood heat, then the carpet would be desrned considerably the 
 coolest instead of the warmest, and the other things would appear hotter in 
 the same order in which they appeared colder in the winter room. Were a 
 bunch of wool and a piece of iron exposed to the severest cold of Siberia, or 
 of an artificial frigorific mixture, a man might touch the first with impunity, 
 (it would merely be felt as rather cold;) but if he grasped the second, his 
 hand would be frost-bitten and possibly destroyed ; were the two substances, 
 on the contrary, transferred to an oven ; and heated as far as the wool would 
 
SPREADING BY CONDUCTION. 265 
 
 bear, he might again touch the wool with impunity (it would then be felt as 
 a little hot,) but the iron would burn his flesh. The author has entered a 
 room where there was no fire, but where the temperature from hot air 
 admitted was sufficiently high to boil the fish, &c., of which he afterwards 
 partook at dinner ; and he breathed the air with very little uneasiness. 
 He could bear to touch woolen cloth in this room, but no body more solid. 
 
 The foregoing considerations make manifest the error of supposing that 
 there is a positive warmth in the materials of clothing. The thick cloak 
 which guards a Spaniard against the cold of winter, is also in summer used 
 by him as a protection against the direct rays of the sun : and while in 
 England, flannel is our warmest article of dress, yet we cannot 'more 
 effectually preserve ice than by wrapping the vessel containing it in many 
 folds of softest flannel. 
 
 In every case where a substance of different temperature from the living 
 body touches it, a thin surface of the substance immediately shares the heat 
 of the bodily part touched the hand generally ; and while in a good con- 
 ductor, the heat so received quickly passes inwards, or away from the surface, 
 leaving this in a state to absorb more, in the tardy conductor the heat first 
 received tarries at the surface, which consequently soon acquires nearly the 
 same temperature as the hand, and therefore, however cold the interior of the 
 substance may be, it does not cause the sensation of cold.. The hand on a 
 good conductor has to warm it deeply, a slow conductor it warms only super- 
 ficially. The following cases further illustrate the same principle. If the 
 ends of an iron poker, and of a piece of wood of the same size, be wrapped in 
 paper and then thrust into a fire, the paper on the wood will begin to burn 
 immediately, while that on the metal will long resist : or if pieces of paper 
 be laid on a wooden plank and on a plate of steel, and then a burning coal 
 be placed on each, the paper on the wood will begin to burn long before that 
 on the plate. The explanation is, that the paper in contact with the good 
 conductor loses to this so rapidly the heat received from the coal, that it 
 remains at too Iowa temperature to inflame, and will even cooi to blackness 
 the touching part of the coal; while on the tardy conductor the paper becomes 
 almost immediately as hot as the coal. It is because water exposed to air 
 cannot be heated beyond 212, that it may be made to boil in an egg-shell or 
 a vessel made of paper, held over a lamp, without the containing substance 
 being destroyed; but as soon as it is dried up, the paper will burn and the 
 shell will be calcined, as the solder of a common tin kettle melts under the 
 same circumstances. The reason why the hand judges a cold liquid to be so 
 much colder than a solid of the same temperature is, that from the mobility 
 of the liquid particles among themselves, those in contact with the hand are 
 constantly changing. The impression produced on the hand by very cold 
 meruairy is almost insufferable, because mercury is both a ready conductor and 
 a liquid. Again, if a finger held motionless in water feel cold, it will feel 
 colder still when moved about; and a man in the air o'f a calm frosty morning 
 does not experience a sensation nearly so sharp as if with the same tempera- 
 ture there be wind. A finger held up in the wind discovers the direction in 
 which the wind blows*by the greater cold felt on one side, the effect being 
 still more remarkable if the finger is wetted. If a person in a room with a 
 thermometer, were with a fan or bellows to blow the air against it, he would not 
 thereby lower it, because it had already the same temperature as the air, yet 
 the air blown against his own body would appear colder than when at rest, 
 because -being colder than his body, the motion would supply heat-absorbing 
 particles more quickly. In like manner, if a fan or bellows were used against 
 
266 HEAT. 
 
 a thermometer hanging in a furnace or hot-house, the thermometer would 
 suffer no change, but the air moved by them against a person, would be 
 distressingly hot, like the blasting sirocco of the sandy deserts of Africa. If 
 two similar pieces of ice be placed in a room somewhat warmer than ice, one 
 of them may be made to melt much sooner than the other, by blowing on it 
 with a bellows. The reason may here be readily comprehended why a person 
 suffering what is called a cold in the head, or catarrh from the eyes and nose, 
 experiences so much more relief on applying to the face a handkerchief of 
 linen or cambric than one of cotton : it is, that the former, by conducting, 
 readily absorbs the heat and diminishes the inflammation, while the latter, 
 by refusing to give passage to the heat, increases the temperature and the 
 distress. Popular prejudice has held that there was a poison in cotton. 
 
 " Heats spreading in fluids chiefly ly the motion of their par ticks." 
 (Read the Analysis, page 256.) 
 
 Owing to the mobility among themselves of fluid particles, heat entering 
 a fluid anywhere below the surface, by dilating and rendering specially lighter 
 the portion heated, allows the denser fluid around to sink down and force up 
 the rarer; and the continued currents so established, diffuse the heat through 
 the mass much more quickly than heat spreads by conduction in any solid. 
 
 Count Rumford's experiments led him at first to conclude that liquids, 
 but for this carrying process, by the particles changing their place, were 
 absolutely impassable to heat. A piece of ice will lie very long at the bot- 
 tom of water which is made to boil at the top by the contact of any hot 
 body y and when it at last melts, Count R. believed that it did so entirely 
 from the heat which passed downwards through the sides of the vessels 
 containing the water. But an ingenious experiment by Dr. Murray decided 
 the question differently. He made a vessel of ice, which, ef course, could 
 Dot carry downwards any heat greater than 32, as ice melts at that degree ; 
 and having put into the vessel a quantity of oil at 32, with the bulb of a 
 thermometer being a quarter of an inch under the surface of the oil, he 
 placed a cup of boiling water in contact with " the surface of the oil : in a 
 minute and a half-the thermometer rose nearly a degree, ancf in seven minutes 
 it rose five degrees, beyond which it did not go. The heat then must have 
 passed downwards through the liquid, proving a conducting power ; unless, 
 indeed, it passed by radiation, as explained in a subsequent page. 
 
 The internal currents or circulation produced by heat in fluid masses, and 
 of which there are so many important instances in nature, were more fitly 
 explained in the chapter on Hydrostatic* and Pneumatics ; we shall here, 
 therefore, allude to them very shortly. 
 
 Perhaps the best experimental illustration of the subject is obtained by 
 
 lacing a tall glass jar, filled with water, in which small pieces of amber are 
 
 iffused to show its movements, first in a warm bath, and then in a cold 
 
 bath. In the first case, the water and amber near the outside of the jar 
 
 where they are heated, will exhibit a rapid upward current, while in the 
 
 centre of the jar they will form an opposite and downward current. In the 
 
 second case, or when the jar is placed in a cold bath, the direction of the 
 
 currents will be reversed. 
 
 Consideration of these currents led the author of this work, some years 
 ago, to propose what he deemed a great improvement on the construction and 
 management of boilers and evaporating pans generally; namely, to convert 
 the upward and downward current in the mass of boiling liquid into a lateral 
 
SPREADING BY CONDUCTION. 267 
 
 current below, constantly and rapidly sweeping the bottom of tlie vessel. 
 In ordinary boilers, when a portion of liquid is converted into steam in con- 
 tact with the horizontal bottom, it does not separate from the bottom im- 
 mediately, but remains until a steam-bubble of considerable size be formed, 
 and in the mean time the part of the boiler defended by it from the con- 
 tact of the liquid, becomes overheated, and the following evil consequences 
 ensue: 1. Rapid destruction of the boiler, and on that account a rapid 
 expense; 2. Necessity for having originally much thicker, and therefore 
 dearer boilers ; 3. The thickness being an impediment to the passage of 
 heat, there is a proportionate waste of fuel; 4. and last, When the liquid is 
 a vegetable juice or extract, as sugar-cane juice, of a nature to be carbonized 
 and blackened when overheated, the quality of the product is often exceed- 
 ingly deteriorated. I'he patentees of an apparatus, described at 184, for 
 boiling sugar in vacuo, and therefore at a low and steady temperature, 
 gained, it was said, more than $40,000 a year by preventing the injury now 
 spoken of. And when the liquid is a saline solution, like the sea-water 
 used in steam ships, the salt soon encrusts the bottom of the boiler; and 
 powerfully both prevents the passage of heat and destroys the boiler. Now 
 a current sweeping the bottom prevents all these consequences, and may be 
 easily obtained. The most obvious method is, to place in the liquid some 
 upright tubes with open tops at a certain distance under the surface of the 
 liquid, and with the bottoms also open, but latterly, and all in one direc- 
 tion ; the consequence will be, that as soon as the liquid begins to boil, the 
 general mass, consisting of liquid mixed with bubbles of steam, becomes of 
 considerably less specific, gravity than the liquid in the tubes, remaining 
 unmixed, because steam will not enter the lateral mouths of the tubes, and 
 the columns of heavier liquid will therefore descend rapidly and issuing by 
 the lateral openings of the tubes all in one direction along the bottom of the 
 boiler, will powerfully and uninterruptedly sweep it. In a long wagon- 
 shaped boiler the tubes, instead of being round, should be made flat and 
 broad enough to reach from side to side ; and if a very rapid current be de- 
 sired, they must be made larger than the spaces between them in which the 
 steam has to rise, for thus the steam bubbles, being driven closer together, 
 will make the rising column so much the lighter, and its ascent consequently 
 the more rapid ; % and there will be a corresponding rapidity of issue of the 
 sweepmg current. In a moderate sized pan or boiler of the usual basin or 
 half-globe shape, the simplest method of producing the current is to have a 
 smaller vessel of similar shape made of thin metal, and placed within the 
 other so as to have about an inch space all round between them, and having 
 one large opening at its bottom, then all the steam mixed with fluid will 
 rise between the outer and inner vessel, while the unmixed liquid will 
 descend through the open bottom of the inner vessel, and spread in every 
 direction, sweeping the bottom of the outer. The sweeping of the bottom 
 of a boiler might also be effected by a wheel kept turning, to cause the 
 liquid to resolve horizontally ( as was done with another view in the large 
 Scotch whiskey stills,) or by a frame made somewhat like a rake or gridiron, 
 kept moving backwards and forwards upon the bottom. 
 
 As stated in a previous section, it is the heating and dilatation of the fluid 
 air over a tropical island while acted upon during the middle of the day by' 
 the powerful rays of the sun, which allow the colder and heavier air from 
 the face of the ocean around to press inwards upon it and force it upwards 
 in the atmosphere the cold current forming the delightful sea-breeze of 
 the climate. And it is the general heating of the air over the whole equa- 
 
268 HEAT. 
 
 torial belt of the earth, which, rendering it specifically lighter than the air 
 nearer the poles, allows this to assume the form of cool trade-winds, con- 
 stantly-blowing towards the sun's path, and pressing upwards the hot air, 
 which then spreads away on the top of the atmosphere towards the poles, to 
 mitigate the severity of the northern and southern cold. In the watery 
 ocean also there is a circulatory motion of the same kind, although less in 
 degree, tending to distribute heat and equalize temperature, and contributing 
 to produce some of the great sea currents known to mariners. 
 
 The vertical currents produced by heat, in the ocean and in-great masses 
 of water generally, preserve in and over them a comparatively uniform tem- 
 perate freshness, while the rocks and soil on the shores around may be 
 either parched under a burning sun, or bound up in frost. A keen frost 
 chills, and soon hardens in its icy grasp the surface of the ground j but of 
 water similarly exposed, the part first cooled descends to the bottom by its 
 increased density, and forces up a warmer water to take its place ; this in its 
 turn is cooled and descends, and a continued circulation is established, so 
 that the surface cannot become ice until the whole mass, of whatever depth, 
 has been cooled down to its greatest density. Hence the very deep sea is 
 not frozen even in the coldest climates, and in temperate climates, the 
 severest winter does not freeze even the ordinary lakes. During this intes- 
 tine movement in the water, that which ascends to the surface to be cooled, 
 by losing one degree of its heat, warms more than 500 times its bulk of air 
 one degree, and thus tempers remarkably the air passing over it. Hence 
 places in the vicinity of the sea and of lakes are warmer in winter than 
 places further inland, although nearer to the equator. England is much 
 warmer in winter than central Germany, which lies south of England ; and 
 the coast of Scotland and of the north of Ireland are warmer than Lon- 
 don : snow never lies long upon these coasts. As continental or inland 
 countries have thus in winter an extreme of cold, so they have in summer 
 an extreme of heat. Water admits the rays of the sun, and absorbs the 
 heat into the whole thickness of its mass, and therefore is warmed very 
 slowly ; but the dry earth retains all the heat near its surface, and is there- 
 fore soon heated to excess. 
 
 The ventilation of our dwellings and halls of assembly (as explained pre- 
 viously) is owing to the motion produced by the changed specific gravity of 
 air when heated. The air which is within the house becomes warmer than 
 the external air, and the latter then presses in at every opening or crevice 
 to displace the other. The ventilation of the person by the slow passage of 
 air through the texture of our clothing is a phenomenon of the same kind; 
 and thicker clothing acts chiefly by diminishing the rapidity of this passage. 
 Hence an oiled-silk or other air-tight covering laid on a bed has greater in- 
 fluence in preserving warmth than one or two additional blankets, and is 
 not generally used, only because it prevents ventilation, and by shutting in 
 the insensible perspiration, soon produces dampness. From the part of bed- 
 clothes immediately over the person there is a constant outward oozing of 
 warm air, and there is an oozing inward of cold air in lower situations 
 around. In many persons the circulation of the blood is so feeble that in 
 .winter, they have great difficulty in keeping their feet warm, even in bed, 
 unless with the assistance of a bottle of hot water or some such means, and 
 in consequence they often pass sleepless nights, and suffer in their general 
 health. In such cases, at the suggestion of the author, a long flexible tube 
 has been used, as of spiral wire, covered with leather or varnished cloth, 
 by which a person can send down to his feet his hot breath, and thus apply 
 
SPREADING BY CONDUCTION. 269 
 
 them effectually a natural animal warmth, as in a cold day he does to his 
 hands by blowing upon them through his gloves. 
 
 The power of fluids to diffuse heat being due to their power of carrying r , 
 and not of conducting it, the consequence should follow, that any circum- 
 stance which impedes the internal motion of the fluid particles, should 
 diminish the suffusing power. Accordingly we find, that fluids in general 
 transfer heat less readily in proportion as they are more viscid. Water, for 
 instance, transfers less quickly than spirits ; oil than water; molasses or syrup 
 than oil ; and water thickened by starch dissolved in it, or which has its 
 internal motion impeded by feathers or thread immersed in it, less quickly 
 than where it is pure and at liberty. Cooling being merely a motion the 
 reverse of heating, it is influenced by the same law. Hence the reason why 
 thick soups, pies, puddings, and all semifluid masses, retain their heat so 
 long so much longer than equal bulks of mere fluid. The same law affords 
 explanation of the facts, that very porus masses and powders, as charcoal, 
 metal filings, saw dust, sand, &c., conduct heat more slowly than denser 
 masses, their interstices being filled with air, which scarcely conducts heat, 
 and which, by the structure of the substance, has no freedom of motion or 
 circulation by which it might carry the heat. 
 
 "Heat spreads, also, partly ~by being radiated or shot light from one Ioc7y 
 to another, through transparent media or space with readiness affected by 
 the material and the state of the giving and receiving surfaces.'' (Read 
 the Analysis, page 256.) 
 
 If a heated ball of metal be suspended in the air, a hand brought in any- 
 direct ion near to it, will experience the sensation of heat ; and beneath it the 
 sensation will be as strong as on the sides, although the heat has to shoot 
 down through an opposing current of air approaching the heated ball, to rise 
 from it, as explained in a previous section. A delicate thermometer substi- 
 tuted for the hand, will equally detect the spreading heat, and if held at 
 different distances, will prove it to diminish in the same ratio as light di- 
 minishesjn spreading from any luminious centre, viz., to be only a fourth 
 part as intense at a double distance, in a corresponding proportion for other 
 distances. If the heated body be enclosed in a vacuum, a thermometer 
 placed near it will still be affected in the same manner. If a screen be inter- 
 posed between the body and the thermometer, the latter will not be affected 
 at all, proving the heat to spread in straight lines. Heat, when diffusing 
 itself in this way, to distinguish it from heat passing by contact and com- 
 munication, as described in the last section, is called radiant heat; that is 
 to say, spreading in radii or rays all around its source as light spreads. 
 
 Radiant heat resembles light yet in other respects. It as rapidly perme- 
 ates certain transparent substances, and its course suffers in them a degree 
 of the bending, termed refraction by ' opticians. It is reflected from many 
 kinds of polished surfaces, just* as light is reflected from a common mirror; 
 and many such surfaces directed to one point or centre (as when Archimedes 
 made the sun his assistant to burn the Roman ships) or a single concave 
 surface, having its own centre or focus, will concentrate heat just as light. 
 Its motion in the sun-beam is so rapid, as for any distance at which men can 
 try the experiment, to appear instantaneous; and the rays of heat from hot 
 iron or burning charcoal concentrated at great distances by suitable mirrors, 
 affect a thermometer as quickly as the heat of the sun similarly reflected. 
 Although light and heat are united in the sun's ray, they are still separable 
 
270 HEAT. 
 
 by our glass prisims or lenses; and the focus of heat behind a burning glass 
 is not precisely the focus of light. Heat, in radiating through air, does not 
 warm the air, and its passage is not sensibly affected by winds or any other 
 motion of the air. These resemblances in the phenomena of light and heat 
 have by some inquirers been held to prove that the two classes of appear- 
 ances are only different modifications of action in the same subtile substance 
 or either, 
 
 The diffusion of heat by radiation, as it takes place in an instant to any 
 distance, and begins whenever there is any inequality of temperature between 
 bodies exposed to each other, would produce instant balance of temperature 
 throughout nature, but that heat leaves and enters bodies with readiness, 
 depending on the condition of their surfaces, and on their internal conduct- 
 ing powers. A black stone-ware ter-pot, for instance, will radiate away 100 
 degrees of its heat in the same time that a pot of polished metal will radiate 
 only 12 degrees. 
 
 Professor Leslie was the first to see the importance of investigating this 
 subject, and he had the merit of contriving well-adapted means, and of de- 
 tecting many of the important facts. As common thermometers are not suffi- 
 ciently delicate to determine very sudden changes of temperature, where the 
 influence is so light as in many cases of radiant heat, he used the beautiful 
 differential thermometer contrived by himself, in conjunction with concave 
 mirrors, (as represented on next page,) to concentrate the heat, and accumu- 
 late its energy. Then taking as his heated body a cubical tin vessel filled 
 with boiling water, and covering it successively with plates or layers of dif- 
 ferent substances and with different colors, and exposing the thermometer 
 to it for a given time under all the changes, he noted the number of degrees 
 which the thermometer rose, (as seen in the table which here follows) and 
 thus ascertained the radiating power of each sort of covering. 
 
 Lamp black . . . . . . . 100 
 
 Writing paper . . . . . . . .98 
 
 Crown glass . . . . . . . 90 
 
 Ice .87 
 
 Isinglass ........ 75 
 
 Tarnished lead 45 
 
 Clean lead 19 
 
 Iron polished . . . . . . . .15 
 
 Tin plate 12 
 
 Gold, silver and copper 12 
 
 He next reversed the experiments by using his hot-water vessel always in 
 the same state, and covering the thermometer bulb with the different sub- 
 stances and colours, and thus he ascertained that the comparative absorbing 
 powers of the substances and colours were very nearly proportioned to their 
 radiating powers : lamp-black, for instance,- absorbed or was heated 100, 
 while the polished metals absorbed or were heated only 12, and so for the 
 others. And, lastly, the absorbing powers being an indication of the oppo- 
 site or reflecting powers (for a body absorbing only a given proportion of 
 the heat which falls on it, must reflect the remainder,) he, by the same 
 experiments, ascertained the radiating, absorbing, and reflective or mirror 
 powers of the bodies, and therefore all the important points respecting radiant 
 heat in its relation to them. 
 
 It seems paradoxical that the putting a clothing of a thin cotton or woolen 
 fabric upon the polished tin vessel, should cause the heat to be received by 
 
SPREADING BY RADIATION. 271 
 
 it or dissipated from it much sooner than if the vessel were naked, but such 
 is the fact. And metal with a scratched or roughened surface radiates or 
 receives much more rapidly than polished metal. 
 
 The property of absorbing heat depends much upon the colour of the 
 substance, and, as a general rule, the dark colours, viz,, those which absorb 
 most light, absorb also most heat. Dr. Franklin proved this by laying 
 pieces of cloth of different colours on snow, and exposing them during a 
 given period to the sun's rays : while he noted the different depths to which 
 by the melting of the snow under them, the pieces sank. Hence comes the 
 importance of having a white dress in summer, that by it, with the sun's 
 light, the heat also may be repelled. And a white dress in winter is good 
 because it radiates little. Polar animals have generally white furs. White 
 horses ars both less heated in the sun, and less chilled in winter, than those 
 of darker hues. 
 
 The rate of cooling in heated bodies must be influenced by all the particu- 
 lars noted above viz., substance, surface, colour, and by the excess of heat 
 in the cooling body as compared with those around it. 
 
 The concentrating apparatus used for experiments on the radiation of 
 heat consists of two concave tin mirrors, here represented at a and b, so 
 formed and placed in relation to each other that all the rays of light or heat 
 issuing from the focus of one, as at c, shall, after a double reflection, be 
 collected in the focus of the other, d. A stand under one focus c is intended 
 
 Fig. 124. 
 
 
 
 ^ 
 
 el 
 
 R'ff^iUJV) 
 
 IT, x M LM 
 
 to support the body giving out or receiving heat, and a stand under the 
 other d is meant to support the thermometer. For farther explanation of 
 the action of such mirrors, we may refer to what was said of the concen- 
 tration of sound in the section on Acoustics^ or to what follows in the sec- 
 tion on Optics, on the concentration of light. The general rationale of such 
 facts is, that heat, light, sound, and elastic ball, &c., when reflected from any 
 point of a surface, returns, if it fall perpendicularly to that point, in the 
 same line by which it approached ; but if it fall obliquely, or from one side 
 of the perpenpicular, it returns in a line deviating as much on the other side. 
 Now the surfaces of concave mirrors are so formed, that every ray issuing 
 from the focus shall, when reflected, become parallel to every other ray as 
 represented by the dotted lines in the figure ; and it is the property of a 
 similar mirror receiving parrallel rays to make them all meet in its focus : 
 thus, any influence radiating from c towards the mirror d, will again, after 
 two reflections, be collected at d. The purpose and effect of such mirrors 
 in experiments on heat, are merely to concentrate feeble influences, so that 
 they may be more accurately estimated. To show the effect and mode of 
 action of such mirrors, they may be placed exactly facing each other at any 
 convenient distance, and then a hot body of any kind, as a metallic ball or 
 a canister of boiling water, being placed in one focus while a thermometer 
 
272 HEAT. 
 
 stands in the other, the thermometer will instantly rise ; although if left in 
 any intermediate situation nearer to the hot body, and therefore not in the 
 focus, it will not be affected. If burning charcoal be placed in one focus, 
 and a readily combustible substance in the other, the latter may be set fire 
 to, at the distance of thirty feet or more. 
 
 If, in one focus of the mirror apparatus described above, there be placed, 
 instead of the canister of hot water, a piece of ice, the thermometer in the 
 .other focus immediately falls. This has been called the radiation of cold, 
 and persons were at one time disposed to think that it proved cold to have 
 a positive existence distinct from heat. The case, however, is merely that 
 the thermometer happens then to be the hotter body in one focus of the 
 mirrors, placed in close relation with a colder body, the ice, in the other, 
 and consequently by the law of equal diffusion, it must share its heat with 
 the ice, and will fall. The mirrors in any case have merely the effect, by 
 preventing the spreading and dissipation of the radiant heat from either 
 focus except towards the other of making two distant bodies act upon each 
 other as if they were very near. All the heat that seeks to radiate from 
 the thermometer d in the direction of the surface of the mirror b, if not 
 met by an equal tension or force of temperature in the other mirror or 
 focus, to which they are directed at a and c, will radiate away to c, and 
 become deficient at d. Some inquirers have believed that heat was constantly 
 radiating in exchange from substance to substance (as light radiates 
 between opposed bodies) only more copiously from the side where the 
 temperature was highest : others have held that motion took place only 
 where there was excess of heat ; that is, when the balances of temperature 
 was destroyed ; and this is the simplest view. 
 
 There is a remarkable difference in one respect between the heat of the 
 sun and that radiated from any other source, namely that the first passes 
 through air, glass, water, and transparent bodies generally, very readily, 
 while the latter, although not obstructed by air, is almost totally intercepted 
 or absorbed, in passing through any of the other substances named. In our 
 drawing-rooms it is common to have plate glass fire-screens, which, while 
 they allow the light to pass, defend the face from the heat; but all persons 
 know that the heat of the sunbeams, as well as their light, enters our green- 
 houses through the glass which covers them. A glass screen interposed be- 
 tween the concave mirrors in the apparatus above described, destroys almost 
 entirely the effect of the heated body placed in one focus, on the thermo- 
 meter in the other, and the trifling effect really produced has appeared to 
 some to be owing to the heat that is absorved by the screen on one of its 
 sides, and then after passng through *it by conduction, is radiated from the 
 other. This conclusion seemed to be supported by the fact that screens of 
 metal or of glass, covered with lamp black, paper, &c., allow transmission 
 nearly in proportion to their several absorbent and radiant powers. More 
 careful experiments, however, have seemed to prove that, even at a low 
 temperature, a certain portion of the heat is suddenly radiated through the 
 glass, and at a high temperature, a much larger portion. A glass mirror 
 reflects the light of a fire, but at first retains nearly all the heat, and only 
 radiates it afterwards as a hot body. 
 
 The -doctrines of radiant heat make us aware of the importance of having 
 vessels of polished metal for containing liquids or other things which we de- 
 sire to keep warm; hence, tea and coffee-pots, dishes for soup, &c., should 
 be 'polished. As a black earthen tea-pot loses heat by radiation nearly in 
 proportion to the number 100, while one of silver or other polished metaJ 
 
SPREADING BY RADIATION. 273 
 
 loses only as 12, there will be a corresponding difference in their aptitude 
 for extracting the virtues of any substance infused in them. Pipes for the 
 conveyance of steam or hot air, if left naked, should be of polished metal ; 
 but after arriving at a place where they have to give out their heat, their 
 surface should be blackened and rough. A (?oat of polished mail is not a 
 cold covering. A mirror intended to reflect "heat should be of highly 
 polished metal, and such, for an obvious reason, the interior of a screen behind 
 roasting meat is attempted to be made. A fireman's mark is usually covered 
 externally with smooth tin foil. It is of advantage that the bottom of a tea- 
 kettle or other cooking vessel be externally black, because the bottom has 
 to absorb heat, but the top should be polished because it has to confine. 
 
 The interesting phenomenon of dew was not at all understood until lately, 
 since the laws of radiant heat have been investigated. At sun-rise, in par- 
 ticular states of the sky, every blade of grass and leaflet is found not wet- 
 ted, as if by a shower, but studded with a row of distinct globules most 
 transparent and beautiful, bending it down by their weight, and falling like 
 pearls when the blade is shaken. These are formed in the course of the 
 night by a gradual deposition on bodies rendered by radiation colder than 
 the air around them, of part of the moisture which rises invisibly from 
 water surfaces into the air during the heat of the day. In a clear night the 
 objects on the surface of the earth radiate heat to the sky through the air 
 which impedes not, while there is nothing nearer than the stars to return 
 the radiation ; they consequently soon become colder, and if the air around 
 has its usual moisture, part of this will be deposited on them, in the form 
 of dew, exactly as the invisible moisture in the air of a room is deposited 
 on a cold bottle of wine when first brought from the cellar. Clouds, by 
 obstructing the radiation spoken of, obstruct the formation of dew. Air 
 itself seems not to lose heat by radiation. A thermometer placed upon the 
 earth any time between sunset and sunrise, generally stands considerably 
 lower than another suspended in the air a few feet above it ; owing to the 
 radiation of heat upwards from it and from the earth, while the surrounding 
 air remains nearly in the same state. During the day, while the sun shines, 
 the earth is warmer than the air. The reason why the dew falls, or is 
 formed so much more copiously upon the soft spongy surface of leaves and 
 flowers, where it is wanted, than on the hard surface of stones and sand 
 where it would be of no use, is the difference of their radiating powers, 
 There is no state of the atmosphere in which artificial dew may not be made 
 to form on a body, by sufficiently cooling it and the degree of heat at which 
 the dew begins to appear is called the dew-point being an important 
 particular in the meteorological report of the day. In cloudy nights heat 
 is radiated back from the clouds, and the earth below not being so much 
 cooled, the dew is scanty or deficient. And it is, when uninformed persons 
 would least expect the dew, viz. : in very warm clear nights, and perhaps 
 when the beautiful moon invites to walking, as in some of the evenings of 
 autumn with the harvest moon and harvest occupations that the dew is 
 more abundant, and the danger greater to delicate persons of taking harm 
 by walking among the grass. 
 
 18 
 
274 HEAT. 
 
 " Heat by entering bodies expands them, and through a range which includes 
 as three successive stages tne forms of solid, liquid, and air or gas ; be- 
 coming thus, in nature, the grand antagonist and modifier of the effects 
 of that attraction which holds corporeal atoms together, and which, if act- 
 ing alone, would reduce the whole material universe to one solid, lifeless 
 mass. (Read Analysis, page 256.) 
 
 If an experimenter take a body which is as free from heat as human art 
 can obtain it a bar of solid mercury, for instance, as it exists in a polar 
 winter and if he then gradually heat such body, it will acquire an in- 
 crease of bulk with every increase of temperature ; first, for a time, there 
 will be simple enlargement or expansion in every direction ; then the mass 
 will in addition be softened ; then it will be melted or fused ; that is to say, 
 in the case supposed, the solid bar will be reduced to the state of liquid mer- 
 cury, with the cohesive attraction of the atoms nearly overcome : if the 
 mass be still farther heated, it will gain bulk until at a certain point, the 
 atoms will be repelled from one another to much greater distances, constitu- 
 ting then a very elastic fluid called an air or gas, many hundred times more 
 bulky than the same matter in the solid or liquid state, and forcibly distend- 
 ing an appropriate vessel as common air distends a bladder ; susceptible, 
 moreover, of dilating indefinitely farther, by farther additions of heat, or by 
 diminution of the atmospheric, or other pressure, against which it had to 
 rise during its formation. A subsequent removal of the heat from the gas- 
 eous mercury, will cause a progress of contraction corresponding to the pre- 
 vious progress of expansion, and the various conditions or forms of the sub- 
 stance above enumerated, will be reproduced in a reversed order, until the 
 solid mass re-appear, as at first. 
 
 What is thus true of mercury is proved, by modern chemical art, to be 
 true also of all the ponderable elements of our globe, and of many of the 
 combinations of these elements, as water, for instance, familiarly known 
 in its three forms of ice, water and steam ; although compound substances 
 generally, by great changes of temperature, are decomposed into their 
 elements. 
 
 A student might at first have difficulty in believing that the beautiful 
 variety of soil, liquid, and air, found among natural bodies could depend 
 upon the quantities of heat in them, because these forms are all seen exist- 
 ing at the same common temperature ; but he afterwards learns that each 
 substance has its peculiar relation or affinity to heat, and that hence, while 
 at the medium temperature of the earth, some bodies contain so little as to 
 be solids like the metals, stones, earths, &c. ; others have enough to be 
 liquids as mercury, water, oils, &c. , and others have enough to be airs- 
 as oxygen, nitrogen, hydrogen, &c. Men, until better informed, are prone 
 to deem the tastes in which bodies are most frequently observed by them, 
 the natural or essential states of such bodies ; and the Indian king reasoned 
 but in the usual way when he held the Dutch navigators, newly arrived on 
 his shores, to be gross impostors, because they said that in their country, at 
 one time of the year, water became so hard that they could walk upon it, 
 and drive -their carriages upon it, and shape it into solid blocks. All per- 
 sons err like this king, who in thinking of the different substances familiarly 
 known to them, regard their accidental state of solid, liquid or gas, which 
 state is really dependent on the temperature of the bodies, and therefore 
 on the particular climate or situation on earth where they are found, to be in 
 them an essential natural character. As well might a person who had never 
 
CAPACITY FOR HEAT. 275 
 
 seen silk, but as a delicate gauze or satin enveloping some lovely human form, 
 refuse to recognize it in the unsightly coil of the worm which produces it. 
 
 The degree in a general state of temperature at which the substances most 
 important to man change their states from solid to liquid, or from liquid to 
 air, will be noticed in a future page. Here we have only to remark, that the 
 differences are very great. Mercury melts at about 80 degrees below the 
 melting point of ice, and porcelain at about 30,000 degrees above. There 
 are some substances which require so high a temperature for their fusion or 
 for their conversion into gas, that human art has difficulty, or even finds it 
 impossible, to produce the changes by simple concentration of heat ; but all 
 such substances are quickly reducible to the liquid form when placed in con- 
 tact with others for which they have a chemical affinity, and which possess 
 already the form of liquid or air; as when gold or platinum are dissloved in 
 mtro-muriatic acid flint in the fluoric acid carbon in hydrogen gas. Now 
 many persons may not have reflected that the dissolving a solid in any fluid 
 menstruum is merely another mode of melting it by heat; yet this is the 
 truth, for the menstruum is itself fluid, only because of the much heat which 
 it contains, and in dissolving the more obdurate substances, it does so merely 
 because its atraction for the substance brings the particles into .union with 
 the heat which already exists in itself. Heat, then, is the one universal 
 solvent, or cause of fluidity. Its influence in this view is interestingly seen 
 in the fact, that a fluid when heated can dissolve much more of solid than 
 when cold. Water, while hot, keeps dissolved twice as much of many salts 
 as it can when its temperature has fallen, as is proved by the crystals of salt 
 formed in any saturated solution as it cools. r There are, again, in nature 
 many substances having such an affinity for heat, that, until lately, they 
 have been known only as airs ; and even in the present advanced state of 
 art, they cannot, by any degree of mere cooling, be reduced to the liquid or 
 solid form ; yet all such, when pressure is added to the cooling, or when the 
 chemical attraction for them of some other substances which already exist 
 in the liquid or solid state is made to co-operate, may be reduced. An instance 
 is afforded when oxygen is made part of a liquid acid, or of a solid ore. 
 
 Of solids, some on receiving heat become very soft before they are lique- 
 fied, as pitch, glue, iron, &c. ; others change completely at once, as ice in 
 becoming water : and some pass at once to the state of air, without, therefore, 
 having assumed at all the intermediate state of liquid they are sublimed, 
 as it is called, and on cooling again may be caught in a powdery state, as 
 seen in that form of sulphur or of benzoin, termed the flower of the sub- 
 stance. Of the latter class, also, are camphor, arsenic, corrosive sublimate, 
 and the substance called iodine, which last, from the state of rich ruby 
 crystals, becomes at once, on being heated, a dense transparent gas of the 
 same hue, and in cooling resumes its crystalline form. 
 
 The reader having arrived at this place, may peruse again with advantage 
 the five pages near the beginning of this work, which treat of the influence 
 of heat on the constitution of masses. 
 
 (( Each particular substance, according to the nature, proximity, &c., of its 
 ultimate particles, takes a certain quantity of heat, (said to mark its CAPA- 
 CITY) to produce in it a given change of temperature or caloric tension.' 9 
 (Read the Analysis, page 256.) 
 
 A pound of water, for instance, that its temperature may be raised one 
 degree, takes thirty times as much heat as a pound of mercury. This may 
 
276 HEAT. 
 
 be proved in various ways. First, if the heat be derived from any uniform 
 source, the water must remain exposed to it thirty times as long as the mer- 
 cury. Second, if both substances, after being equally heated, be placed in 
 ice until cooled to the freezing point, the heat which escapes from the water 
 will melt thirty times as much ice as that which escapes from the mercury, 
 instead of the two becoming of a middle temperature, as in the case when 
 equal quantities of hot and cold water are mixed, and every degree of heat 
 lost by the one quantity becomes just a degree gained by the other the pound 
 of hot water, by giving up one degree to the pound of cold mercury, raises 
 the temperature of the latter thirty degrees ; and in the same proportion for 
 other differences : or on reversing the experiment, a pound of hot mercury 
 will be cooled thirty degress by warming a pound of water one degree. 
 
 Now each particular substance in nature, just as water or mercury, has 
 its peculiar capacity for heat ; and experiments made by the modes of mix- 
 ture and of melting ice above described have led to the construction of tables 
 which exhibit the relations. The following short table is an abstract, show- 
 ing the comparative capacities of equal weights of some common substances. 
 Water, for reasons of convenience, has been chosen as the standard of com- 
 parison. It appears that a pound of hydrogen gas takes about twenty times 
 more heat "to produce it in a given change of temperature, than a pound of 
 water, while a pound of gold takes about twenty times less, and, therefore, 
 four hundred times less than the hydrogen. The figures in the table, by 
 marking the comparative capacities for heat of various substances necessarily 
 indicate, also, the comparative quantities of ice melted by equal weights of 
 the substances in cooling through an equal number of degrees. A pound 
 of water, the standard, must cool 140 degrees, that is ; must give up 140 
 degrees of its heat, to melt one pound of ice. 
 
 Gases. 
 
 Hydrogen - - 2H 
 
 Atmospheric air - - - 1| 
 
 Carbonic acid gas - - - - 1| 
 
 Common steam - - li 
 
 Liquids. 
 
 Solution of carbonate of ammonia 2 
 
 Alcohol - - - - - lj! 
 
 Water ..... 1 
 
 Milk 1 
 
 Olive oil - - - - - | 
 
 Linseed oil- .... \ 
 
 Sulphuric acid - - - i 
 
 Quicksilver .... ^ 
 
 Solids. 
 
 , Ice ft 
 
 Wheat - - * 
 
 Charcoal ..... 
 
 Chalk i 
 
 Glass - J 
 
 Iron ... 
 
 Zinc ... T ' ff 
 
 Gold - 
 
CAPACITY FOR HEAT. 277 
 
 We may remark here that some late researches, by another mode of trial, 
 make the capacity of air to be only a quarter that of water, although in the 
 preceding table it appears to be one and three-quarters. Now as the other 
 aeriform fluids have been compared with water through the medium of atmo- 
 spheric air, if there be an error with respect to this, it must run through all 
 the figures noting the capacity of other aeriform substances. 
 
 If we seek a reason or reasons why there should be among bodies the dif- 
 ferences of capacity here stated, the circumstances chiefly attracting attention 
 are the following. 1st. Equal weights of the various substances have very 
 different bulks or volumes, and therefore have different room in which the 
 heat may lodge ; as a pound of mercury, for instance, is only one-fourteenth 
 part as bulky as a pound of water. That the bulk, however, is not the only 
 influencing circumstance appears in the fact, that mercury only has one 
 thirtieth of the capacity of water. 2d. In equal bulks of different sub- 
 stances, the space may be more completely occupied by the particles of one 
 than of another as is probably true of the particles of mercury compared 
 with those of water. 3d. But as the facts are not fully accounted for by 
 these two circumstances, we must infer that there is some difference in the 
 ultimate particles of bodies affecting their relations to heat. We shall now 
 review more particularly the various circumstances mentioned. 
 
 First. The influence of bulk or volume, in determining the capacity for 
 heat, is proved by the facts stated in the preceding table, and by many others. 
 In the table, for instance, it is seen that hydrogen and the gases generally, 
 with their great comparative bulk, have also great capacity ; that liquids 
 have less capacity than gases ; that solids have less than liquids but the 
 capacity, as already stated, is not in strict proportion to bulk; for hydrogen 
 which is many thousand times more bulky than an equal weight of water, 
 has only twenty-one times the capacity. Again, if any body whatever be 
 suddenly compressed into less bulk, heat will issue from it as if squeezed 
 out. Thus iron or other metal, suddenly condensed by the heavy blow of 
 a hammer, is thereby rendered hotter, that is expelled heat gradually spreads 
 from it. Because water and spirit, on being mixed, occupy less space than 
 when separate, there is from the" mixture a corresponding discharge of heat. 
 But the truth is most remarkably exemplified in airs or gases, owing to their 
 great range of elasticity. They may be condensed or dilated a hundred-fold 
 or more, and there will be a simultaneous concentration or diffusion of their 
 heat, that is to say, the production, in the space occupied by them of intense 
 heat or cold. The heat of air just condensed, or the cold of that which has 
 just expanded, is much greater than even the most delicate thermometer can 
 indicate, for there is so little heat altogether even in a considerable volume of 
 air, that the mass of a mercurial thermometer, although absorbing a great 
 part of it, would be little affected. The extent, however, of the change of 
 temperature is seen in the facts, that, by the sudden condensation of air we 
 may inflame tinder immersed in it, and by allowing air suddenly to expand, 
 we may convert any watery vapour diffused through it into ice or snow. 
 Nay, air, containing carbon in perfect solution, as is true of the common coal 
 gas, if first condensed to expel heat, and then allowed suddenly to expand, 
 will be so cooled that the carbon will be separated like a black cloud, as snow 
 is separated in the case before described. The cold which separates or 
 freezes carbon from a gas holding it in solution, must be very intense. It 
 might be expected that air suddenly compressed into half its previous volume, 
 should become just twice as hot as before, or if suddenly dilated to double 
 volume, should be only half as hot, thus enabling us to ascertain the whole 
 
278 HEAT. 
 
 quantity of heat contained in it ; but the facts are not so ; the temperature 
 changes, near the middle degrees of the scale at least, much less than the 
 density. Air in doubling its volume from a common density, becomes colder 
 only by about 50 of Farenheit's thermometer. 
 
 The different capacity for heat of air in different states of dilation, pro- 
 duces effects of great importance in nature as well as in the arts. Thus, 
 
 On the surface of the earth near the sea-shore, the air of the atmosphere 
 has a certain density (a cubic foot weighs about one ounce and a quarter) de- 
 pendent on the weight and pressure of the superincumbent mass; but on a 
 mountain top 15,000 feet -high, as half the mass of the atmosphere is below 
 that level (see " Pneumatics,") the air is bearing but half the pressure, and/ 
 consequently any quantity of it has twice the volume of an equal quantity 
 at the sea-side, with a temperature consequently many degrees inferior. The 
 air which is at any time on a mountain-top, may, a little while before, have 
 been on an adjoining plane or shore, and in gradually climbing the mountain 
 side as a wind, it must have been gradually expanding and becoming cooler 
 in proportion to the diminishing pressure. It is found that air, on rising 
 from the sea-shore, becomes one degree colder nearly, for the first 200 feet 
 of perpendicular ascent, and that air becomes altogether about 50 colder in 
 rising 15,000 feet; so that at this latter elevation, water exposed to the air 
 is frozen even near the equator, where the temperature of low plains is at 
 least 80. It thus appears that if a man could travel with the wind so as to 
 remain always surrounded by the same air, he might begin his journey 
 with it from the summer vineyard of the Rhine, might soon after find it 
 the piercing blast of the Alpine summits ; and again, a little after, without 
 any change having occurred in the absolute quantity of its heat, might feel 
 it as the warm breath of the flowers on the plains of Italy. 
 
 The explanation is thus given of why very elevated mountains in all parts 
 of the earth are hooded in perpetual snows. We have just said that even at 
 the equator, where the average temperature hear the sea is 84, water will 
 be frozen when carried to an elevation of 15,000 feet. A line, therefore, 
 traced round a mountain at this level would divide the portion of it destined 
 to sleep under lasting ice and snow from the portion below covered with 
 green herbage. This line, wherever found, is called the snow line, or line 
 of perpetual congelation. At the equator it is high in the atmosphere, be- 
 cause there is a difference of about 40 between the average temperature of 
 the country and the freezing point of water, viz., the difference between 84 
 and 32, and an elevation of 15,000 feet corresponds to this difference; but in 
 a progress towards the poles, the line is met with gradually nearer to the earth, 
 as the difference between the average temperature and the freezing point is 
 less. In Switzerland, the snow line is at 6,500 feet above the sea ; in Norway, 
 it is below 5,000. With respect to the line of congelation, it is farther to be 
 remarked, that in tropical countries, because the temperature of the air is nearly 
 uniform during the whole year, the line or limit of frost and snow is distinct 
 and unvarying, that is to say, is narrow, particularly where the acclivity is 
 considerable ; but in countries to the north and south, which have strong con- 
 trast of summer and winter, the line rises in summer and falls in winter, and 
 thus becomes broad and less evident ; in the hot season much snow is melted 
 or half melted above the middle of the line or belt, while in winter much snow 
 and ice is accumulated below that, to be melted again when summer returns. 
 
 In the breadth of the line of congelation for changeable climates, we have 
 the reason of the formation of what are called glaciers around snow-capped 
 mountains situated in such climates, and around such only. The snow near 
 
CAPACITY FOR HEAT. 279 . 
 
 the upper part of ths broad line having been only softened or half thawed in 
 the preceding summer, becomes in winter almost as solid as ice, and in the 
 succeeding summer vast masses of it r detached by the action of the sun and 
 of the internal heat of the earth, and loaded with more recently deposited snow, 
 are constantly falling down into the neighbouring valleys within the broad 
 line of congelation ; where, beingaccuraulated, and the crevices filled up with 
 snow or with water which hardens to ice, they form at last the huge glaciers 
 or seas of ice, mers de glace, which renders certain regions so remarkable. 
 The falling of the masses above described (called in Switzerland avalanches,} 
 is what renders the ascent to snow-clad mountains so terrific and dangerous. 
 Around Mount Blanc, in the awful solitudes of the elevated valleys, the 
 avalanches are thundering down almost without interruption during the whole 
 summer, in which season only the attempt to ascend the mountain can be 
 made : and a pistol shot, or any considerable agitation of the air, may suffice 
 to set loose masses that would sweep away a whole convoy. Beneath gla- 
 ciers there is always going on a melting of that part of the ice which is in 
 contact with the earth, and hence a stream of water constantly issues from 
 the bed of every glacier. These streams in Switzerland are the beginnings 
 of the magnificent rivers the Rhine and Rhone Like the avalanches break- 
 ing loose in summer among the mountains, there are in polar seas vast masses 
 of ice detached from the shores, and which afterwards drift into warmer seas 
 to be melted. These often become as rafts to the arctic bear, and to his sur- 
 prise, carry him to new latitudes, and leave him at last to perish in the midst 
 of the wide ocean, when his support has vanished from beneath him. 
 
 Although the proofs are not so immediately apparent, the line of congela- 
 tion exsits as truly every where in the open sky, over sea and plane, as 
 where there are mountain heights to wear its livery; and considerably below 
 the line, the cold, aided by electrical agency, is sufficient to produce in the 
 form of mist or clouds, a deposition from the air of the watery vapour con- 
 tained in it. There is thus in nature an admirable provision to shade the 
 earth at proper times from the too powerful rays of the sun, or to supply rain 
 as wanted, without the transparency of the inferior regions of the atmosphere 
 being much affected. As the watery vapour rising from sea or lake, and in- 
 visibly diffused in the atmosphere, can only reach to the height where the cold 
 is great enough to condense it, the clouds may in general be regarded as the 
 top of that atmosphere of watery vapour or aeriform water, which is always 
 mixed more or less with the atmosphere of mere air; and as the quantity of 
 watery vapour which can exist invisibly in a given space, depends altogether 
 on the intensity of heat present, the clouds in a cold or humid atmosphere 
 will be low, and in a warm or a dry atmosphere will be high, or there may 
 be none. An aeronaut mounting in bis balloon through a clear sky, often 
 enters a dense cloudy stratum, and for a time is surrounded by the gloom 
 almost of night, the face of earth being hidden from him below, while the 
 heavenly bodies are equally veiled from him above ; but rising still higher, he 
 again emerges to brightness, and looks down upon the fleecy ocean rolling 
 beneath him, as the climber to a lofty peak looks down from the ever pure 
 atmosphere around on the inferior region of clouds and storms. 
 
 The diminished temperature of air in the higher regions of the atmosphere, 
 often enables the natives of temperate climates, when forced to reside in hot 
 tropical countries, inimical to their health, to find near at hand, on some 
 mountain height, the congenial temperature of their early home.". The 
 author of this work, during a visit to the then not long inhabited Island of 
 Penang in the strait of Malacca, examined this fact with pleasure not readily 
 
280 HEAT. 
 
 forgotten. The centre of the island is occupied by a lofty mountain ridge 
 thickly wooded, on the northern summit of which a few residences visible 
 from the sea-shore like eagles' f nests on a cliff, had just been constructed. 
 Towards these one morning at sunrise, on an active little horse of the 
 country, and along a tolerable road, he began to climb from the hot plain 
 below. At first there were around him purely tropical objects, inspiring 
 tropical feelings, the latter modified indeed by the reflection that his track 
 lay through a forest, in which until lately the foot of man never penetrated, 
 and where the trees nursed through ages to their greatest growth, and the 
 stupendous precipices and the sublime waterfall had so recently been exposed 
 to human observation ; but as he gradually ascended, he perceived the 
 character of the vegetation to be changing and the air to be becoming so light 
 and cool as strongly to awaken in him thoughts of distant England nay, 
 almost the illusion that he was there. When he had reached the summit, 
 however, 'and a clear space opened to view the whole country around, his 
 attention was soon recalled to the fervid land of the sun. At first, from the 
 elevation being so great, the eye took account only of the grander features of 
 the scene, and which were such nearly as might be met with on a Grecian 
 or Italian shore : the expanse of sunny water in that beautiful strait, the 
 opposite continent with its river winding seaward across' the plain, the town 
 and the roadstead near it crowded with ships, which appeared only as specks 
 on a wide-spread map ; but, on closer inspection, and particularly with the 
 aid of the telescope, were described the rich groves or cocoa-nut and banana, 
 the plantations of spice, and cotton, and sugar-cane, the tawny labourers, the 
 bamboo dwellings, the fanciful canoes, or prows/ and other objects of the 
 like character. And such was the scene which even under the equator, a 
 person could place under his eye, while the thermometer near him stood as 
 in an English month of May. 
 
 The interiors of the islands of Jamacia and Hayti have many situations 
 of great extent, which combine, as above described, the advantages of tropi- 
 cal situation and temperate climate, and which might well be inhabited by 
 English labouring colonists. The vast plain of Mexico, and much of the 
 central land of South America, are similarly circumstanced : and it is not 
 uncommon, where the ascent to the gigantic Andes is gradual, to find at the 
 bottom of the ridge a town, whose markets are stored only with the produc- 
 tions of the equator, while in a town higher up will be seen only what 
 belongs to the temperate skies of Europe : climates of the earth naturally 
 distant, thus meeting, as it were, in amicable vicinity, on the same rising 
 plain. 
 
 The facts detailed in the preceding paragraphs are intended to illustrate 
 the subject, of the relation of volume in a body to the capacity for heat. We 
 now proceed to speak of Density in the same respect. 
 
 Second. It might be anticipated that a dense body, or one in which the 
 constituent particles may be supposed to fill more completely the space occu- 
 pied by it than the particles do in a rarer body, would have smaller capacity 
 for heat, in proportion to the smaller space left vacant in its mass : and in a 
 general comparison of the capacities of equal bulks of different substances, 
 such anticipation is partly verified, as when a pint of dense mercury is found 
 to have only about half the capacity which a pint of lighter water has. The 
 relation, however, is by no means universal, nor at all in proportion to the 
 differences of density. Water, which is denser than oil, and according to 
 the hypothesis should have less capacity, yet has nearly double the capacity ; 
 and mercury, which being nearly fourteen times denser than water, might 
 
EXPANSION OF BODIES. 281 
 
 be expected to have only a fourteenth of the capacity, has really for equal 
 volumes a half, or, as formerly stated, for equal weights, a thirtieth. 
 
 Third. We are at last, therefore -compelled to admit that the relation be- 
 tween various substances and heat) which we call capacity for heat, depends 
 much more on the nature of the ultimate particles of the substances than 
 either on the absolute bulk or comparative density of the masses. Throwing 
 much light on this subject, it has been ascertained in late times, that all 
 material substances are composed of extremely minute unchangeable atoms, 
 of which, in different substances, the comparative weights have been deter- 
 mined, although not the absolute weights ; that is to say, for example, the 
 atom of gold is known to weigh four times as much as the atom of iron, 
 although we do not know how many thousands or millions of atoms are 
 required to form a grain of either. Now very recent researches seem to 
 prove that for each ultimate atom, no matter of what substance, nearly the 
 same quantity of heat is required to produce in a mass of the atoms a given 
 change of temperature. Thus an ounce of iron, which has four times as 
 many atoms as an ounce of gold, has four times the capacity for heat. This 
 law seems to hold for all simple substances ; but for compounds there seems 
 to be another law not yet ascertained. 
 
 Instead of the term capacity for heat used in the preceding pages, with 
 respect to particular substances, that of specific heat has by some authors 
 been preferred ; but as the latter gives to a commencing student the idea 
 rather of kinds of heat than of quantities, the term capacity has been 
 retained. 
 
 ' ' Each substance in nature, for a given change of temperature, undergoes 
 expansion in a degree proper to itself, the expansion generally increasing 
 more rapidly than the temperature, as the cohesion of the particles be- 
 comes weaker from increased distance j being remcwkably greater, there- 
 fore, in liquids than in solids, and in airs than in liquids; the rate being 
 quickened, moreover, near the points of change." (See the Analysis, 
 page 256.) 
 
 The following table, containing the names of some common substances, 
 solid, liquid, and aeriform, shows, by the figures following each name, how 
 much the substance increases in bulk, by having its temperature raised from 
 that of freezing to that of boiling water. A lump of glass, for instance, 
 would gain one cubic inch for every 416 cubic inches contained in it; while 
 a mass of water would gain one inch for twenty-three, dilating thus for the 
 same range of temperature eighteen times more than glass. 
 
 Solids, 
 
 Glass gains one part in - 416 
 
 Deal 416 
 
 Steel .... - 283 
 
 Iron - 271 
 
 Brass 177 
 
 Silver - - - -' - - 375 
 
 Lead 117 
 
 Liquids. 
 
 Mercury gains one part in - 55 
 
 Water ... - 23 
 
 Fixed oils ----- 12 
 
 Alcohol ...... 9 
 
282 HEAT. 
 
 Airs. 
 
 Common air,") 
 
 all gases and I gain one part in - - 3 
 
 vapours J 
 
 We have to warn readers here not to confound the increase by heat of 
 the general bulk of a solid body with the increase of its length. The latter 
 is only one-third as great as the former. This will be understood by consi- 
 dering that the increase of bulk is divided between the length, breadth, and 
 depth (or thickness. If the substance of a metalic square rod or wire, be 
 dilated by heat a one-hundreth part of its bulk, it does not gain all that 
 hundreth at its end, becoming 101 inches (or other measure) long, instead 
 of 100 ; but every part becomes deeper and broader in the same proportion 
 as it becomes longer (we may suppose it divided into a row of equal little 
 cubes,) and the rod gains in length only the third part of an inch. A fluid 
 enclosed in a tube unchangeable by heat (if such tube there were) would 
 show its whole dilatation in an increase of length, because there could be no 
 swelling laterally, and' its extremity, therefore, from any variation of tempe- 
 rature, would have a triple extent of motion. A degree of this consequence 
 is obtained in our common thermometers, because the containing glass, al- 
 though dilatable by heat, is so much less dilatable than the fluid within. 
 As regards solids, we have to inspire so much more frequently respecting the 
 dilatation in length, breadth, &c., that is'to say, the linear dilatation, in one 
 direction, than the increase of general bulk, that tables are frequently made 
 stating only the linear dilatation. It may be found at once from the above 
 table, by recollecting that it is one-third of the increase of bulk: thus, as 
 glass, in passing from the freezing to boiling heat of water, dilates one part 
 in 416 of its bulk, it will dilate only one-third of a part in length, or a whole 
 part in an extent of three times 416 or 1,248. 
 
 The expansion of solids by heat has been ascertained by bringing micro- 
 scope instruments to bear on rods of the different substances heated to various 
 degrees, in troughs of oil or water. The expansion of fluids, again, is found 
 by filling a glass vessel with a known weight of any fluid, and then ascer- 
 taining how much is made to run over or escape by a given increase of heat ; 
 or how much the fluid rises into a long tubular neck like the stock of a ther- 
 mometer. This quantity, added to what is required to fill the increased di- 
 mensions of the heated glass vessel, (which from the ascertained expansion 
 of glass is known,) forms the whole of the increase 
 
 The general and comparative expansion of solids by heat is exemplified in 
 the following cases : 
 
 An iron bullet, when heated, cannot be made to enter an opening, through 
 which when cold it passes readily. 
 
 A glass stopper sticking in the neck of the bottle, often may be released 
 by surrounding the neck with a cloth taken out of warm water ; or by im- 
 mersing the bottle in the water up to the neck ; or by making strong friction 
 on the neck by a tape or any soft rope put around it, and then pulled back- 
 wards and forwards. By any one of these means the binding ring of the 
 neck is heated and expanded sooner than the stopper, and so becomes for a 
 short time slack or loose upon it. 
 
 Pipes of cast-iron for conveying hot water, steam, &c., if of considerable 
 length, must have joinings which allow a degree of shortening and 
 
EXPANSION OF BODIES. 283 
 
 lengthening, otherwise a change of temperature may destroy them. An in- 
 competent person undertook to warm a large manufactory by steam from one 
 boiler. He laid a rigid main pipe, along a passage, with lateral branches 
 passing through holes into the several apartments ; but on his first admitting 
 steam, the expansion of the main pipe tore it away from all its branches. 
 
 In an iron railing, a gate which during a cold day may be loose and easily 
 shut or opened, in a warm day may stick, owing to their being greater expan- 
 sion of it and of the neighbouring railing than of the earth on which they 
 are placed. Thus also the centre of the arch of an iron bridge is higher in 
 warm than in cold weather; while on the contrary, in a suspension or chain 
 bridge, the centre is lowered. 
 
 The iron pillars now so commonly used to support the front walls of those 
 houses of which the ground stories meant to serve as shops have spacious 
 windows, in warm weather really lift up the wall which rests upon them, 
 and in cold weather allow it again to sink or subside considerably more than 
 if the wall were brick from top to bottom. 
 
 In some situations (as lately was seen in the beautiful steeple of Bow- 
 church, in London) where the stones of a building are held together by 
 clamps or bars of iron driven into the stones, the expansion in summer of 
 these clamps will force the stones apart sufficiently for dust or sandy partt- 
 cles to lodge between them : and then, on the return of winter, the stones 
 not being at liberty to close as before, will cause the ends of the shortened 
 clamps to be drawn out, and the effect increasing with each revolving year, 
 the structure will at last be loosened and may fall. 
 
 The pitch of a piano-forte or harp is lowered in a warm day or in a warm 
 room, owing to the expansion of the strings being greater than of the wooden 
 frame-work j and in cold the reverse will happen. A harp or piano which 
 is well tuned in a morning drawing-room cannot be perfectly in tune when 
 the crowded evening party has heated the room. 
 
 Bell-wires too slack in summer, may be of the proper length in winter. 
 
 One admirable contrivance for keeping the pendulum of a clock always 
 of the same length, by making the greater expansion by heat of a middle 
 bar of brass counteract the smaller expansion of two side-rods of steel, was 
 explained under the head of " Pendulum," as was also the construction of a 
 balance-wheel having a corresponding property. A difference of a 100th of 
 an inch in the length of a common pendulum, causes a clock to err ten 
 seconds in twenty-four hours, and a rise or fall of 25 of Farenheit's ther- 
 mometer produces this difference. Another kind of compensation pendulum, 
 not less admirable, distinguished by the name of its inventor Graham, is 
 obtained by substituting for the solid bob or ball at the bottom, a glass ves- 
 sel containing mercury. The mercury on expanding by heat, has its centre 
 of gravity raised just enough to compensate for the lengthening of the rod 
 of the pendulum. 
 
 Crystals when heated, do not expand quite equally in breadth and in 
 length. The same is true of fibrous substances, as wood which expands 
 and contracts more in breadth than in length. This is instanced in the 
 leaking, during cold weather, of a ship's deck which, in warm weather, is 
 tight; an occurrence which the author, in rounding the Cape of Good 
 Hope, had to regret as the cause of destruction to some valuable specimens 
 of natural history which he had collected among the Eastern Islands. 
 
 Bodies expanded by heat, unless when their intimate composition is 
 changed by it, regain exactly their former dimensions on being cooled. 
 
284 HEAT. 
 
 As is seen in the preceding table, the expansion of liquids by heat is much 
 greater than of solids. 
 
 A cask quite filled with liquid in the winter, must, in summer, force its 
 plug or burst ; and a vessel which has been filled to the lip with warm liquid, 
 will not be full when the liquid has cooled. Hence a cunning dealer in 
 liquids has tried to make his chief purchases in very cold weather, and his 
 chief sales in warm weather. 
 
 There exists in the case of water, an extraordinary exception, already men- 
 tioned to the law of expansion by heat and contraction by cold, producing 
 unspeakable benefits in nature. Water contracts only down to the tempera- 
 ture of 40, while, from that to 82, which is its freezing point, it again 
 dilates. One curious consequence of this peculiarity is exhibited when a 
 pool or well happens to be formed on the upper surface of a mass of ice, as 
 on one of the glaciers of Switzerland and elsewhere, namely, that the well 
 goes on quickly deepening itself, until it penetrates to the earth beneath. 
 Supposing the surface of the water originally to have nearly the temperature 
 of the melting ice, or 32, but to be afterwards heated by the air and sun, 
 instead of the water being thereby dilated or rendered specifically lighter, 
 and detained at the surface, it becomes heavier the more nearly it is heated 
 to 40, and therefore sinks, down to the bottom of the pit or well; but there, 
 by dissolving some of the ice, and being consequently cooled, it is again 
 rendered lighter, and rises to be heated as before, again to descend ; and this 
 circulation and digging cease only when the water has bored its way quite 
 through. 
 
 Airs are expanded by heat still more than liquids. 
 
 The expansion of aeriform bodies by heat produces many important effects 
 in nature. Some of these have ah-eady been considered in the preceding 
 parts of this work, as the rising of heated air in the atmosphere, causing the 
 winds all fe over the earth; the same in our fires and chimneys supporting 
 combustion and ventilating and purifying our houses; the same again from 
 around animal bodies, removing the poisonous or contaminated air which 
 issues from the lungs, and insuring a constant supply of fresh air for the 
 support of life, &c. % 
 
 It is remarkable with respect to aeriform bodies, that, unlike solids and 
 liquids, they are all equally dilated by the same change of temperature, re- 
 ceiving an increase of about a third part of their bulk (37 parts in 100) on 
 being heated from the freezing to the boiling point of water, viz., 180, 
 their bulk being therefore doubled from the same standard point by about 
 500. This general truth holds, not only with respect to the more perma- 
 nent airs or gases, but also with respect to all steams or vapours in the dry 
 state, that is, when not in contact with the liquid producing them. The 
 probable reason of this uniformity is, that cohesive attraction which varies 
 so much in different solids and liquids, modifying the effects of heat upon 
 them in aeriform fluids does not exist at all. 
 
 The extent of this dilatation for airs is so much greater than for liquids 
 or solids, that it forces itself much more strikingly upon the common atten- 
 tion. Thus, a bladder containing considerably less than its fill of air, be- 
 comes tense immediately on being held to the fire. The air in a ballooa just 
 escaping from a cloud, has been so suddenly expanded by the direct rays of 
 the sun, as to injure the texture of the balloon; and probably some of the 
 fatal accidents among aeronauts have been owing to this occurrence. Burn- 
 ing fuel conveyed into a vessel or case which can be suddenly and strongly 
 
EXPANSION OF AIR. 285 
 
 closed, will produce an expansion of the air confined with it, capable of burst- 
 ing any vessel of ordinary strength in short, will produce an explosion. 
 
 Now, if not before, at any rate soon after steam-engines began to be used, 
 and had so strikingly shown to what important purposes the force of an 
 expanding aeriform fluid might be applied the thought would naturally 
 occurr that the force of com'mon air dilating by heat might also be rendered 
 useful. Accordingly a variety of air-expansion engines have been proposed, 
 but as yet no one has been reduced to profitable practice. Had the truth been 
 generally known, which very recent investigations have proved, that a given 
 quantity of heat, when used to dilate air, produces several times as mifch 
 expansive power as when used to form steam, the attempts to bring such an 
 application of heat under control would probably have been more numerous, 
 and possibly, by this time, more successful. The subject is so interesting 
 that we shall subjoin a few remarks upon it. 
 
 To produce a cubic foot of common steam from water originally cold, 
 about 1,160 degrees of heat are required, as will be explained a few pages 
 hence. The same quantity of heat would double the volume of about five 
 cubic feet of atmospheric air, as is known from the comparative capacities 
 for heat of the two substances, and the rate of dilatation of air when heated. 
 Now the value for work of the foot of steam passing from the boiler into a 
 working cylinder would be, to press up the piston of the steam-engine 
 through a foot, as from c d to a b, with a force all the way of 15 Ibs. per 
 inch of the piston surface ; while the working valve of the five feet of air in 
 dilating to double bulk, would be to lift up the piston five times as far as 
 the steam, viz., from g h to e f, but with a force 
 gradually diminishing (represented here by the Fig. 125. 
 
 shaded part of the figure) as the expansion went 
 on, from 15 Ibs per inch at the begining until the 
 air had dilated to its destined volume, when the 
 force would altogether cease , its whole effect, 
 therefore, would be five feet impulsion of the pis- 
 ton with a pressure averaging between 15 Ibs. and 
 nothing, viz., 7 Ibs. per inch ', and the friction in 
 the two cases and the varying intensity of the lat- 
 ter pressure being neglected, the force of the air 
 would be 2 times as great as that of the steam. 
 But it is farther to be considered, that only about 
 half the heat of a fire is applied to use in a steam- 
 engine, viz., that part which enters the boiler, 
 while the remainder passed up the chimney ; and 
 
 in an air-engine probably the whole might be applied. In an air-engine, 
 moreover, there might be a great increase of power from the combustion, or 
 semi-explosion of the inflammable gas evolved from the fuel. We see from 
 this of what importance the discovery would be of a means enabling us 
 effectually to apply the force of expanding air. 
 
 If we suppose a fire a to be placed on a grate near the bottom of a close 
 cylinder, d a, and the cylinder to be full of fresh air recently admitted, and 
 if we farther suppose the loose piston g d to be pulled upwards, it is evi- 
 dent that all the air in the cylinder above d will be made to pass by the 
 tube e through the fire, and will receive an increased elasticity tending to the 
 expansion or increase of volume, which the fire is capable of giving it. If 
 there were only the single close vessel da, the expansion might be so strong 
 as to burst it; but if another vessel b c of equal size were provided, com- 
 
286 
 
 HEAT. 
 
 Fig. 126. 
 
 d\ 
 
 Fig. 127 
 
 municating with the first through the passage b, 
 and containing a cZose-fitting piston e /, like that of 
 a steam-engine, the expansion of the air in the 
 first cylinder would act to lift the said piston, and 
 so might work water-pumps, or do any other ser- 
 vice which a steam-engine can perform, At the 
 end of the lifting stroke of the piston fc, it might 
 be made to open an escape- valve for the hot air, 
 placed in any convenient part of the apparatus, 
 and to cause the descent of the blowing piston d 
 to expel that air, while a new supply of fresh air 
 would enter by another valve into the cylinder 
 above d. The engine would then be ready to 
 repeat its stroke as before, and the working 
 would be continued as in a steam-engine. 
 
 The preceding simple conception of an air-engine occurred to the author's 
 thoughts while considering the application of a condensed air-furnace to some 
 chemical purposes. It appeared to him that, in applying any such engine 
 to use, the chief difficulties to be surmounted would be, to prevent the very 
 heated air and dust from injuring the valves and other working parts of the 
 engine, and to obviate the inconvenience of the inequality of power at differ- 
 ent parts of the stroke. Various expedients occurred to him. The over- 
 heating might be prevented by surrounding the cylinder, 
 &c., with water; and both cylinder and piston would 
 suffer less from dust, if, instead of the common piston c. 
 represented above, a great hollow plunger a were used, 
 (such as is here represented, and is now common in 
 water-pumps for mines) embraced by an air-tight neck or 
 collar at b c, which neck would be the only part of the 
 cylinder requiring to be made with nicety. But a more 
 complete security would be obtained by interposing water 
 between the hot air and the piston, as* represented in this 
 other sketch, where the working cylinder d has a water- 
 vessel b connected with it, and the heated air is admitted 
 to b to press upon a float on the water-surface, to lift the 
 working piston d e. This construction, too, if desired, 
 would, allow the fire chamber a to be made larger than 
 the cylinder, and to be kept constantly filled with highly 
 expansive air, each discharge of which into the space b would be replaced by 
 
 cold air either from the space 
 above the piston d, driven in 
 through a tube as the piston 
 ascended, or from a distinct 
 blowing cylinder worked by 
 the beam. And if it were 
 wished to apply the same 
 principle to an engine work- 
 ing with double strokes, that 
 is, forcing the piston alter- 
 nately up and down, as in the 
 double stroke steam-engine, 
 the object might be attained, 
 by having a second water-ves- 
 sel f communicating with the 
 
 Fig. 128. 
 
EXPANSION OF AIR. 
 
 287 
 
 Fig. 129. 
 
 part of the working cylinder above the piston d; and the air would pass 
 alternately to the one or the other vessel b or /, by the operation of the cock 
 c, as steam passes in a steam-engine ;, the supply of fresh air to the chamber 
 a would be given by a blowing cylinder worked through a connection with 
 the engine, as the air-pump of a steam-engine is worked: 
 
 The sketch of an air-engine, as here given, was include^ in the specifica- 
 tion of a patent for another object engaged some years ago by a friend of 
 the author's ; but that friend being almost immediately called to other busi- 
 ness, and the author's professional engagements forbidding his attention to the 
 subject, it was not prosecuted. In the specification, drawn up by an engi- 
 neer in the town, some minor adaptations were described. One experiment 
 has lately been made by a Swedish engineer with the simple form of dry 
 apparatus described at page 285, for the purpose of ascertaining its power, 
 and the effect was found to be several fold greater than of steam from the 
 same quantity of fuel ; but the apparatus was rude, and only calculated to 
 prove in a short trial, the existence of the power, but not the fitness of the 
 machine to endure uninjured, or to be rendered easily obedient to control; a 
 complete experiment, therefore, remains still to be made. Could an obedi- 
 ent and durable engine be contrived, at all approaching in simplicity to the 
 plan given above, its advantages over the steam-engine would be very con- 
 siderable. First, its original cost would be much less, by reason of its small 
 comparative size, its simplicity, and the little nicety of workmanship re- 
 quired. Secondly, it would require much less room, and would be very 
 light; hence its peculiar fitness for purposes of propelling ships and wheel- 
 carriages. Thirdly) the quantity of fuel required being 
 so much less, would not load the ship or carriage leav- 
 ing little room for anything else. Fourthly, the ex- 
 pense of fuel and repairing would be little. Fifthly, 
 the engine could be set to work in a few minutes, 
 where a steam-engine might require hours. Sixthly, 
 little or no water would be required for it. 
 
 Another modification of air-engine, called a gas va- 
 cuum engine, has lately been proposed, and many 
 expensive trials have been made of it; but it is in its 
 nature a most wasteful machine, evidently throwing 
 away at least nine-tenths of the power which the 
 combustion generates, It was of this nature in an ex- 
 periment which the author witnessed. A little of the 
 common coal-gas was admitted by the cock b at the 
 bottom of the cylinder a, and was there inflamed, the 
 lid e being at the time raised. The combustion rari- 
 fied the lower stratum of air, so that the air above was 
 expelled, and about one-fifth of the original contents 
 of the cylinder was caused to occupy the whole". The 
 lid was shut down, as nearly as could be judged, at 
 the moment of greatest expansion, so that when the 
 small portion of air and vapour remaining within was 
 again cooled, the interior of the cylinder approached 
 nearly to the state of vacuum. It, in fact, retained 
 only a fifth of the air. A communication being then 
 opened from the vacuous cylinder by the tube e to a 
 water reservoir ten feet below, the water was driven 
 up by the atmospheric pressure, until it filled more 
 
288 
 
 HEAT. 
 
 than half of the cylinder. The water so raised was then made to turn a 
 common water-wheel, and to do work. A much larger quantity of water, 
 however, could be raised to the same height at less expense by a steam- 
 engine. The proposer also hoped that he would be able to make the atmos- 
 phere pressing into its imperfect vacuum, act directly upon a piston as steam 
 does, and with power cheaper than that of steam ; but in this anticipation 
 too he was completely in error. To produce his imperfect vacuum cost him 
 very nearly at the same rate as it costs to produce the perfect vacuum in 
 a steam-engine, and his vacuum for equal bulks was worth 
 as a working power, only about one-fourth as much as the 
 steam vacuum. This may be understood by considering, 
 that in a perfect vacuum a piston rises all the way with the 
 same force, which if common steam be used, is 15 Ibs. 
 per inch, the piston may be supposed to rise from c d to 
 a b, but if the vacuum were only three-fourths towards 
 being complete, the pressure on the piston would be only 
 three-fourths of 15 Ibs. at the commencement of the stroke, 
 and then rapidly diminishing, would have ceased altogether 
 when the piston had made three-quarters of its journey, or 
 to/. The force in the first case would be represented by 
 the whole line c d and the whole space c d b a, and in the second by the 
 shortening lines and the triangular space c ef. 
 
 On considering the foregoing diagrams, we may perceive that in the vacuum 
 engine, by far the greater part of the force produced by the combustion of the 
 gas is absolutely wasted, or put to no use, namely, the whole expansive force 
 during the sudden combustion or explosion. It is evident that if a tenth 
 part of the aeriform contents of a cylinder acquire elasticity enough, (a four- 
 teenth part of a nice experiment does so) to be able afterwards to occupy the 
 whole cylinder, that tenth must begin its expansion with the force of a ten- 
 fold atmospheric condensation, or pressure of 150 Ibs. on the square .inch of 
 a piston withstanding it, which pressure will then gradually diminish as the 
 piston rises, but will amount. to an average of five times the atmospheric 
 pressure, or 75 Ibs. per inch all the way; being therefore quadruple, or more, 
 that of steam against a perfect vacuum, and, therefore, again, by our former 
 calculation, more than twelve times greater than the force obtained from the 
 imperfect vacuum of the engine under consideration. 
 
 It is a question which the author thinks will one day be answered in the 
 affirmative, whether nearly the whole force of exploding gas may not be con- 
 verted into a calmly working power, producing from a 
 Fig. 131. given expenditure, ten times or more the effect obtained 
 
 in the vacuum-engine described above, and, therefore, an 
 effect more than equal to that of a steam-engine incur- 
 ring the same expense. There are probably various ways 
 in which the object may be obtained. The following 
 sketch is offered merely to give the reader an idea of a 
 machine for such a purpose. 
 
 Suppose b to be a very heavy close-fitting piston sliding 
 in the cylinder containing it; and suppose the space d 
 \ open to the cylinder, to be filled with atmospheric air of 
 
 double or greater density; then if a mixture of explosive 
 Lil. ' gases admitted by a cock to the chamber a (formed be- 
 lle tween the piston and end of the cylinder) be inflamed, 
 the heavy piston will be shot forward Lke a cannon-ball, 
 
EXPANSION OF AIR. 289 
 
 against the condensed air in d ', and owing to the momentum acquired in the 
 first instance, it will advance much beyond the point where the exploded gas 
 and air in d would balance each other at rest. The quantity of gases admitted 
 would be just such as to carry it to the end of the cylinder. The piston rod 
 e would then by a catch or racket, be connected with the work to be done, 
 and after the condensation of the exploded gases in a cylinder, would be 
 pressed back again, with the greater than atmospheric force in d, as if urged 
 by high pressure steam. Figure 127 at page 286 represents a form of cylin- 
 der which might also answer for this purpose, the heavy plunger being 
 thrown up, to work by its weight in descending. 
 
 It is to be remarked that the first modification of air-engine described at 
 page 285, is partly an explosive engine such as contemplated above, for the 
 gas separated from the coal during the moment of slackened combustion while 
 the lately used air is passing out, becomes an explosive accumulation for the 
 fresh air about to enter, The trial alluded to above proved this to be the fact. 
 
 " The expansion of bodies by heat increases more rapidly than the tempera- 
 ture, and particularly near the melting and boiling points, that is, their 
 points of changing into liquid or air being, however, exactly proportioned 
 to the temperature after the change into air. (See Analysis, page 256.) 
 
 If a given quantity of heat, that, for instance, contained in some measure of 
 boiling water or of common steam, be added to a mass of cool water, it will 
 produce in this a certain increment of bulk ; and if other equal quantities of 
 heat be afterwards successively added under the nice management which 
 such an experiment requires, each new addition will produce a greater incre- 
 ment of bulk than the preceding, particularly when the water approaches to 
 boiling ; but after the water is converted into steam, any farther increase of 
 bulk will be exactly proportioned to the increase of temperature. The same 
 truths may be proved by the converse experiment of abstracting successively 
 equal quantities of heat from steam or water (as by making it melt equal 
 quantities of ice,) and noting the rate of contraction. What is thus true of 
 water in relation to heat, is true also of bodies generally, each, however, 
 having a rate of expansion and temperature for melting and boiling proper 
 to itself. The quickened rate of expansion in solids and liquids might have 
 been anticipated from reflecting, that each successive quantity of heat added 
 to a mass, meets with less resistance to its expanding power than the pre- 
 ceding quantity, owing to the diminishing force of cohesion of the particles 
 as the mass enlarges ; while in an air or gas, again, as cohesion has alto- 
 gether ceased, each addition of heat is at liberty to produce its full and equal 
 effect. If the capacity of substances for heat did not increase with their 
 bulk, the terms "increase of heat' 7 and "increase of temperature" would 
 have the same meaning, and the subject would be more simple. 
 
 The reflection will naturally occur here, that as in the common thermo- 
 meter, the mercury must rise or expand more for a given quantity of heat 
 added at a high than at a low temperature, the scale should be divided to 
 correspond with the inequality. Now this reasoning is good, but the diffi- 
 culty of complying with it in practice is such, that the inconvenience of the 
 slight error arising from an equal division is commonly submitted to. An 
 air thermometer with equal divisions is very correct, but from wanting many 
 of the advantages of the mercurial thermometer, is little employed ; and 
 fortunately in the mecurial thermometer there is such a counterbalancing 
 relation between the expansion of the mercury and of the containing glass, 
 
 19 
 
290 HEAT. 
 
 as to render the error alluded to, at least for any middle range of temperature, 
 very trifling. The subject of unequal thermometric dilatation in the same 
 liquid, and of the differences in that respect in different liquids, depending 
 on the proximity to their boiling points, &c., is well illustrated by Du Luc's 
 experiment of filling various thermometer-glasses with different liquids, and 
 while they are being heated through the same range of^temperature, noting 
 their comparative indications. He marked on each tube the points at which 
 the liquid in it stood when the bulb was placed, first in freezing and after- 
 wards in boiling water, and he then divided the intervening space into eighty 
 parts or degrees. The discordance of the dilatations in the different tubes 
 when the instruments were afterwards placed together, and heated from the 
 freezing to the boiling degrees of water, was as here detailed. 
 
 Mercury. Olive Oil. Alcohol. Water. 
 
 - - - 
 
 10 9.5 - 7.9 - - 0.2 
 
 20 - - 19.3 - - 16.5 - - 4.1 
 
 30 - - 29.3 - - 25.6 - - 11.2 
 
 40 - - 39.2 - - 35.1 - - 20.5 
 
 50 - - 49.2 - - 45.3 - - 32 
 
 60 - - 59.3 - - 56.2 - - 45.8 
 
 70 - - 69.4 - - 678 - - 62 
 
 80 - - 80 - 80 80 
 
 The singular discrepancy in the case of water is owing to the peculiarity 
 described in former pages, of its contracting by cold only down to 40 of 
 Fahrenheit, and then dilating again until it freezes. 
 
 Laborious investigations have been made by the French chemists to dis- 
 cover a comprehensive law determining the rate of expansion in all bodies, 
 but the object is not yet satisfactorily accomplished. 
 
 " To melt a solid body, or to vaporize a liquid, a large quantity of heat 
 enters it, but in the new arrangement of the particles and generally in- 
 creased volume of the mass, the heat becomes hidden from the thermometer 
 and is called LATENT HEAT. It reappears during the contrary changes, 
 after ichatever interval." (See the Analysis, page 256.) 
 
 The expansion of bodies by heat, instead of proceeding throughout in 
 some nearly uniform or gradual manner, exhibits in its course two singular 
 transformations of the body : the first, when the solid breaks down into a 
 liquid ; the second, when the liquid swells out into an air or gas ; so that 
 there are, in all, three very distinct modifications or states of existence for 
 the body dependent on the agency of heat. The substance of water, for 
 instance when at a low temperature, exists in the solid form called ice ; but 
 at 32 of Fahrenheit, on receiving more heat it gradually becomes liquid or 
 water, and on receiving more at 215, even under the resisting pressure of 
 the atmosphere, it acquires a bulk nearly 2,000 times greater than it had as 
 a liquid, (gradually as regards the whole, but suddenly as regards each sepa- 
 rate portion,) being then called steam or aeriform water. And other bodies 
 under analogous circumstances, undergo similar changes. It is farther 
 remarkable, that although during the changes a large quantity of heat enters 
 the mass, producing in one case liquidity, in the other the form of air, the 
 temperature is the very same, immediately after, as immediately before the 
 
LATENT-HEAT. 291 
 
 change, the last received heat becoming hidden or latent in the mass : thus 
 water running from melting ice affects the thermometer but as the ice does, 
 and steam over boiling water appears no hotter than the water. The glory of 
 originally discovering the facts, to recall which the terms latent heat 
 or caloric of fluidity, have since been used, belongs to the illustrious Dr. 
 Black. The construction of the modern steam-engine was an early result of 
 kindred investigations made by his friend, James Watt. 
 
 We select the following instances as serving to display the subject of latent 
 heat in its various bearings. 
 
 A mass of ice brought into a warm room, and there receiving heat frojji 
 every object around it, will soon reach the temperature of melting or 32 
 but afterwards both the ice and the water formed from it will continue at 
 that temperature until all be melted ; the heat which continues to enter, 
 effecting a change only in the form of the mass. And in the case supposed, 
 whatever time was required for heating the mass of ice one degree, just one 
 hundred and forty times as much will be required for melting it ; proving 
 that 140 is the latent heat of water. 
 
 If two similar flasks, one filled with ice at 32, and the other with water 
 at 32 be placed in the same oven or over the same flame, the water will 
 gain 140 degrees of heat while the ice is nearly being melted into water at 
 32 : and in the course of the experiment, .a correspondence will always exist 
 between the phenomena; for instance, when the water has gained 14 of 
 heat, it will be found that just a tenth part of the ice is melted. 
 
 If equal quantities of hot. and cold water- be mixed together, the whole 
 acquires a middle temperature, each degree lost by the hot water becoming 
 a degree gained by the cold ; but if a pound of ice at 2, and a pound of 
 water 140 hotter be mixed together, the 140 of heat will go merely to melt 
 the ice, for there will result two pounds of water at 32. 
 
 If a flask of water at 32, or its freezing point, and a similar flask of strong 
 brine (which does not freeze until cooled to near zero) also at 32, be 
 exposed together in the same cold place, it will be found that when the brine 
 has lost 10 of its heat the water flask will still exhibit an undiminished 
 temperature, but a fourteenth part of its contents will be converted into ice. 
 Now as in such a case the water flask must continue to radiate away heat just 
 as much as the other, it can maintain its temperature by absorbing into its 
 general mass the heat which was latent in the portion of water frozen. 
 
 It is possible, by slowly cooling water which is kept in perfect repose, to 
 lower its temperature, while yet liquid, ten degrees below its ordinary freez- 
 ing point; but then on the slightest agitation, ice will be formed. It might 
 be expected, in such a case, that the whole water will instantly freeze, 
 because all is colder than common ice ; but no, only a fourteenth part freezes ; 
 and singularly, both that fourteenth and the remaining liquid are rendered 
 in the moment ten degrees warmer rising to 32. Here the 140 of latent 
 heat escaping from the fourteenth part of the water which freezes, become 
 10 of sensible heat for the whole mass, so that the remaining water has the 
 temperature at which it only begins to freeze. 
 
 Strong solutions in hot water of various neutral salts, if allowed to cool 
 while exposed to atmospheric pressure, soon deposit crystals of the salts ; but 
 in a close vessel, which protects them from such pressure, they will remain 
 liquid even when cold. Now at the moment of opening such a vessel to 
 admit the pressure, the salt immediately crystallizes, and the latent heat 
 
292 HEAT. 
 
 given out by the solidifying particles warms very sensibly the remaining 
 liquid and the vessel. 
 
 From the preceding facts it may be perceived, that the quantity of ice 
 formed or melted in any case, becomes a correct measure of the quantity of 
 heat transferred. From this consideration, the illustrious Lavoisier con- 
 structed his calorimeter, or heat measure. It is a case or Vessel lined with 
 ice, and the quantity of heat given out by any body placed in it is indicated 
 by the quantity of water collected from the melted ice. 
 
 Had the latent heat of water been only 1 or 2 instead of 140, the earth, 
 except in its tropical regions, would have been scarcely habitable. The cold 
 of a single night might have frozen an ocean, and the heat of a single day 
 ijight have converted the accumulated snows of a winter into one sudden 
 and frightful inundation. As the fact is, however, both changes are beauti- 
 fully gradual, and easily controlled or prepared for. 
 
 The fact of latent heat in other liquids than water is familiarly exhibited 
 in the slow melting of various substances as of the metals j lead or pig-iron 
 for instance of butter or oils of glass, &c. ; and on the other hand, in the 
 slow solidification of any melted masses when heat is again abstracted. 
 
 The substances below enumerated, while passing from the solid to the 
 liquid state, absorb and render latent the quantities of heat here noted ; 
 which quantities are therefore called the latent heats of the liquids. 
 
 Ice - ... 140 
 
 Mercury - - 142 
 
 Bees' wax - - 170 
 
 Tin - - 442 
 
 Zinc - - - - 492 
 
 If a piece of frozen mercury (the temperature of which is at least 40 
 below zero) be thrown into a little water at 32, the latent heat of the water 
 immediately passes into the mercury and melts it ; but, singularly, the water 
 in the act of melting the mercury, is itself frozen. 
 
 " Latent Heat of aeriform fluids" 
 
 Water in a vessel placed over a fire gradually attains the boiling temper- 
 ature or 212, but afterwards its temperature rises no more, for the farther 
 addition of heat becomes latent in the steam escaping during the ebulition. 
 One way of determining the quantity of heat which becomes latent in steam 
 is to note how much more time is required for boiling a quantity of water to 
 dryness, than for merely heating it to the boiling point, or through any cer- 
 tain number of degrees. The experiment indicates about 1,000 ; that is to 
 say, 1,000 times as much heat is latent in any quantity of water formed 
 into steam, as would raise the temperatnre of the liquid water one degree. 
 Watt had found that water in a vessel placed over a lamp was about six times 
 as long in being completely evaporated, as in being originally heated from 
 an ordinary temperature to that of boiling. 
 
 If we place in the same oven, or over similar flames, two like vessels con- 
 taining water, one of which is open at the end and the other is strongly closed, 
 the two will gain heat equally up to the boiling point, but afterwards the 
 open vessel from giving out steam will remain at the same temperature, while 
 the other, by confining the heat which enters, will show the temperature 
 continuing to rise as before, until the increasing tendency of the water to 
 
LATENT HEAT. 293 
 
 dilate forces the vessel open. Supposing the water in the latter vessel, be- 
 fore vent is given, to have become 100 hotter than common boiling water, 
 instead of the whole, when at liberty, being immediately converted into 
 steam, as might be expected, only a tenth part will be so changed (the same 
 quantity as will be found to have already escaped from the other vessel,) for 
 the tenth part requiring in the form of steam 1000 of latent heat, will take 
 the excess of 100 from the other nine parts, and will leave them as common 
 boiling water. If, however, water heated considerably beyond the boiling 
 point be allowed to expand very suddenly, the whole is blown out of the 
 vessel as a mist, by the steam formed at the same instant through every part 
 of the mass; but the whole mass in such a case is* no more converted into 
 true steam than the whole of very brisk soda-water is converted into gas 
 when similarly thrown out by the sudden extrication of the carbonic acid 
 gas, on uncorking the bottle. Misconception of this matter has led to most 
 wasteful experiments on steam-engines of very high pressure. Mr. Perkins, 
 for instance, thought he truly described what was accomplished by saying of 
 the water that it had " flashed into steam." 
 
 The same indication of the latent heat of steam is obtained by the converse 
 experiment of first converting a quantity of water into steam, and then admit- 
 ting it to cold water or to ice. A pound of steam will raise the temperature 
 of ten pounds of cold water 100 degrees, or will melt about 8 pounds of ice. 
 
 In the great quantity of heat which becomes latent in steam, we perceive 
 the reason why water projected upon a raging fire so powerfully represses it, 
 and hence, again, why^re and water are so often adduced proverbially as 
 exemplifying a fierce antagonism. t 
 
 It was when Watt had discovered how much heat was lost when steam 
 was lost, that he contrived the separate condenser for his steam-engine, by 
 which he at once saved three-fourths of the fuel formerly used 
 
 Substances diifer among themselves in regard to the latent heat of their 
 vapours as much as in their other relations to heat. Thus the latent heat 
 of the vapor or steam of 
 
 Water . is 1,000 
 
 Vinegar 900 
 
 Alcohol ....... 442 
 
 Ether 300 
 
 Oil of turpentine 177 
 
 From the less latent heat in these last-mentioned vapours than in that of 
 water, we might at first suppose that there would be great advantage from 
 using them in steam-engines. Accordingly numerous experiments have been 
 made, and patents secured under this idea ; but the fact is, that in the same 
 proportion as the heat is less, the volume of the vapour is less, and therefore 
 no mechanical advantage is obtainable. 
 
 The influence of external pressure in keeping the particles of liquids together, 
 in opposition to the repulsion of heat seeking to render their mass aeriform, 
 was considered in the chapter on " Pneumatics ;" but to make the present 
 section complete, the subject must be shortly resumed. 
 
 Because water or any liquid, under the pressure of the atmosphere, while 
 receiving heat, remains tranquil, and apparently unchanged, until it reaches 
 what is called its boiling point, at which a bubbling or conversion into vapour 
 takes place, we might suppose its ordinary boiling temperature necessary to 
 
291 
 
 HEAT. 
 
 Fig. 132. 
 
 enable it under any circumstances, to assume or to maintain the form of air. 
 33ut this is no more true than that a common spring compressed against any 
 obstacle or force, has no tendency to expand or recover itself till the moment 
 when at last it overcomes the obstacle. Liquid water with its heat is really 
 a spring compressed by the powerful weight of the atmosphere, and seeking 
 to expand itself into steam with force proportionate to its temperature. Even 
 at 32, or its freezing point, as. is found by placing it in a vacuum, it seeks 
 to assume the form of air, with a force of pressure 1 J ounce on each square 
 inch of its surface, and can be restrained only by a counter-pressure of that 
 amount; and at any higher temperature, to correspond with the greater 
 dilating tendency, the restraining force must also be greater: at 100, for 
 instance, it must be 13 ounces; at 150, 14 Ibs.; at212,151bs.; at 250, 30 
 Ibs., and so on as stated in the preceding part of this work : and whenever 
 the restraining force is much weaker than the expansive tendency the forma- 
 tion of steam takes place rapidly and far below the surface of the liquid, so 
 as to produce the bubbling and agitation called boiling. Now it is because 
 the atmosphere or ocean of air which surrounds the earth happens to have in 
 it 15 Ibs. weight of air over every square inch of the earth's surface, and 
 presses on all things there accordingly, that 212 happens to be called the 
 boiling point of water. An atmosphere less heavy 
 would have allowed liquid to burst into vapour at 
 lower temperatures, and one more heavy would 
 have had a contrary effect. The exact degree of 
 expansive force for every degree of temperature in 
 water and other liquids, has been ascertained by 
 heating them in vessels furnished either with pro- 
 perly loaded valves, as at /in this figure, or with a 
 tall upright tube, as d 6, into which the liquid c may 
 force a column of mercury to an elevation marking 
 the expansive tendency ; the valve and mercury 
 being, of course, protected from the external atmo- 
 spheric pressure, or the necessary allowance being 
 made for that pressure. Boiling at the bottom of a 
 deep vessel is resisted by the weight of the liquid 
 in addition to that of the atmosphere, as already ex- 
 plained, and consequently the temperature at which 
 it occurs there, is higher than near the surface of the 
 vessel. Boiling heat is greater, also, in a deep 
 mine, where of course there is additional depth and 
 weight of atmosphere over any exposed liquid, 
 
 at times when the barometer is unusually high, that is to say, when the 
 atmosphere is unusually heavy in cases where air or steam is confined over 
 the boiling surface so as to press more upon it, as when brewers for a time 
 shut the lid or valve of their great boilers, &c. Water placed on the fire in 
 a strong vessel, from which steam cannot at all escape, may be rendered even 
 red-hot, without a bubble forming or one particle being dissipated ; but the 
 tendency to expand into steam is then great enough to burst any known 
 material of moderate thickness. The Marquis of Worcester exploded a 
 cannon by shutting up water in it and then surrounding it with fire. Boiling 
 temperature is lower again when the experiment is made on mountains or in 
 other situations above the level of the sea, where there is less height of air 
 resting over the boiler. In the city of Mexico, which is 7,000 feet above the 
 sea, water boils before it reaches the heat of 200, instead of, as in places 
 
BOILING. 295 
 
 near the sea-level, at 212. Wollaston's thermometer, beautifully adapted 
 for determining the height of mountains, balloon ascents, &c., by merely 
 indicating the heat of boiling water in any situation, is a fine illustration of 
 this truth. If in any place we take off the atmospheric pressure from a liquid, 
 as by placing it in the receiver of an air-pump, it will boil at very low tempera- 
 tures indeed. Water thus treated boils at a temperature many degrees 
 below the heat of English summer days ; and ether boils when colder than 
 common ice. Generally, in a vacuum, substances boil at a temperature 
 124 lower than while restrained by the atmospheric pressure. 
 
 Consequences of these truths respecting the boiling temperature are the 
 following. 
 
 As water at any temperature is tending to dilate itself into steam, with 
 force proportioned to the temperature, the steam rising from any mass of 
 water presses on the surface of the vessel containing it with that force; and 
 in a steam-engine, therefore, the temperature of the water in the boiler tells 
 the degree of force with which the steam is acting on the piston. 
 
 Because in the case of steam the same law holds as for aeriform fluids 
 generally, viz., that the outward elasticity or spring increases in proportion 
 as the fluid is more condensed high pressure steam is merely condensed 
 steam, just as high-pressure air is condensed air; and to obtain a double 
 or triple pressure, we must have twice or thrice the quantity of steam under 
 the same volume. 
 
 The reason that high pressure steam issuing from a boiler heated to 300 
 or more is not hotter than low-pressure steam from a boiler at 212, is,-that 
 in the instant when the high-pressure or condensed steam escapes into the 
 air, it expands until balanced by the pressure of the atmosphere, that is, until 
 it becomes low-pressure steam, and it is cooled by the expansion, as air is 
 cooled on escaping from any condensation. 
 
 The vessel called Papin's Digester, is merely a metallic pot or boiler, 
 which can be kept closed in spite of the force of the steam formed within 
 it ; and in such a vessel, water can be heated far beyond the ordinary boiling 
 point, sufficiently for instance, to dissolve and extract all the gluten or 
 jelly of bones, and to form from them a rich soup where common boiling 
 would procure nothing .-or even to melt lead lying in water. 
 
 The person who increases the fire under a boiling pot with the hope of 
 making the water hotter, is foolishly wasting the fuel, for the water can only 
 boil, and it does boil at 212 of the thermometer. 
 
 As different substances under any given pressure, become aeriform at dif- 
 ferent temperatures, mixtures of such may be decomposed by heat. If a 
 mixture of spirit and water, for instance, be placed over a fire, the spirit will 
 boil off long before the water. If the spirituous vapour be caught apart and 
 condensed, the operation is called distillation. All distillations are of the 
 same nature. 
 
 The instrument here represented consists of a glass tube blown into bulbs 
 at the two ends a and 6, and hermetically sealed 
 after receiving into it some water, but no air. Fig. 133. 
 
 There will always be in the apparently empty 
 
 part a stream or aeriform water of density pro- J--5tf & 
 
 portioned to the temperature. If one of the bulbs C S^ 
 be heated more than the other, the steam or va- ^ < 
 pour in that one will, for the reasons stated above 
 be denser and stronger than in the other, and will 
 
296 HEAT. 
 
 therefore be forcing its way into the other; where, owing to the lower tem- 
 perature, a part of it will be relapsing into the state 6f water, making room 
 for more. Hence, if the difference of temperature between the bulbs be long 
 maintained, the whole water will, by a sort of distillation, gradually pass 
 into the colder bulb. If the difference of temperature become at any time 
 considerable, the liquid will boil in the warmer bulb, even although the 
 source of heat be only the living hand grasping it. 
 
 To the author of this work it appears that by a larger apparatus made on 
 this principle, fresh water might be conveniently obtained from salt water on 
 board ship, or on islands deficient in fresh springs. Suppose any two 
 air-tight vessels like a and b, of large size, communicating by a tube furnished 
 with a stop-cock near b : then if the vessel a were filled with salt water, and 
 were heated by being exposed to the sun, (its surface being blackened to ab- 
 sorb heat, and protected by glass from the cooling effect of the air,) and if 
 the other vessel b were made a vacuum by pumping out from its bottom the 
 water with which it had been previously filled, and were then kept as cold 
 as possible by wetted coverings and a current of air, on opening the cock 
 at 6, vapour would pass over from the warmer vessel to be condensed in the 
 colder, and there would be a distillation from sea-water by the natural action 
 of the sun alone, of a water naturally fresh and pure. Cases have .occurred 
 where a knowledge of this fact would have saved shipwrecked crews from 
 perishing by thirst; and there are rocky islands in the ocean where there is 
 no supply of fresh water but from precarious rains or importation from abroad, 
 but which might be rendered pleasantly habitable by the adoption of such a 
 means. 
 
 When a substance has reached the temperature at which it boils, that is to 
 say, at which its power of emitting vapour becomes rather more than a balance 
 to the" atmospheric pressure, its dilating force is strong indeed. Persons may 
 not reflect that 1 5 Ibs. on a square inch is about a ton on a square foot. 
 and such is the power with which the vapour of all boiling substances rises 
 from them sufficient in a single Cornish steam-engine to urge the piston 
 with the force of 600 horses ! But even at temperature much below boil- 
 ing, the tendency to expand, as already stated, is still very great, and although 
 not attracting common attention, is silently working many beautiful and im- 
 portant ends in the economy of nature. As in a perfect vacuum, freezing 
 water gives out a steam or vapour that would lift an opposing weight with 
 force of 1 J ounce per inch, or 16 Ibs. on a square foot: and even solid ice 
 gives out its vapour of nearly equal strength, so also do many other liquids 
 and solids give out their vapours. Thus in the apparently empty space called 
 the Terricellian vacuum, over the mercury in a barometer tube, there is 
 always an aeriform mercury, dense in proportion to the temperature ; and 
 around camphor, and the essential or volatile oils, &c v there is similarly an 
 atmosphere of the substance in the form of air. 
 
 It had for a considerable time been known that into a perfect vacuum 
 many bodies emitted almost instantly in the form of air, a quantity of their 
 substances proportioned to their temperature ; but it was reserved for Mr. 
 Dalton to make the admirable discovery, that even into any space filled with 
 air, these vapours arise in quantity and density the same as if air were not pre- 
 sent the two fluids seeming to be independent of each other, with the ex- 
 ception that in a vacuum the equal diffusion of vapour takes place at once, 
 while in a situation already occupied by air, it proceeds more slowly as the 
 vapour can force its way through the particles of the air, and in general takes 
 place by a tranquil evaporation from the surf ice instead of the agitation of 
 
EVAPORATION. 297 
 
 ebullition. In an apartment with an open vessel of water in it, there is soon, 
 although invisible, a steam of watery vapour mingled with the air ; as dense 
 as if the room were a vacuum at the same temperature. 
 
 Consequences of this important truth are the following : 
 
 That it is only an atmosphere of the substance of each body, which, by 
 pressing on the body, can prevent its farther dissipation by heat. Thus we 
 can save camphor, musk, smelling oils, spirits, water, &c., only by placing 
 them in closed bottles or vessels, in which, additionally to the air present, an 
 atmosphere of their own substance is formed, involving the remaining masses 
 with pressure proportioned to their temperature and its density. 
 
 The important process of drying things is merely the placing them under 
 an elevated temperature if attainable, and in an atmosphere not containing so 
 much of the liquid as to be saturated, at the temperature. The effect of 
 wind or motion of the air in quickening evaporation, is owing to its removing 
 air saturated with the moisture, and substituting air which is not thus 
 producing nearly the case of the substance placed in a vacuum. 
 
 If air at a certain temperature, contain mixed with it as much water as can 
 be sustained in the form of invisible vapour at that temperature, and if then, 
 by any cause, as by rising in the atmosphere, the air be cooled, it will abstract 
 heat from the vapour, and cause a portion to be precipitated or visibly con- 
 densed into a fog or rain. Water rising as invisible vapour from the surface 
 of a lake or river, often, when it has reached a certain height, is condensed 
 into the stratum of clouds which there appears, and which for a time may 
 remain usefully protecting the fields from the intense meridian sun, or may 
 fall again as refreshing showers over the country. 
 
 It is the tranquil and invisible evaporation of which we are now speaking, 
 which lifts from the surface of the wide ocean all the water which, after 
 condensation, returns to the ocean in the form of the myriads of river streams 
 which give life and beauty to the face of nature. 
 
 In warm climates there are inlets of the sea, occasionally shut off from the 
 parent ocean, and where, after the sun's rays have drunk up all the water, 
 the deposited salt remains to be carried away in loads for the uses of man, as 
 sand is carried away from any ordinary shore. There are in the bowels of the 
 earth prodigious accumulations of salt, some of which may have been formed 
 in this way, during the revolutions of the world in remote past time, and 
 which are now turned to man's account as salt-mines. When the Nile over- 
 flows its banks with water holding in solution, although in almost imper- 
 ceptible proportion, mineral substances brought from the interior of Africa, 
 some of that water admitted into reservoirs, and afterwards dried up by the 
 sun's heat, leaves a rich store chiefly of chrystallized natron or soda. 
 
 The following are other instances of vapour which is invisible while at a 
 higher temperature, but is thickly precipitated when air, with which it is 
 mixed, is cooled, or when it touches a colder solid body : the steam observed 
 at night and morning hovering over brooks and marshes heated by the sun 
 during the day : the frost-smoke, as it is called, which lies on the whole face 
 of the greenland seas in the beginning of winter, where the water, warmed 
 by the long day of the polar summer, continues to emit its vapour for a con- 
 siderable time after summer is past, into an atmosphere, become too cold to 
 preserve it invisible : the breath or perspiration of animals, of horses in 
 particular, after strong exertion, becoming so strikingly visible in cold and 
 damp weather, or even in warm weather, when the air is already charged with 
 
298 HEAT. 
 
 'moisture: in cities where there are deep drains communicating with 
 kitchens, manufactories, &c., and constantly filled with moist warm air/the 
 vapour-loaded air, although clear or transparent in the drain, immediately 
 on escaping into a frosty atmosphere lets go its moisture, with the appearance 
 of steam issuing from a great subterranean caldron. Steam over water in 
 any boiler, is transparent or perfectly aeriform as may be seen when water 
 is made to boil in a vessel of glass, but as soon as it is cooled by contact or 
 admixture of colder air, it ceases to be true steam, and is condensed into 
 small particles of water suspended in the air. Many persons while thinking 
 of steam, figure it only in this latter state, as particles of water mixed with 
 air nearly as a subtile powder might be mixed, and its substance occupying 
 really no more space than the original water did. Now uiAil steam is cool 
 and condensed, it is of a nature to fill alone any appropriate vessel and 
 powerfully distend it, just as air fills and distends a bladder. Steam issuing 
 from the spout of a kettle is hardly seen near the mouth, but as its distance 
 from the spout increases, it is cooled into a thick cloud or vapour. 
 
 In a vessel from which air and atmospheric pressure are excluded, even 
 the temperature of freezing air being sufficient to maintain permanently in 
 the state of gas or air, many substances which exist as liquid under the 
 atmospheric pressure and the whole mass of such a substance, when placed 
 in a vacuum, not being instantly converted into gas, because the portion 
 which first rises becomes an atmosphere weighing upon the remaining mass, 
 and because, moreover, that portion, by absorbing from the mass much heat 
 into the latent state, cools the mass much below the freezing point; we see 
 why the liquids now spoken of are so rapidly cooled to at least the freezing 
 point if placed where a vacuum can be maintained, that is to say, where, 
 after common air has been removed, the aeriform matter arising from them, 
 and absorbing their heat, is also promptly, and in a continued manner, 
 abstracted. It is thus that water placed in an exhausted receiver of an air- 
 pump is so rapidly cooled, and that when there is beside it a vessel of con- 
 centrated sulphuric acid, or other substance capable of absorbing the watery 
 vapour as formed, it is soon reduced to the state of ice'; or again, that 
 water, or even mercury, surrounded by ether evaporating in a vacuum, is 
 so quickly frozen. It is thus, also, that if one bulb of the instrument 
 described at page 296, be immersed in a freezing mixture, the water in the 
 other and distant bulb will soon become ice ; for the vapor arising from 
 that water in the vacuum maintained throughout the apparatus by the 
 freezing mixture, is immediately condensed again in the immersed bulb, 
 and leaves the vacuum still free for the ascent of more vapour, to carry 
 away more heat from the water as latent heat, and to make it freeze. 
 
 As we have explained, also, that in a liquid there is the same tendency 
 to evaporation, whether it be or be not exposed to the air, we see the reason 
 why all evaporation is a very cooling process. The effect, however, in air 
 is neither so rapid nor so great as in a vacuum : first, because the presence 
 of the air impedes the spreading of the newly-formed vapour from the liquid 
 surface, and keeps it where its pressure resists the formation of more vapour ; 
 and, secondly, because the air in contact with the liquid, shares its higher 
 temperature with the liquid. Still, in India, flat dishes of water, placed 
 during the night on beds of twigs and straw kept wet and in a current of 
 air, soon exhibit thin cakes of ice and thus, ice is procured in India for 
 purposes of luxury. 
 
 The absorption of latent heat in the evaporation which goes on from the 
 sea and earth in all warm climates, greatly tempers the heat of these climates, 
 
EVAPORATION. 299 
 
 and the vapour afterwards spreading to the poles, as explained in "Pneuma- 
 tics" under the head of Winds, carries warmth thither to be given out 
 when it is recondensed into the form t of rain, or is solidified as snow. The 
 formation any where of mist or rain warms the air most sensibly, by the 
 liberation of the latent heat from the precipitated vapour. Again, the liquid 
 water, which, during winter, is converted into snow or ice, had been a re- 
 servoir of latent heat stored to temper the frosty air of the commencing cold 
 season ; and in the following spring, such ice and snow serve as empty re- 
 ceptacles, in which the first violence of the returning sun hides or expends 
 itself; allowing the temperature to change more gradually, and for many 
 living beings, therefore, more safely. The vast stores of ice and snow 
 among high mountains, as among the Alps and Pyrenees, are often, during 
 the summer, stores of mild temperature to regions around : for besides cool- 
 ing the air near them, they are the never-failing sources of the rivers which 
 run from them during the whole of summer, carrying freshness through dis- 
 tant lands : from the Alps, for instance, proceed the Rhine and the Rhone 
 most romantic and beautiful of European streams ; and from the Pyrenees, 
 the rapid Gave, &c., which, while channels around from lower regions are 
 almost dried up by the summer Jieat, flows only the more freshly as the heat 
 is greater, and the feeding snows are more abundantly dissolved. 
 
 Men in artificially raising temperature, are generally causing the liberation 
 of heat which had been previously latent ; and in lowering temperature 
 or producing cold, they affect their purpose almost solely by rendering a 
 quantity of heat latent. 
 
 Lavoisier thought that the heat of all combustion was merely the latent 
 heat of the oxygen gas concerned in the combustion, given out during its 
 combination with the burning body. It is so in part, but we now know that 
 it depends more on the intensity of the chemical action between thecombin- 
 'ing substances. The water thrown upon quick-lime to slake it, becomes 
 solid in combination with the lime, and gives out its latent heat so remark- 
 ably as often to set fire to a wooden vessel or ship containing it. 
 
 When dwelling-houses, green-houses, manufactories, &c., are warmed, as 
 is now common, by the admission of steam into systems of pipes which 
 branch over them, the heat is chiefly that previously latent in the steam, 
 and which spreads around as soon as the steam, by touching pipes of a lower 
 temperature, is condensed to a state of water. The modes of most profitably 
 effecting these purposes have to be considered in a future chapter. 
 
 For producing artificial cold, our processes generally involve the circum- 
 stance either of a solid changing into a liquid, during which it absorbs, and 
 hides in its new constitution much of the heat previously sensible in it and 
 in the liquid dissolving it ; or of a liquid changing into vapour, during which 
 heat equally becomes latent. Thus by dissolving a salt, nitre, for instance, 
 in water, we obtain a solution which is very cold. 
 
 In India, the common mode of cooling wine for table is to surround the 
 bottles with nitre thus melting ; and the water of the solution being evapor- 
 ated again before next day, the salt is left ready for use as before. Such is 
 the mutual attraction of water and many salts, that they readily combine, 
 assuming the liquid form, even when water is used in the solid state of 
 ice ; and as, in that case, both the water and the salt render heat latent, the 
 fall of temperature is very great. Thus common salt and snow (or ice) 
 when mixed, dissolve into liquid brine 37 colder than freezing water, or 5 
 below the zero of Fahrenheit. The last mentioned fact explains the com- 
 
300 HEAT. 
 
 mon practice of sprinkling salt on an ice-covered pavement before a street 
 door to clear away the ice. The salt and ice quickly combine and form 
 liquid brine, which either of itself runs off into the gutter and disappears, or 
 is easily swept of, or its water evaporates, leaving only the salt behind. It 
 is true that the brine is at first a refrigerating mixture, which cools still more 
 the pavement and the neighboring ice, but all which is touched by the salt 
 is melted. Servants usually err in using a pickaxe or spade immediately 
 after the sprinkling, instead of waiting, and with a broom spreading the 
 melting salt couipleatly over the place. 
 
 Frigorific Mixtures. 
 Substances mixed. Thermometer sinks. 
 
 Common salt .... 1 part j From any temperature to 5 below 
 Snow or pounded ice . . 2 j zero. 
 
 Common salt 5 \ ^ ^e 
 
 Snow or ice ..... 12 I From an ^ tem P erature to 25 
 Nitrate of ammonia . . 5 j 
 
 MuTed sulphuric acid ! 2 Z } From ' 32 abo to 23 bel * 
 
 Fused Potass . . . . 4 ) 000 , Kno , , 
 
 g o -from 32 above to 51 below zero. 
 
 Nitrate of ammonia 1 - ^ 
 
 From 50 to 4 above zero. 
 
 Sulphate of soda . . . 8 ) K A0 , A0 
 Muriatic acid . . . . 5 - j From 50 to or zero " 
 
 We have already described under other heads the frigorific effect of 
 evaporating in a vacuum or in the air, and of the operation of condensing 
 a gas to squeeze the heat out of it before letting it expand again to a great 
 volume. 
 
 for any given substance, the changes of state from solid to liquid, and from 
 liquid to air, happen under similar circumstances, so precisely at the 
 same temperature, that they mark fixed points in a general scale of tem- 
 perature, and enable us to regulate and compare our various thermome- 
 ters. (See Analysis, page 256.) 
 
 As we can neither weigh heat, nor measure its bulk, nor see it, and as, 
 even if our sense of touch were a correct judge in the matter, which it is 
 not, we dare not touch things which are very hot or cold, some other means 
 were wanted for estimating the presence in bodies of this very subtile prin- 
 ciple ; and a mean has been found in the measurement of its most obvious 
 and constant effect, namely that dilatation or expansion of bodies, which 
 again ceases when the heat is withdrawn. Any substance so circumstanced 
 as to allow this expansion to be accurately measured, becomes to us a ther- 
 mometer or measure of heat. 
 
 In solid substances the direct expansion by heat is so small as to be seen 
 or measured with diffculty. In airs, again, the expansion is very extensive ; 
 but there is the objection that in any apparatus yet contrived which will 
 allow their expansion completely to appear, they cannot be protected from the 
 varying pressure of the atmosphere an influence which affects their volume 
 even more than common changes of temperature. But liquids are free from 
 both disadvantages, and when placed in a glass bulb, as , having a long 
 
TABLE OF TEMPERATURE. 
 
 301 
 
 o 
 
 neck or stalk a b, into which the liquid may rise when expanded by heat ; to 
 be measured, they form the most generally convenient 
 of thermometers. Then, among liquids, mercury is, Fig. 134. 
 
 on several accounts, singularly pre-eminent. In mer- 
 cury, the range of temperature between freezing and 
 boiling reaches a higher point than in any other liquid, B 
 and a lower point than in all others except alcohol; its 
 little capacity for heat and ready conducting power, 
 causes it to be very quickly affected by change of tem- 
 perature ; its expansion is singularly equable for equal 
 increase of heat through the important middle part of 
 the scale, which includes the common temperature on 
 earth, namely, from freezing to the boiling heat of F 
 water ; and it is easy to proportion the bulb and the \\ 
 
 stalk to each other, so that a small difference of tern- a O 
 perature shall cause the mercurial column in the stalk 
 to rise or fall very conspicuously. 
 
 Now, when the important fact was ascertained that solid water or ice melts 
 in every case at precisely the same temperature, and that pure liquid water 
 in a metallic vessel, and under a given atmospheric pressure, boils always at 
 the same temperature, it followed that by placing such a thermometer, as 
 above described, first in melting ice and then in boiling water, and marking 
 upon the stalk the two points at which the mercury stood, as represented 
 here by F and B, two fixed or variable points would be obtained, and the 
 interval between them might be divided on the glass, or on a suitable scale 
 to be attached to the glass, into any convenient number of parts to be called 
 degrees ; it followed farther, that by continuing the division to any extent 
 both above and below the fixed points, a general scale of temperature would 
 be obtained, with respect to which all thermometers made on the same prin- 
 ciple would perfectly agree, although the size of the divisions on the stalks 
 would vary according to the comparative capacities of the bulk and stalk in 
 the different instruments. Our Newton had the honor first to propose the 
 regulating points of freezing and boiling, and they are now universally 
 adopted, but the interval between them has been variously subdivided ; that 
 is to say, there has not been agreement among philosophers as to what they 
 would call a degree of heat. In the Centigrade thermometer, which is the 
 most simple, the division is into 100 equal parts ; in Reaumur's, which is 
 commonly used in France, it is into 80 parts ; and in Fahrenheit's, which is 
 used in England, it is into 180. In Fahrenheit's, moreover, the freezing 
 point, instead of being called zero, as in the others, is called 32, because 
 the maker choose to begin counting from the lowest heat which he met in 
 Iceland, and which was 32 below freezing of his scale. To turn the 
 degrees of any one of these thermometers into degrees of any other, we 
 have only to recollect that 9 of Fahrenheit are equal to 5 of the Centi- 
 grade, and to 4 of Reaumer. Therefore, multiplying by 9 and dividing by 
 5 or 4, or the reverse, and adding or subtracting the 32 of Fahrenheit, 
 gives, as^the result, the degree desired. , 
 
 The bulb of a mercurial thermometer is formed by heating to fusion in 
 the flame of a lamp, the end of a glass tube, which has a very small and 
 equable bore, and then blowing into the tube until the softened end swells 
 like a soap-bubble, to the size desired. The mercury is foreed into such a 
 bulb through its long stalk by the pressure of the atmosphere, thus. First, 
 a portion of the air originally in the bulb being expelled by warming the 
 bulb, the open end of the stalk is immersed in mercury, and when the air 
 
302 
 
 HEAT. 
 
 remaining in the bulb cools and contracts, a little mercury enters. Secondly 
 this admitted mercury having been made to boil, so as to fill with its vapour 
 the whole capacity of the bulb and tube, and to expel the air, on the open 
 end being again immersed in mercury, and the mercurial vapour within being 
 condensed, the atmosphere presses in fresh mercury to fill the whole vacu- 
 um. To complete the making of the thermometer, the bulb is again heated 
 to expel so much of the mercury as that when cold, the tube shall be about 
 one-third full of it, and then, before the heated mercury begins to recede, 
 the end or opening is hermetically closed by directing upon it the point of 
 a blow-pipe flame which fuses the glass. 
 
 Although the direct expansion of any solid body by a moderate change 
 of temperature is so inconsiderable as to be with difficulty measured, M. 
 Breguet, of Paris, lately with much ingenuity contrived a thermometer 
 which makes it very evident. Having soldered side by side two very small 
 flattened wires of silver and platinum, or of any other metals having different 
 expansibility by heat, he found that all changes of temperature made such 
 compound wires bend to a great extent, as a sheet of damp paper curls on 
 being held before the fire. The metal most shortened or least lengthened 
 acting like a bow-string to pull the other into the arched form ; and he then 
 found, on giving to the compound wire a spiral or cork-screw form, and 
 securing the upper end of it to a fixed stand, while the lower was left free to 
 move, that an index like the hand of a watch attached to the lower end was 
 turned completely round by a certain change of temperature, and afforded 
 on a circle of degrees placed like a watch face below it, indications which 
 perfectly agreed with those of good mercurial thermometers. Other modifi- 
 cations of the same principle have since been successfully tried, so simplified 
 anfl reduced in bulk as to be introduced into the structure of a pocket watch. 
 Air is a substance on several accounts admirably adapted to the formation 
 of a thermometer ; for it has great extent of dilatation from small increase of 
 heat ; it quickly receives impressions, and its dilatation is equal for equal 
 increments of heat at all temperatures; but, as already stated, there is the 
 strong objection that the pressure of the atmosphere cannot be excluded, 
 without at the same time confining the air, and effecting its expansion. Mr. 
 Leslie, however, has used for particular purposes an air thermometer, which 
 he calls the differential thermometer. It consists of two bulbs a and b, filled 
 with air and connected by a bent tube d c, containing 
 liquid, the instrument being hermetically sealed, so that 
 the atmosphere cannot affect the air within. The greater 
 heat in the bulb b than in the other, as when that bulb is 
 touched by the warm hand or is exposed to the sun's ray, 
 is marked by the descending of the liquid in the tube d, 
 which has a scale attached to it. We may observe that 
 equal divisions or degrees marked on the scale of this 
 thermometer, do not mark equal changes of temperature, 
 for the increasing condensation and resistance of the air in 
 the cold bulb require the force overcoming it progressively 
 to increase. If the resistance, on the contrary , 4 were un- 
 varying, as in an air thermometer open to a steady atmo- 
 sphere, equal extent of motion in the fluids would mark 
 equal increments of heat. An air-thermometer made of a 
 simple bulb and long stalk of semi-transparent porcelain, 
 with the mouth downwards, and containing in its neck 
 melted lead or other fusible metal instead of mercury, is 
 well adapted for measuring very high temperatures. 
 
 Fig. 135. 
 
TABLE OF TEMPERATURE. 
 
 303 
 
 Temperatures, below that of freezing mercury, are usually measured by 
 alcohol which substance has not yet been frozen ; and temperatures higher 
 than of boiling mercury, are measured by the expansion of air or of metals, 
 as above described, or by the contraction of pieces of baked clay, which, 
 when highly heated, lose water and become semi-vitrified, The use of baked 
 clay was proposed by Wedgewood, and the apparatus has been called 
 Wedgewood's Pyrometer, or fire measure. All contrivances for measuring 
 heat may be graduated so as to correspond with the scale adopted for the 
 mercurial thermometer. 
 
 It is most interesting, while considering the vast number and importance 
 of the phenomena, produced by heat, to observe the degrees in the general 
 scale of temperature at which these severally take place. In the following 
 table, a selection of the facts is classified, the temperatures being all referred 
 to the scale of Fahrenheit's thermometer. 
 
 Table of facts connected with the influence of heat corresponding to certain 
 
 temperatures. 
 
 Wedgewood. 
 
 Highest temperature measured . . . 240 
 
 Chinese porcelain softened . . . 156 
 
 Cast-iron thoroughly melted . . . 150 
 
 Greatest heat of a'commen smith's forge . . 125 
 
 Mint glass surface .... 11-4 
 
 Stone ware baked in . . . 102 
 
 Welding heat of iron . . . 92 to 95 
 
 Delft ware baked in . . .41 
 
 Fine gold melts . . . . .32 
 
 Settling heat of flint glass . . . 29_ 
 
 Fine silver melts . . . .28 
 
 Brass melts . . . . .21 
 
 Full red heat (tlie beginning of Wedgewood 1 s 
 
 Pyrometer) . . . . .0 
 
 Heat of a common fire ...... 
 
 Iron red in the dark . 
 
 Quicksilver boils . . . . " . 
 
 Jiinseed oil boils ...... 
 
 Lead melts . 
 
 Sulphur melts . . . 
 
 Water boils ....... 
 
 A compound of three parts tin, five of lead, and eight of bismuth 
 
 melts . ..... 
 
 Alcohol boils . . . ... 
 
 Bees'-wax melts ...... 
 
 Ether boils ....... 
 
 The present medium temperature of the globe ... 
 Water freezes ....... 
 
 Milk freezes ....... 
 
 Vinegar freezes ...... 
 
 Strong wine freezes ...... 
 
 Weak brine freezes .... zero 
 
 Quicksilver freezes . . . . below zero 
 
 The air sometimes at Hudson's Bay .... 
 
 Greatest artificial cold yet measured .... 
 
 Fahrenheit. 
 
 32,277 
 21,357 
 20,577 
 17,327 
 15,897 
 14,337 
 13,427 
 6,407 
 5,237 
 4,847 
 4,717 
 3,807 
 
 1,077 
 790 
 
 750 
 660 
 600 
 594 
 226 
 212 
 
 210 
 174 
 142 
 
 98 
 50 
 32 
 30 
 28 
 20 
 
 
 40 
 50 
 91 
 
304 HEAT. 
 
 There is reason for thinking that the higher temperature noted in this 
 table are considerably too high, owing to the insufficiency of the thermometer 
 or Pyrometer (Wedgewood's) by which they were estimated. 
 
 It is a curious inquiry, suggested by contemplating the preceeding table, 
 how much heat may yet remain in bodies at the- lowest temperature which 
 we know ? No conjecture was hazarded on the subject until Dr. Irving thought 
 it might be elucidated by comparing the quantity of heat which becomes 
 latent in a body on changing form, with the capacity of the body before and 
 after the change. For instance, with respect to water, he said : as it requries 
 one-tenth more heat to make a certain change in the temperature of water 
 than of an equal quantity of ice, probably ice-cold water contains altogether 
 just one-tenth more heat than of an equal quantity of ice at the melting point : 
 then as we know the water to contain exactly 140 more heat than the ice, 
 viz., its latent heat, the whole or absolute quantity of heat in it will be ten 
 times 140, or 1,400. By applying this reasoning, however, to other sub- 
 stances than water, it evidently is fallacious ; and the conclusion follows that 
 we have as yet no means of solving the question ; the thermometer no more 
 telling us the absolute quantity of heat in any body than the rising and falling 
 of the tide between any two rocks tells us the total depth of the rocky chasm. 
 
 From what is said in the last and in preceding paragraphs, it is evident 
 that the thermometer gives very limited information with respect to heat :it 
 merely indicates, in fact, what may be called the tension* of heat in bodies, 
 or the tendency of the heat to spread from them. Thus it does not discover 
 that a pound of water takes thirty times as much heat to raise its temperature 
 one degree as a pound of mercury ; nor does it discover the caloric of fluidity 
 absorbed when bodies change their form, and which is called " latent heat" 
 only because hidden from the thermometer ; nor does it tell that there is more 
 heat in a gallon of water than in a pint ; and if an observer did not make 
 allowance for the increasing rate of expansion with increasing temperature, 
 in the substance used as a thermometer, he would believe the increase of heat 
 to be greater than it is; and lastly, when a fluid is used as a thermometer, 
 the expansion observed is only the excess of the expansion in a fluid over 
 that in the containing solid, and subject to the irregularities of expansion in 
 both substances ; all proving that the indications of the thermometer, unless 
 interpreted by other circumstances and our knowledge of the general laws 
 of heat, no more disclose the true relations of heat to bodies, than the money, 
 accidentally in a man's pocket tells his rank and riches. 
 
 t) ~by its different relations to different substances, has a powerful influ- 
 ence on their chemical combinations." (See Anaylsis, page 266.) 
 
 By observations made and recorded through past ages, man has now come 
 to know that the substances constituting the world around him although 
 appearing to differ in their nature, almost to infinity, are yet all made up of 
 a few simple elements variously combined ; and he has discovered that the 
 peculiar relations of these elements to heat, particularly their being unequally 
 expanded by it, and their undergoing fusion and vaporization at different 
 temperatures, furnish him with ready means of separating, combining, and 
 new-modifying them to serve to him most useful purposes. Where the primi- 
 tive savage, looking around on rocks and soils, saw in their diversified aspect 
 almost as little meaning as did the inferior animals which participated with 
 him the shelter of the wood or cave, his descendant, with penetration sharp- 
 ened by science, descries at once the treasures of the mine, and aided by 
 
ABSOLUTE QUANTITY OP HEAT. 305 . 
 
 heat, whose wonderful energies he has learned to control, pursues through 
 all the Protean disguises of ores, and salts, *and solutions, each of the wished- 
 for substance, until he secures it apart/ For instance, in what to his fore- 
 fathers for thousands of years appeared but a red dross, he knows that there 
 lies concealed the precious iron king of metals ! and soon forcing tjiis in 
 his ardent furnace to assume the metallic form, he afterwards, with imple- 
 ments made of it, moulds all other bodies to his will : the trees from the 
 forest, and the rocks from the quarry, in obedience to these, are fashioned 
 by him as if they were of soft clay, and at his command rise into magnificent 
 structures of palaces and ships, with which the earth and the ocean are now 
 so thickly covered. The minute detail of the relations to heat of particular 
 substances forms a great portion of the department of science called chemistry 
 (a name taken from an Arabic word signifying fire;) but a general review 
 of the subject belongs to this work. 
 
 The most common ores of metals are combinations of the metals with 
 oxygen, carbonic acid, or sulphur, substances all of which are volatilized at 
 much lower temperatures than the metals. Now simply roasting, as it is 
 called, or strongly heating the ores, suffices often to drive away great part 
 of these adjuncts; and where additional assistance is required, it is obtained 
 by mixing with the ore something which when heated attracts the substance 
 to be expelled more strongly than the metal does. Charcoal, for instance, 
 heated with oxide-ore, takes the oxygen, and flying off with it as carbonic 
 acid, leaves at the bottom of the furnace or crucible the vivified or pure metal. 
 
 Mercury mixed with the dross of a mine, dissolves any particles of gold 
 or of silver existing in the dross, and the ingredients of the solution may 
 afterwards be obtained separate by mere heating the mercury passing away 
 as vapour to where it is cooled and again condensed for subsequent use, and 
 the more fixed gold or silver remaining in the vessel and just as in all other 
 distillations, like that of spirit from wine, or of essential oils from water, &c., 
 there is the separation by heat of a more volatile from a less volatile sub- 
 stance. The only difference between^what is called drying by heat and dis- 
 tilling, is that in the one case the substance vaporized, being of no use, is 
 allowed to escape or become dissipated in the atmosphere; while in the other, 
 being the precious part, it is caught and condensed into the liquid form. 
 The vapour from drying linen, if caught would be distilled water. The 
 abundant vapour from wheaten bread while being baken, is chiefly a spirit 
 like what is obtained from malt, and by an ingenious apparatus lately con- 
 trived, may be caught and preserved. 
 
 A piece of cold charcoal lies in the air for any length of time without 
 change : but if heated to a certain degree, the mutual cohesion of its particles 
 is so weakened, in other words, the particles are so repelled and separated 
 from each other, that their attraction for the oxygen in the air around is 
 allowed to operate, and they combine with that oxygen, so as to produce the 
 phenomenon of combustion. The same is true, under similar circumstances, 
 of almost any dry vegetable or animal substances, and of several of the 
 metals. 
 
 Nitre, sulphur and charcoal, while cold, may be mixed together most 
 intimately without any change taking place ; but if the mixture or any part 
 of it, be heated to a certain degree, the whole explodes with extreme violence, 
 for it is gunpowder. By the change of temperature, and the consequently 
 altered relative attractions of the different substances, a new chemical 
 arrangement of them then takes place with the intense combustion and 
 expansion, which constitute the explosion. 
 
 20 
 
306 HEAT. 
 
 Sea sand and soda very intimately mixed, and even ground together, if 
 remaining cold, remain also merely an opaque and useless powder j but if 
 the mixture be heated, to diminish the cohesion of the particles of each sub- 
 stance to those of its own kind, so that the mutual attraction of the two sub- 
 stances may come into play, the two substances melt together, and unite 
 chemically into the beautiful compound called glass ; a product than which 
 art has formed none more admirable which, in domestic use alone, is fash- 
 ioned into the brilliant chandelier and lustre, into the sparkling furniture of 
 the side-board, into the magnificent mirror plate, and when extended across 
 the window opening, admits the light while it repels the storm. 
 
 Perhaps the influence of temperature on chemical union is nowhere more 
 remarkably exhibited, than in retarding or hastening the decomposition of 
 dead vegetable and animal substances. The functions of life bring into 
 combination to form the various textures of these organic or living bodies, 
 chiefly four substances, viz., carbon, or coal ; the ingredients of water, or 
 oxygen and hydrogen ; and lastly, nitrogen which substances, when in the 
 proportions found in such bodies, have but slight attractions for each other, 
 and all of which, except the carbon, usually exist as airs. Their connection, 
 therefore, is easily subverted ; and particularly by a slight change of tem- 
 perature, which either so weakens their mutual hold as to allow new arrange- 
 ments to be formed, or altogether disengages the more volatile of "them. At a 
 certain temperature, a solution of sugar (which consists of the three substances 
 first mentioned, carbon, oxygen, and hydrogen) undergoes a change into a 
 spirituous wash, from which spirit or alcohol may then be obtained by dis- 
 tillation : but if the heat be continued under certain circumstances the liquid 
 undergoes a second change, or new arrangement of constituent particles, and 
 becomes vinegar : under still other circumstances, it undergoes a third change, 
 which is a destructive decomposition, or rotting, as we call it, and the oxygen 
 and hydrogen ascends away as airs. But sugar and many similar vegetable 
 compounds, preserved at a low temperature, remain unchanged for ages. 
 
 Again, as regards dead animal substances, we find that although at a certain, 
 not very elevated temperature, they undergo that change in the relations of 
 their elements which we call putrefaction, during which nearly their whole 
 substances rise again to form part of the atmosphere, still at or below the 
 temperature of freezing water, they remain unaltered for any length of time. 
 
 In the middle of summer, recently caught salmon, or other fish, packed in 
 boxes with ice, may be conveyed fresh from the most remote parts of Britain 
 to the capital. In our warmest weather, any meat or game may be long 
 preserved in an ice-house. In Russia, Canada, and other northern countries, 
 on the setting in of the hard frosts, when food for the cattle and poultry is 
 with difficulty procured, the inhabitants kill their winter supply, and store up 
 their provender of frozen flesh or fowl, as in other countries men store that 
 which is salted or pickled. The most striking illustration which we can 
 adduce of this kind is the fact that on the shore of Siberia, in 1801, in a vast 
 block or island of ice, then accidentally broken and partially melted, the car- 
 cass of what has been called the antediluvian elephant was found, perfectly 
 preserved an elephant differing materially from those now existing on earth, 
 and having a skelton exactly similar to the fossil specimens found deep 
 buried in various countries. The carcass was soon discovered by the hungry 
 bears of the district, which were seen eagerly feeding on its flesh, as if it had 
 died but yesterday, although it must have been of an era long anterior to that 
 of any existing monument on earth, of human art, or even of human being. 
 After it had fallen from the ice to the sandy beach, and its tusks had been 
 
INFLUENCE OP HEAT ON ANIMA'TED BEINGS. 307 
 
 carried away for sale by a Tunguisian Fisherman, and much of its flesh had 
 been devoured, a naturalist, from St. Petersburg who visited it foumd an ear 
 still perfect, and its long mane, and apart of its upper lip, and an eye with 
 the pupil, which had opened on the glories of a former or younger world ! 
 About 30 Ibs. weight of its hair, which had been trodden into the sand by 
 the bears while eating the carcass, and part of the skin, were preserved, and 
 are now distributed in different museums of natural curiosities. A piece of 
 the skin with the hair upon it is to be seen in the museum of the London 
 College of Surgeons. 
 
 " Heat has powerful influence also on animated nature, both vegetable and 
 animal." (Read the Analysis, page 256.) 
 
 As the detail of the relations of heat to particular inanimate substances 
 belongs to the province of chemistry, so does the detail of its relations to 
 particular living vegetables and animals belong to the department of 
 Physiology ; but a general review of the subject is required in a treatise on 
 Natural Philosophy. 
 
 The influence which heat exerts on inanimate nature, is, by the common 
 mind, more immediately and completely perceived than its influence on beings 
 which have life. Thus, to all it is obvious, that the contrast between a winter 
 and summer landscape, is owing chiefly to the effect of heat on the water of 
 the landscape ; that during the absence of heat, there is the dry barren defor- 
 mity of accumulated ice and snow, covering every thing, the roads impassable, 
 the rivers bound up, perhaps hidden, the air deprived of moisture, and loaded 
 often with powdery drift : but that on heat returning, the gliding streams 
 again appear, the cascades pour, the rills murmur, the canals once more offer 
 their bosom to the boats of commerce, the lake and pool again show their 
 level face, reflecting the glories of the heavens, and the genial shower falls 
 upon the bosom of the softened earth, become ready to receive the spade or 
 the ploughshare. But this change is not at all greater than what happens 
 to a winter tree acted upon by the warmth of spring. Again, it may be said 
 with truth, that heat applied to the cold boiler of a steam-engine, is the cause 
 of all its succeeding motions ; of the heaving of its beam and pumps, the 
 opening and shutting of its valves, the turning of its wheels, and its ultimate 
 performance of any work, as of spinning, or weaving, or grinding, or propel- 
 ing vehicles by land and water; but as truly may it be said, that heat coming 
 to a seed which has lain cold for ages, is the cause of its immediate germina- 
 , tion and growth ; or coming to a lately frozen tree is the cause of the rising 
 of its sap, the new budding and unfolding of its leaves and blossoms, the 
 ripening of its fruits. And what is true of one seed or tree, is true of the 
 whole of the vegetable creation. When the warm gales of spring have once 
 breathed on the earth, it soon becomes covered, in field and in forest, with 
 its thick garb of green, and soon opening flowers or blossoms are every where 
 breathing back again a fragrance to heaven, among these the heliotrope is 
 seen always turning its beautiful disc to the sun, and many delicate flowers 
 which open their leaves only to catch the direct solar ray, closing them often 
 even when a cloud intervenes, and certainly when the chills of night approach. 
 On the sunny side of a hill, or in the sheltered crevice or a rock, or on a 
 garden wall, with warm exposure, there may be produced grapes, peaches, 
 and other delicious fruits, which will not grow in situations of an opposite 
 character all acknowledging heat as the immediate cause, or indispensable 
 condition, of vegetable life. 
 
308 HEAT. 
 
 And among animals, too, the effects of heat are equally remarkable. The 
 dread silence of winter, for instance, is succeeded in spring by one general 
 cry of joy. Aloft in the air the lark is every where caroling; and in the 
 shrubberies and woods, a thousand little throats are similarly pouring forth 
 . their songs of gladness during the day, the thrush and blackbird are heard 
 above the rest, and in the evening the sweet nightingale : for all birds it 
 is the season*of love .and of exquisite enjoyment. And it is equally so for 
 animals of other kinds : in favoured England, for instance, in April and May, 
 the whole face of the country resounds with lowings, and bleatings, and bark- 
 ings of joy. And even man, the master of the whole, whose mind embraces 
 all times and places, is far from being insensible to the change of season. 
 His far-seeing reason of course draws delight from the anticipation of autumn, 
 with its fruits; and his benevolence rejoices in the happiness observed among 
 all inferior creatures; but independently of these considerations, on his own 
 frame the returning warmth exerts a direct influence. In his early life, when 
 the natural sensibilities are yet fresh and unaltered by the habits of artificial 
 society, spring to man is always a season of delight. The eyes brighten, the 
 whole countenance is animated, and the heart feels as if new life were come, 
 and has longings for fresh objects of endearment. Of those who. have passed 
 their early years in the country, there are few who, in their morning walks 
 in spring, have not experienced, without very definite cause, a kind of tumul- 
 tuous joy of which the natural expression would have been, how good the 
 God of nature is to us. Spring, thus, is a time when sleeping sensibility is 
 roused to feel that there lies in nature more than the grosser sense perceives. 
 The heart is then thrilled with sudden ecstacy, and wakes to aspirations of 
 sweet acknowledgment. 
 
 Besides the effects of heat now mentioned, and which are comparatively 
 transient as being connected with the seasons, there are other effects on ani- 
 mated nature of a more prominent character. Certain species of vegetables 
 and animals, by their relations to heat, are confined to certain latitudes or 
 climates; as the orange tree and bird of paradise, to warm regions; the fir tree, 
 and arctic bear, to those that are colder : and when individuals of either 
 class can support diversity of climate, they acquire a certain character accord- 
 ing to the climate as seen in the sheep and dogs of the various regions of 
 the earth. In this latter respect there is no instance more interesting than 
 that furnished by the varieties of the human race. If we assume that the 
 whole sprung from one stock, what a contrast is there between the native of 
 equatorial Africa, of temperate Europe, and of the Polar zone : between the 
 Negro, the Greek, and the Esquimaux: or, again, between the dark slender 
 children of Hindostan, the strongly-knit fairer Roman or Spaniard, and the 
 taller, ruddy, powerful Briton. And in the female sex of the last-named 
 countries, we may remark the gentleness and singular devotedness of the 
 Indian woman, the more commanding dark eye and gesture of the graceful 
 nymph of Italy or Spain, and the happily attempered mixture of qualities in 
 the fair and much-favoured daughters of Britain. 
 
 The very important influence of heat upon the temporary bodily state of 
 animals, becomes an object of much study to the physician. It explains 
 among many other facts, the connection of temperature with the rise of fevers 
 and other pestilences, the powerful remedial efficacy of hot and cold bathing, 
 of changes of climates, of regulating the temperature of air breathed by in- 
 valids, the protection from clothes, houses, &c. 
 
THE SUN THE GREAT SOURCE. 309 
 
 " The great natural source of heat is the sun" (See Analysis, page 256.) 
 
 To admit this, it is only necessary to think of the comparative tempera- 
 tures of night and day, of seasons and of climates, and to reflect that the sun 
 is the sole cause of the differences. We need not wonder, then, that to 
 many savage nations, seeking the course of their life and happiness, the sun 
 has been the object, not only of admiration, but of worship. 
 
 The heat comes from the sun with all his light. If a sunbeam enter by a 
 small opening an apartment otherwise closed and dark, it illuminates intensely 
 the spot or object on which it first falls, and its light bein^then scattered 
 around, all the objects in the room become feebly visible. Again, a cold 
 thermometer, held to receive the direct ray, rises much, while, in any other 
 situation, it is less affected ; proving the heat to be, like the light, at first 
 concentrated, and then widely diffused, losing proportionally of its intensity. 
 Light passes from the sun to the earth in about eight minutes of time, as 
 will be fully explained in a future chapter ; and there is every reason to 
 conclude that heat travels at the same rate. 
 
 Human art can gather the sunbeams together, and the intense heat 
 existing in the focus of their meeting, is another proof that the sun is the 
 great source of heat. A pane of glass in a window, or a small mirror will 
 reflect the sun's rays so as strongly to affect the eye at a distance of miles 
 and the heat accompanies the ray, for by many such mirrors directed towards 
 one point, a combustible object placed there may be inflamed Archimedes 
 set fire to the Roman ships by sunbeams, returning from many points to 
 one, his God-like genius thus rivaling by natural means, the supposed feats 
 of fabled Jupiter with his thunderbolts. Again, when the light of a broad 
 sunbeam is made by a convex glass or lens to converge to one point or focus, 
 the concentrated heat is also there for a piece of metal held in the focus 
 will drop like melting wax : and if the glass be purposely advanced, its 
 focus will similarly advance, and will pierce through the most obdurate 
 substances, as red-hot wire pierces through paper or wood. A hunter on the 
 hill, and travelling hordes on the plains, often conveniently light their fires 
 at the sun himself, by directing his energies through a burning glass. 
 
 The direct ray of the sun, simply received into a box which is covered 
 with glass to exclude the cold air, and is lined with charcoal or burned cork 
 to absorb heat, and to prevent the escape of heat once received, will raise a 
 thermometer in the box to the temperature of 230 of Fahrenheit, a tem- 
 perature considerably above that of boiling water. And the experiment 
 succeeds in any part of the earth where there is a clear atmosphere, and 
 where the sun attains considerable apparent altitude. We see, therefore, 
 that a solar oven might in some cases be used. In opAating with the 
 apparatus suggested by the author, and described at page 295, for distilling 
 water by the heat of the sun, the vessel intended to absorb the heat, and to act 
 as the still, should be enclosed in a case covered and lined as above described . 
 
 Reflecting on such facts as now recorded, and on the globular form and the 
 motions of our earth, we have a reason and the measure of the differences of 
 climate and of season found upon the earth. It is evident that the part of the 
 globe turned directly to the sun, receives his rays as abundantly as if it were 
 a perfect plane, directly facing him, while on parts, which, as viewed from 
 the sun, would be called the sides of the globe, with the increasing obliquity 
 of aspect, an equal breadth or quantity of rays is spread over a larger and 
 a larger surface; and at the' very edge the light passes level with the surface, 
 and altogether without touching. The sunny side of many a steep hill in 
 
310 HEAT. 
 
 England receives the sun's rays in summer as perpendicularly as the plains 
 about the equator ; and such hill-sides are not heated like these plains, only 
 because the air over them is colder just as mountain tops, even at the 
 equator, owing to the rarified, and, therefore, cold air around them, remain 
 for ever hooded in snow. In England, at the time of equinoxes, a level 
 plain receives only about half as much of the sun's light and heat as an equal 
 extent of level surface near the equator : and in the short days of winter it 
 receives considerably less than a third of its summer allowance. 
 
 There are few contrasts in nature more striking than between the conse- 
 quences of diffeilnt intensity of the sun's influence : for instance, the inha- 
 bitants of India, at midday, with the thermometer at 120, are running to 
 the shade of their bangalows, darkening their windows, hanging wetted mats 
 upon the walls and roofs, sprinkling water on the floors, fanning themselves 
 with ever moving punkas, and feeling the slightest covering or exertion too 
 much while, on the other hand, the dwellers in Greenland, with the ther- 
 mometer below zero, are loaded with furs, and are seeking the direct sun- 
 shine or heat from a fire, as their life and comfort. Again, there is the 
 contrast observed on passing, as the author once did, very rapidly, from 
 such a paradise as Rio de Janeiro, with all its vegetable riches, to Tristan 
 de Cunha, and the Isle of Desolation in the Southern Ocean, which exhibit 
 only cold and naked rocks ; but yet, where the scene swarms with its appro- 
 priate inhabitants the sea with seals, and the air with clouds of sea-fowl, 
 playing over the never-resting waves like flakes of eddying snow. "Were a 
 person for a moment to doubt whether the sun be the real cause of such 
 differences, and of the fact that certain creatures are found only in certain 
 zones of the earth, let him reflect on the extraordinary migration of animals, 
 which have their home, not in any fixed region, but wherever the sun has 
 for a time particular degree of influence, and which acccordingly follow the 
 sun in the changes of season. We have the swallow in vast numbers coming 
 to visit the British isles in the spring, to play over our woods and waters, 
 in pursuit of the insects which the heat breeds to fill the air, welcome 
 harbingers of the coming summer and its riches ; and in autumn, the same 
 creatures are seen congregating on our shores, to wing their flight back in 
 united multitudes to more southern countries, where, in turn, there is a 
 temperate influence of the sun. The same reason brings to England the 
 nightingale, and makes our woodlands resound with the notes of the cuckoo. 
 In the waters of our bays and coasts, again, there appear with the seasons 
 the vast shoals of fish, as of the herring and mackeral, which prove such 
 abundant food for millions of human beings ; and we have salmon, at stated 
 times, penetrating from the ocean far up the mountain streams, to deposit 
 its spawn for further supply ; all by their movements contributing to the 
 harmonious and beneficent system of the universe. 
 
 With respect to the sun as a source of heat, there have been two opinions 
 among philosophers, one class believing that the sun is an intensely heated 
 mass, which radiates its heat and light around, like a mass of intensely 
 heated iron ; and another class holding that heat is merely an affection or 
 state of an ethereal fluid, which occupies all space, as sound is an affection 
 or motion of air, and that the sun may produce the phenomena of light and 
 heat without waste of its temperature or substance, as a bell may produce 
 the phenomenon of sound; holding, farther, that the sun, below its luminous 
 atmosphere may be habitable, even by such animals as live on this earth. 
 Those who take the first view, are awakened to the dread contemplation of 
 a universe carrying in itself the seeds of its own decay, or at least of great 
 
COMBUSTION. 311 
 
 periodical revolutions : the others may view the universe as destined to last 
 nearly unchanged, until a new act or will of its Creator shall again alter or 
 destroy it. 
 
 Of one fact there can be no doubt, namely, that the pregfcnt temperature 
 of the surface of the earth is much lower than the temperature in remote 
 past time. The rocks called primitive, as granite and gneiss, constituting 
 the interiors of our great mountain masses and the substrata of our plains, 
 bear evident marks of there having been at one period a molten state, from 
 which they have been solidified by a very gradual cooling : and even the 
 whole mass of the earth at some time must have been so fluid or soft, as, in 
 obedience to gravity, to have assumed its rounded form, and in obedience to 
 the centrifugal force of its whirling, to have bulged out, at its great circum- 
 ference or equator, the thirty-four miles which its equatorial diameter exceeds 
 the polar ; the same, by the by, in degrees corresponding to the various 
 speed of rotation, being true of all the other planets belonging to the solar 
 system. Again, while in excavating below the surface of the globe, or in 
 examining its structure as exposed to view by volcanic or other convulsions, 
 men encounter, in very many situations, a thickness of more than a mile, of 
 the wreck and remains of former states of the world as, on digging eighty 
 feet under vineyards near Mount Vesuvius, they encounter the more recently 
 buried cities of Herculaneum and Pompeii they farther discover that the 
 animal and vegetable remains buried, without number, in the present cold 
 climates of the earth, and evidently near where the creatures lived, are all 
 of kinds now inhabiting only the warmer or tropical regions. Lastly, in 
 the operations of mining, the deeper men go, the higher they find the tem- 
 perature to be, at the rate of a degree for about 200 feet of descent ; which 
 fact as heat tends to equable diffusion, proves both that the central heat of 
 our earth must have had another source than a radiation from the sun of the 
 present intensity ; and that the surface of the earth is now radiating away 
 more heat than it receives from the sun. The conclusion then follows, that 
 the temperature of the world is still falling although perhaps so slowly that 
 a change may not be detected even within centuries. Possibly, in very 
 remote antiquity, that may have been true which the early Greeks errone- 
 ously thought true in their day, viz., that the equator of the earth, by reason 
 of its great heat, the sun's influence there being joined to the heat from 
 within, was a barrier, impassable by man, between the northern and 
 southern hemispheres. 
 
 " Electricity a source of heat." (See the Analysis.) 
 
 This subject can only be satisfactorily entered upon in the chapter devoted 
 exclusively to electricity, and is, therefore deferred. Suffice it here to say, 
 that while an electrical discharge of current passes from one situation to 
 another, the substance serving as a conductor is often heated, melted, or dissi- 
 pated, in such a manner as to make it doubtful whether human art has any 
 more powerful means of producing these effects. We may remark, too, that 
 in certain cases of the electrical current, the heat is accompanied by as 
 intense a light as art can exhibit. 
 
 lt Combustion and other chemical actions as sources of heat. "(See Analysis, 
 
 page 256.) 
 
 Of the phenomena of nature there is perhaps none which, to the unin- 
 gtructed, appears so inexplicable and so wonderful as that of fire or combus- 
 
312 HEAT. 
 
 tion whether contemplated in its beauty or in its terrors. Fire is seen in 
 its beauty when used by man for his domestic purposes, as when it blazes 
 cheerfully over his parlour hearth, or beams a steady light around from his 
 lamps and chandeliers. Tt is seen again in its terrors, when spreading by 
 accident from some focus, it envelopes in sudden flame and quickly consumes 
 the surrounding objects, perhaps the draperies and other furniture of a single 
 apartment; or wider spread, theTaluable contents of a spacious mansion ; or 
 still wider spread, and deafening uproar, a whole town or a forest : and it 
 is fire which, labouring within the bowels of the earth, first prepares and then 
 urges up to heaven the volcanic eruptions of flames and red-hot rocks, during 
 which the region around often quakes and is uptorn, so that the cities are de- 
 molished into the sudden tombs of the inhabitants, the course of the rivers is 
 changed, the plains are converted into lakes, or the lakes'-beds into dry land. 
 Fire is awfully seen also in some meteors, and when, intentionally lighted by 
 human hands, it bursts from the cannon to produce the carnage of battle. Fire 
 among many nations of antiquity was regarded with awe and holy reverence, 
 the sun himself being honoured chiefly as its concentration or supposed abode. 
 There were sacred fires in many of the temples, and fire was used to complete 
 the splendour of the most august ceremonies. Nay, even Moses, a worshipper 
 of the one true God, has given records of the Burning Bush and of burnt- 
 offerings made to that God : and at the present day, in many Christian churches 
 there are ever-burning lamps and frequent magnificent illuminations. Now 
 this principle of fire, which, when the savage man first saw it spreading after 
 the thunder-clap or the rubbing of forest branches in a storni, so as to threaten 
 universal destruction, he sonatuarally accounted the demon, if not the God of 
 nature ; this principle man's art has now tamed to be a most obedient and by 
 far the most useful of his servants. Fire being in truth but a concentration of 
 the element of heat, which in its tranquil and in-visible diffusion we have al- 
 ready contemplated as the beneficent life or soul of the universe the cause of 
 seasons and climates and of all the changes or activity which distinguish a 
 living world from a dead and frozen mass ; man, by acquiring command over 
 it, commands heat when and where he wills, and thus truly becomes in a 
 second degree the ruler of nature. Fire in man's service may be figured 
 as a legion of serving spirits to whom no labour is difficult, who, in any 
 particular case have power or magnitude exactly proportioned to the quan- 
 tity of food or fuel afforded ; of whom, moreover, man can, at any moment, 
 conjure up one or many by the magic stroke of his flint and steel. In every 
 private dwelling he has of these fiery spirits as domestic slaves in the 
 kitchen and in the parlour. In his manufactories they are melting glass for 
 him, and reducing ores, and boiling and evaporating for a hundred purposes. 
 But it is chiefly when chained to the steam-engine, that they show their 
 prodigious powers : ras when, putting forth a giant's strength, they heave a 
 river from the bottom of a mine, or urge up a vast ship through the winter 
 storni ' } or when equaling, if not surpassing, in nice dexterity, the work 
 of human hands, they twist the silken or cotton threads, and weave them 
 into most delicate fabrics. Men, grown familiar with such prodigies, have 
 almost ceased to be moved by them ; but even now few persons can resist a 
 feeling of wonder and admiration when chemistry is calling forth the hidden 
 spirit of combustion in some new and less familiar guise : as, for instance, 
 when a piece of iron wire, lighted as a taper in oxygen gas, burns with such 
 resplendent brilliancy ; or when phosphorus similarly placed, throws around 
 its overpowering flood of flame \ or when small portions of the metal called 
 potassium, cast upon the surface of water, become as beads of most intense 
 
COMBUSTION. 313 
 
 light running about there, and crossing as in a merry dance ; or, lastly, 
 when flames produced from particular substances are seen rising deep-tinged 
 with most vived and beautiful colours. 
 
 Singularly interesting, then, to philosophers, as in such particulars the 
 phenomena of combustion must always have appeared, one may wonder 
 that its true nature could remain to them so long a mystery ; but until the 
 admirable researches of Davy, made only a few years ago, their conjectures 
 had scarcely approached the truth. An opinion long prevailed, that in every 
 combustible substance there was present a certain quantity of a something 
 denominated phlogiston, which, on being disengaged or separated, became 
 obvious to human sense as light and heat. The white oxide of zinc, for 
 instance, named the flowers of zinc, and into which the metal is changed by 
 burning, was supposed to be the metal deprived of its phlogiston; and when 
 on this oxide being heated with charcoal, the metal again appeared, it was 
 supposed simply to have recovered phlogiston from the charcoal. The illus- 
 trious Lavoisier had the merit of most clearly disproving this hypothesis, by 
 showing that the flowers or powders obtained from a metal by burning it, 
 were heavier than the piece of metal from which they were produced, by the 
 exact weight of the oxygen gas, which disappeared in the combustion, &c. ; 
 and he showed farther, that in this and many other cases, combustion was 
 merely the act of two substances combining chemically ; but he fell into an 
 error almost as great as that which he overthrew, by supposing that for com- 
 bustion, oxygen had always to be one of the combining substances, and that 
 the heat andlight given out in everycase had been previously latent in the 
 oxygen. 
 
 When Sir Humphrey Davy began his labours on the subject, than which 
 labours there is not, perhaps on record a more perfect specimen of scientific 
 research, it was already known, first, that bodies when compressed or by 
 any means reduced in bulk, generally give out a part of their heat, as when 
 air condensed in the match-syringe lights tinder, or when water and sul- 
 phuric acid, uniting into a compound of smaller volume than the separate 
 ingredients, becoming very hot, or when water, poured upon quick-lime to 
 slake it, and becoming solid with it, produces heat sufficient to inflame wood, 
 as has been fatally proved by the burning of many lime-loaded ships : and 
 that in such cases, the heat produced during the chemical union depended 
 more upon the energy of the action which united the substances than upon 
 the change of volume produced. 
 
 Farther, it was known that any substance having its temperature raised, 
 by whatever means, to 800 or more of Fahrenheit's thermometer, became 
 incandescent or luminous as when iron, or stone, or any substance not dis- 
 sipated by heat, is placed in a common fire; in the first degree the substance 
 being said to be red-hot, and at higher temperatures to be white-hot. 
 
 Out of these two truths Davy constructed his explanation. He asserted 
 that in any case, combustion is merely the appearance produced when sub- 
 stances, having a still stronger attraction for each other than quick-lime and 
 water, are, with intense energy, combining chemically, so as to become 
 heated at least to the degree of incandescence , and that during the pheno- 
 menon there is not, as was formerly supposed, something altogether con- 
 sumed or destroyed, or something called phlogiston escaping, but that the 
 substances concerned are only assuming a new form or arrangement. Thus, 
 if a piece of charcoal be enclosed in a glass vessel filled with air, of which 
 vessel the mouth dips into a liquid to confine the air, and if the charcoal be 
 then heated to a certain degree, by means of a burning-glass or otherwise, 
 
314 HEAT. 
 
 the cohesion of its particles yields to their attraction for the oxygen of the 
 air around them, and they immediately begin to combine with the oxygen so 
 energetically as to produce a heat still much greater, accompanied by the 
 light or incandescence of combustion. The charcoal, under these circum- 
 stances, soon entirely disappears, or is dissolved in the air, as sugar may be 
 dissolved in water ; but if the air be afterwards weighed, it is found to have 
 gained in weight the exact weight of the charcoal which has disappeared ; 
 and a chemist can again separate the charcoal from the air, and use either 
 for any purpose as before. In like manner, if a piece of iron wire be heated 
 at one end, which wr then plunged into a jar of oxygen gas, it will burn as a 
 most brilliant taper, and will gradually fall in the form of oxidized drops or 
 scales of iron, to the bottom of the vessel. Now during this process, the 
 quantity of oxygen will be diminished, but if the scales mentioned be col- 
 lected, they will be found to weigh just as much more than the original wire 
 expended, as there is of oxygen lost or combined with them. A chemist 
 can separate this iron and oxygen, and exhibit them apart as before, without 
 loss. Again, if iron and sulphur, in certain proportions, be heated together, 
 they unite with vivid combustion, but the product weighs exactly as much 
 as the original ingredients. 
 
 "While every instance of combustion is thus only a case of chemical union 
 going on with such intensity of action as to produce incandescence, still, 
 according to the nature of the substances combining, the appearance varies 
 much. It may be, for instance, with flame or without flame. The great 
 combining substance in nature, that is to* say, the most universally distributed, 
 is oxygen, of which the name is now become familiar even to the ears of the 
 unlearned. It forms four-fifths of the substance of water and one-fifth of our 
 atmosphere, being on the latter account present wherever man can be, and 
 ready to unite itself with any matter exposed to it at the necessary tempera- 
 ture. Now, of substances burning in air, those which are originally aeriform, 
 as coal-gas, or which on being heated, are rendered aeriform, or vapourized 
 before the union takes place as oil or wax, assume the appearance of flame 
 which means that the aeriform particles usually invisible are raised to the 
 incandescent temperature; but when the substance combining with the 
 oxygen remains solid, while its particles are gradually lifted away by the 
 oxygen acting only at the surface of the mass, it appears during the whole 
 time, only as a red hot stone. The latter is the case of charcoal, coke, Welch 
 stone-coal, &c., while in the case of wood, common coal, &c., a greater or 
 less portion of the inflammable matter is by the heat of the combustion con- 
 verted into vapour, and produces the beautiful appearance of flame. 
 
 Of the substances called combustible, and thus called because they combine 
 with oxygen so energetically as to become incandescent, there are only a 
 few, as the metals called potassium, sodium, &c., which will begin to unite 
 with oxygen, or to burn at the common temperature of our globe, the others 
 requiring to be at some higher temperature. Thus phosphorous begins to 
 burn at 150, sulphur at 550, charcoal at 750, hydrogen at 800, &c., it 
 appearing that up to these temperatures the attraction of the atoms of the 
 substances among themselves is sufficient to resist the other attraction or that 
 of oxygen. But when the combustion once begins, the temperature, from 
 the effect of the combustion itself, rises instantly much beyond the degree 
 necessary for the commencement of the process. Oxygen and hydrogen, 
 which begin to burn or combine at 800, produce a flame of as intense heat 
 as human art can excite. 
 
 On the circumstance that bodies require to have certain preparatory tern- 
 
COMBUSTION. 315 
 
 perature before beginning thus to combine with oxygen, depend many im- 
 portant facts of nature and art. Hence the safety with which most com- 
 bustibles may be exposed at ordinary temperatures to the contact of atmo- 
 spheric air : otherwise, coal, wood, &c., in the moment of being exposed to 
 the air, would catch fire, as really happens to phosphorated hydrogen gas ; 
 or to the metal called potassium, even when thrown into cold water, the 
 metal attracting the oxygen from the water instantly, and with intense com- 
 bustion. If a fire or flame be so small that, from the rapid absorption or 
 heat by bodies around, it does not produce heat enough to maintain the 
 inflaming temperature of the substance, the combustion will soon be extin- 
 guished. Thus a common coal fire, if not watched, and the remaining fuel 
 occasionally gathered together to reduce the surface of wasteful radiation, 
 will be extinguished long before the whole fuel is expended : but not so 
 with a fire of wood or of paper, which substances burn more readily than 
 coal. The Welch stone-coal can only be made to burn in very large masses, 
 or when mixed with a more inflammable coal or other fuel, or when fed by 
 air already heated. Some of our manufactures have lately been improved 
 by causing the air which feeds the working fire to pass through metal tubes 
 heated by lying in another fire. In common fires much coal is burned at 
 temperatures so low as to be nearly useless. A substance placed in pure 
 oxygen gas burns with much greater intensity, and will begin burning at a 
 lower temperature than if placed in atmospheric air, which contains only 
 one-fifth of oxygen and four-fifths of another substance, nitrogen, which 
 does not aid the combustion because, in the latter case, the nitrogen by 
 absorbing much of the heat of the combustion, lowers the temperature. Iron 
 wire will burn as a taper in oxygen, but not in common air; and a common 
 taper or flaming piece of wood just extinguished by blowing on it, will imme- 
 diately be rekindled if placed in oxygen. Again, a lamp with a very small 
 wick, as of one thread, and producing, therefore, very little heat, will not 
 burn in cold weather, or in an ice-house, and at any time will be extinguished 
 by a foreign body brought near it so as to cool it a piece of ice, for instance, 
 or a small metallic nob, presented to it on the end of a wire, or a metallic 
 ring let down over it ; but if the ball or ring be hot, the effect will not fol- 
 low. By more powerful refrigerating processes, even a considerable lamp or 
 candle may be put out. These discoveries led Davy to the construction of 
 his miner's safety-lamp, which is merely a lamp surrounded by a wire gauze, 
 of which the meshes are of such size that aflame of hydrogen gas attempting 
 to pass through is so cooled by the heated absorbing and heat-conducting 
 power of the metal as to be extinguished. A wire gauze gradually let down 
 upon any common flame, annihilates the part of it which should appear above 
 the gauze ; but the combustible vapour passing invisibly through the gauze, 
 may be lighted afresh on its upper side. Oxygen and hydrogen, which are 
 the constituents of water, when uniting, produce such intense heat that the 
 momentary expansion of the newly formed water, then in the state of steam, 
 is such as to constitute a violent explosion ; and when a jet of the two gases 
 mixed gives a continued flame, the most refractory substances melt in it like 
 wax in a common taper yet these gases may be kept mixed together in the 
 cold reservoir of a blow pipe without combining, and when they are set on 
 fire, while issuing as a jet, the flame does not travel inwards through the 
 opening, as might be feared, because it is cooled by the metal of the orifice. 
 A mixture of oxygen and hydrogen passing through the small channels left 
 in a tube filled with fire, may be lighted at the end of the tube without 
 danger of explosion. 
 
316 HEAT. 
 
 While solid bodies become very visible or incandescent at about 100 of 
 Fahrenheit, airs, owing to their tenuity of condition, require to be heated 
 much farther before they take on the vivid appearance of flame ; and airs of 
 light atoms, like hydrogen, require to be heated still more than heavier airs. 
 Thus the flame of pure hydrogen is pale and blue, but a wire held in it be- 
 comes much more luminous than the flame itself; and the flame of mixed 
 oxygen and hydrogen escaping from a very minute orifice in a glass tube, 
 may itself be scarcely visible, while the extremity of the tube heated by it 
 becomes like a brilliant star. Hence the light of many flames may be in- 
 creased by placing a wire gauze or other solid body in the flame ; as is seen 
 when a piece of lime is placed in a flame of mixed oxygen and hydrogen. 
 Consideration of this subject enables us to explain why common coal gas, 
 which consists of hydrogen holding a quantity of carbon in solution, gives a 
 stronger light than pure hydrogen ; and why oil gas, which contains about 
 twice as much carbon as the coal gas, gives also about twice as much light; 
 or it appears that the atmospheric air, which first mixes with these gases 
 as they issue to burn, is sufficient to combine with all their hydrogen (which 
 it most strongly attracts) but not at the same time with all their carbon : 
 the particles of the carbon, therefore, are first separated or precipitated in 
 the flame, and become so many solid particles most intensely heated and 
 luminous ; and afterwards, when they have ascended a little higher, they 
 meet with new oxygen and burn in their turn, giving a second quantity of 
 light. That this decomposition of the gas really occurs is proved by placing 
 a wire gauze in the flame, for then we find that if it be held near the middle 
 of the flame, it becomes immediately loaded with particles of charcoal sepa- 
 rated there, and cooled by it so as to cohere ; while if held at the bottom of 
 the flame where the carbon is not yet separated, it retains none, and if held 
 at the top of the flame, where they are already burned, it retains none. A 
 candle or lamp is said to smoke when the heat produced by it, or the quan- 
 tity of oxygen allowed to approach the flame, is not sufficient to effect the 
 total combustion of the carbon which rises in the flame. The hollow or cir- 
 cular wick of the common Argand lamp, and the similar form given to gas 
 flames is useful, by admitting air to the inside as well as to the outside of 
 the flame, and the lofty glass chimney is to quicken the currents of air. 
 
 When oxygen mixed with certain of the inflammable gases or vapours is 
 heated, although only to a temperature considerably below that of common 
 burning or explosion, a union still takes place, but very slowly, so that the 
 temperature never rises to what is necessary to exhibit flame. This pheno- 
 menon has been called invisible combustion. It is remarkably exemplified 
 on plunging plantinum or gold wire moderately heated into such a mixture : 
 the combination then goes on in the immediate vicinity of the r hot wire ; 
 and although without flame, still with sufficient disengagement of heat to 
 maintain the wire in an incandescent or luminous state, so long as there are 
 gases left to combine. Thus the vapour always arising at a common tem- 
 perature from the mouth of a phial of ether, (ether consists chiefly of hydrogen 
 and carbon), if made to pass through a coil of heated plantinum wire, while 
 combining, by this slow combustion, with the oxygen of the air around it, 
 gives out heat enough to keep the wire so luminous as to serve as a little lamp 
 by which to read from the dial-plate of a watch through the night. A beau- 
 tiful modification of this principle has been adopted in the miner's safety- 
 lamp ; and when the air of the mine is too impure to maintain the flame, it 
 still suffices thus to produce a continued light from the incandescent metal. 
 

 
 FUEL. 31T 
 
 "fuel" 
 
 Heat being, in the sense already explained, the life of the universe, and 
 man having command over nature chiefly by his power of controlling heat, 
 which power again comes to him with the ability to produce combustion, it 
 is of great interest to inquire what substances he can most easily procure as 
 food for combustion, or fuel, and how these may be most advantageously em- 
 ployed. To speak on this subject at all fully in reference to the various arts 
 of life would be to compose an extensive work, but an interesting sketch 
 may be comprised within narrow limits. 
 
 Although there are a great number of substances, which, in the act of 
 their chemical union, occasion the heat and light which constitute the com- 
 bustion, still by far the greater part of these, in an uncombined state, are so 
 sparingly distributed in nature, and are therefore procurable with such diffi- 
 culty, that heat obtained by sacrificing them would be much too expensive 
 to be within common means. Providence, however, has willed that the ele- 
 mentary substance in nature which has the most energetic attraction for 
 almost all other substances, and which therefore produces in uniting with 
 them the most intense heat, is also of all the most universally distributed. 
 This substance is oxygen. It forms part of our atmosphere, and therefore 
 penetrates, and is present wherever man can breathe, offering itself at once 
 at his service. Then for the purpose of combining with the oxygen, there 
 are chiefly two other substances also very widely scattered, and therefore 
 easily procurable and cheap. These are carbon and hydrogen, the great ma- 
 terials of all vegetable bodies, and therefore of our forest trees, and of coal 
 beds, many of which are evidently the remains of antediluvian forests. Car- 
 bon is found nearly alone in hard ctrnl, but it is united with a large proportion 
 of hydrogen in caking coal, and in wood, wax, resins, tallow and oils. The 
 gases obtained from these last-mentioned substances and now used for illu- 
 mination are merely hydrogen, holding in solution certain proportions of 
 carbon ; and all bodies which burn with flame give out such gases in the act 
 of combustion. In the mass of the earth, as far as known to man, the 
 stones, earths and water, forming its surface, are already combinations of 
 oxygen with other substances, and are therefore not in a state to produce 
 fresh combustion ; but carbon and hydrogen, by various processes of vege- 
 table and animal life, are in numberless situations becoming accumulated, 
 so as to be fit for fuel ; as by other processes the atmosphere is always 
 preserved with its due proportion of oxygen. 
 
 The name fuel is given only to the substances which combine with oxygen, 
 and not to the oxygen itself, probably because the former being solid or 
 liquid, and therefore more obvious to sense, had attracted human notice as 
 producers of combustion long before the existence of the aeriform, agent, 
 oxygen, was even suspected. 
 
 Oils, fat, wax, &c., from becomin^eriform in their combustion, exhibit 
 the appearance of flame, as already explained, and hence these substances are 
 chiefly used for the purpose of giving light ; while wood and coal are more 
 frequently used for mere heating. But the chemist's lamp, by which he 
 distills and evaporates, and his common blow-pipe for directing the point of 
 any flame upon a substance to melt it, and his blow-pipe fed with mixed 
 oxygen and hydrogen, whose flame is capable of melting the most refractory 
 substances prove that it is chiefly the expense of the former kinds of fuel 
 which has limited them so much to the office of light-giving. Lately an 
 
318 HEAT. 
 
 
 
 important application of coal-gas and of oil or fat as heat-giving fuel has 
 been made in a general cookiDg apparatus, which promises to effect a con- 
 siderable diminution of house-keeping expense. 
 
 Wood was the common fuel of the early world when coal-mines were not 
 yet known, and still in many countries it is so abundant as to be the cheap- 
 est fuel. Charcoal is the name given to what remains of wood after it has 
 been heated in a close place; during which operation the hydrogen and other 
 minor ingredients are driven away in the form of vapour. Charcoal is nearly 
 pure carbon. Coke, again, is the charcoal obtained by a similar process from 
 coal. The wood and coal, if much heated in the air, would burn or combine 
 with the oxygen of the air ; but heated in a vessel or place which excludes 
 air, they merely give out their more volatile parts. 
 
 Good coal, where it abounds, is now for ordinary purposes by much the 
 cheapest kind of fuel ; and since within, a few years men have learned to 
 separate from it, and to use, instead of oil and wax, its illuminating gas, 
 namely, its hydrogen holding in solution a little carbon, it has become dou- 
 bly precious to them. A person reflecting that* heat is the magic power 
 which vivifies nature, and that coal is what best gives heat for the endless 
 purposes of human society, cannot without admiration think of the rich stores 
 of coal which exists treasured up in the bowels of the earth for man's use. 
 And Britain, in this respect, is singularly favored. Her extensive coal mines 
 are in effect mines of latent labour or power vastly more precious than the 
 mines of gold and silver in Peru, for a hundred men in England, of whom 
 a part dig coal, and the remainder apply it in manufactures, can get in ex- 
 change for the merchandize they produce more of either gold or silver than 
 a hundred men working in the American mines are able to extract. These 
 coal-mines, therefore, may be said to produce abundantly every thing which 
 labour and ingenuity can produce, or \fhich our money can buy, and they 
 have essentially contributed to render Britain the mistress of the industry 
 and commerce of the earth. Britain has become to the civilized world around, 
 nearly what an ordinary town is to the rural district in which it stands, and 
 of this vast and glorious city the mines in question are the coal-cellars, 
 stored at the present rate of consumption for above 1,000 years; a supply 
 which, as coming improvements in the arts of life will naturally bring 
 economy of fuel, or substitution of other means to effect similar purposes or 
 changes in the channels of industry, may be regarded as exhaustless. 
 
 The coal in many mines is evidently the remains of forests, which have 
 existed in very remote time, swept together either during convulsions of 
 nature or by the more gradual operation of rivers, (as may be seen at present 
 where the great Mississippi is carrying an uninterrupted stream of floating 
 timber to the sea, and depositing it at the bottom of the Mexican gulf) the 
 accumulated wood being afterwards compressed and solidified by superin- 
 cumbent deposit sof earthy matters, aided probably by the action of heat. In 
 many coal-beds the trees yet retain their form, so that their species can be 
 easily distinguished, and there are buried among them other vegetable and 
 animal remains of cotemporaneous inhabitants of the earth. Coal is found 
 'of different qualities. In some places it is almost unmixed carbon, ^nd 
 exceeding solid, as if it had been coked by subterranean heat. Such is 
 the stone-coal of Wales, which in 100 parts contains 97 of pure carbon, with 
 only three of hydrogen and earthy matter. In other places the coal con- 
 tains hydrogen in nearly as large proportion as wood does, so combined with 
 part of the carbon as to form the oily or pitchy substances existing in the 
 
FUEL. 319 
 
 coal which, when burning, produce flame, and when rising unburned, con- 
 stitute smoke. 
 
 The comparative values, as fuel, of different kinds of carbonaceous matter, 
 have been found to be as in the following tables. 
 
 One pound of Melts of ice 
 
 Good coal ----- 90 fbs. 
 
 Coke 84 
 
 Charcoal of Wood - - 95 
 
 Wood 32 
 
 Peat ----- 19 
 
 Lavoisier, in making experiments on combustibles generally, to ascertain 
 the quantities of oxygen expended, and of heat given out during the combus- 
 tion of a given quantity of each, obtained the following results : 
 
 One pound of Melts of ice Takes of oxygen 
 
 Hydogen gas 370 Ibs. 7J Ibs. 
 
 Carburetted hydrogen - 85 4 
 
 Olive oil --- 120 3 
 
 Wax --- - 110 3 
 
 Tallow - - 105 3 
 
 Charcoal ... 95 2f 
 
 Phosphorus- - 100 H 
 
 Sulphur - 25 1 
 
 The following remarks with respect to fuel, and the modes of using it, seem 
 to demand a place here. 
 
 A pound of coke produces nearly as much heat as a pound of coal ; but 
 a pound of coke is the produce of a pound and a third of coal, although the 
 coke is more bulky than the coal. 
 
 It is wasteful to wet fuel, because the moisture in being evaporated carries 
 off with it as latent, and therefore useless heat, a considerable proportion of 
 what the combustion produces. It is a very common prejudice, that the 
 wetting of coal, by making it last longer, is effecting a great saving ; but it 
 does so merely by restraining the combustion or producing a smaller fire, and 
 with the bad fire, there is also much waste of heat. To illustrate the influ- 
 ence of watery vapour upon combustion, we may mention the fact, that a 
 manufacturer who tried to blow his fire by forcing steam into the furnace 
 with the air, extinguished his fire ; and the analogous fact, that ordinary fires 
 burn better in winter than in summer, although the temperature be lower, 
 because cold air is generally much drier than that which is warm. 
 
 Coal which contains much hydrogen, as all flaming coal does, is used 
 wastefully when any of the hydrogen is allowed to escape unburned ; for, 
 first, the great heat which the combustion of such hydrogen would produce 
 is not obtained ; and, secondly, the hydrogen while becoming gas, absorbs 
 into the latent state still more heat than an equal weight of water would. 
 Now the smoke of a fire is the hydrogen of the coal rising in combination 
 with a portion of carbon. We see, therefore, that by causing a fire to 
 destroy or burn its smoke, we not only prevent a nuisance, but effect a great 
 saving. The reason that common fires give out so much smoke is, either 
 that the smoke or vaporized pitch is not sufficiently heated to burn ; or that 
 
320 HEAT. 
 
 the air mixed with it as it ascends in the chimney, has already, by passing 
 through the fire, been deprived of its free oxygen. If the pitch be very 
 much heated, its ingredients assume a new arrangement, becoming transpa- 
 rent, and constituting the common coal-gas of our lamps ; but at lower tem- 
 peratures, it is seen jetting, from cracks or openings in the coal, as a dense 
 smoke a smoke, however, which immediately becomes a brilliant flame if 
 lighted by a* piece of burning paper or the approximation of the combustion. 
 The alternate bursting out and extinction of these burning jets of pitchy 
 vapour, contribute to render a common fire the lively and agreeable object 
 which it is in the winter ^evenings. When coal is first thrown upon a fire, 
 a great quantity of vapourized pitch escapes as a dense smoke, too cold to 
 burn, and for a time the flame is smothered, or there is none ; but as the 
 fresh coal becomes heated, its vapour reproduces the flame as before. In 
 close fire-places, as those of steam-engines, brewing and dyeing apparatus, 
 &c., all the air which enters after the furnace-door is shut, must pass through 
 the grate and the burning fuel, so that its oxygen is consumed by the red 
 hot coal before ascending to where the smoke is. The smoke, therefore, 
 however hot, passes away unburnt; unless, sometimes, as over foundry fur- 
 naces, where the heat is very great indeed, and it burns as a flame or great 
 gas lamp at the chimney-top on reaching the oxygen of the open atmosphere. 
 There have been many modes proposed of destroying smoke : one has 
 been to admit to the space above the fire, by a suitable opening, a certain 
 quantity of fresh air, the oxygen of which may inflame the smoke. This 
 plan is efficient at a certain point of time after the addition of fresh fuel, 
 and then for a time effects the saving of fuel ; but the difficulty of admitting 
 just the quantity of air required to suit the varying demand for it, has not 
 been overcome, and hence, from there being no saving on the whole, the^ 
 plan has been abandoned. When just enough air entered, the flame pro- 
 duced gave so intense a heat as in several cases to have burned or destroyed 
 the parts of valuable boilers exposed to it ; and when, on the contrary, too 
 much air entered, it injuriously cooled the boiler. A contrivance at present 
 commonly adopted for burning smoke, is that* of Mr. Brunton, namely, a 
 circular fire-grate, kept turning like a horizontal wheel, on which, by ma- 
 chinery, coal is made to fall in a gradual manner, so as to be uniformly 
 spread over it. The coal falls so gradually, that although there is generally 
 a 'little smoke from it, there is never much, the oxygen which finds en- 
 trance through and around the grate, being always in quantity the same, 
 and nearly sufficient. A smoke-consuming fire would be perfect, in which 
 the fuel were made to burn only at the upper surface of its mass, so that 
 the pitch and gas disengaged from it, as the heat spread downwards, might 
 have to pass through the burning coals where fresh air were mixing with 
 them ; and thus the gas and smoke, being the most inflammable parts, 
 would burn first and be all consumed. This was the principle proposed 
 in a fire-place suggested by the author for the great brewery of Mr. Meux 
 in his neighbourhood, and tried at the time when attempts were extensively 
 made to abate the nuisance of srnoke in towns. The experiment proved 
 the theoretical perfection of the method, and that it would produce a saving 
 of 15 or 20 per cent, on the expenditure of coal ; but before a durable grate 
 of the kind was completed, the Welch stone-coal was introduced, which 
 has 97 per cent, of pure carbon, and therefore has no pitch to evaporate, 
 and no smoke, and it was at once adopted there and in many other places. 
 A little common coal is added to it to make it more easily combustible. 
 
cl 
 
 FUEL. 321 
 
 Coal in a deep narrow trough, as a b c d, if 
 lighted at its surface a b } burns with a lofty Fig. 136. 
 
 flame as if it were the wick of a large oil- 
 lamp ; for all the gas given out from the coal 
 below, as that is gradually heated, passes 
 through the burning fuel and becomes a flame. 
 Now, if we suppose several such troughs 
 placed together with intervals between them, 
 like the fire bars of a common grate or furnace, 
 there would be a perfect non-smoking fire- 
 place. Such was that made on the occasion 
 
 mentioned ; and although, as a mere experimental apparatus, it was flimsy 
 and imperfect, it put beyond a doubt the possibility of accomplishing its 
 object. The reason of the vast saving of fuel by such a grate is, that the 
 smoke instead of stealing away latent heat being itself also the most com- 
 bustible and precious part of the fuel, here gives all its powers and worth to 
 the purpose of the combustion. The coal rested on moveable bottoms in 
 the troughs (here shown by dotted lines) and was moved up like the wick 
 of a lamp by its screw. The bottoms may be lifted in many ways. Or, 
 without bottoms the coal may be raised in each trough successively by an 
 iron lever or bar introduced by the fire-man under the coal, fresh coal being 
 at the same time put into the trough to fill the space under the lever. The 
 author believes that this principle of construction will still be extensively 
 adopted for the Newcastle or flaming coal, the consequences would be so 
 important. The principle has been already introduced for common parlour 
 fires by Mr. Cuttler in his stove, which is merely a common grate, having, 
 instead of bottom-bars, a deep box to hold the coal for a whole day, with a 
 moveable bottom, which lifts the coal up as wanted. From such a fire there 
 is always ascending a long beautiful flame ; and much more heat is given 
 out than from the same quantity of coal burned in the common way ; the 
 chimney never requires sweeping, for there is absolutely no smoke and there- 
 fore no soot. 
 
 Smoke is effectually consumed also in a fire-place in which the air feeding 
 the combustion is caused to pass downwards through the burning fuel, so as 
 to carry the smoke with it, instead of upwards as usual. This result is at- 
 tained by having the chimney to communicate with the ash-pit. The chief 
 objection is, that the fire-bars are quickly destroyed by the intense heat to 
 which they are exposed ; and to obviate this instead of solid bars, tubes 
 have been used with water passing through them, and admitting -the feeding 
 air only above. 
 
 It is evident that if a house or apartment with the air in it, were once 
 warmed to a certain degree, it would for ever retain its temperature, but for 
 the escape of heat through the walls and windows, or with the air from 
 within, whether passing away as necessary ventilation or as waste. A per- 
 fect system of heating, therefore, would consist in diminishing as much as 
 possible these causes of loss, with reference, however, both to the expense 
 of the means and the salubrity of the dwelling, and in producing and distri- 
 buting the heat judiciously. It may be asserted that a fourth part of the fuel 
 generally expanded in English houses, if more skilfully used, would better 
 secure comfort and health than all that is now expanded. But it does not 
 accord with the character of this general work to enter into minute detail on 
 the subject. Remarks were made upon it in the chapter on " Pneumatics," 
 
 21 
 
322 HEAT. 
 
 under the head of " warming and ventilating/' and more minute informa- 
 . tion may be obtained from Mr. Tredgold's work, expressly devoted to it. 
 
 The detailed consideration of furnaces, blow-pipes, &c., may appear to 
 some so clo?ely connected with our present subject as to demand a place 
 here, but by so treating of them we should be encroaching on the province 
 of the chemist, &c. We may state generally, that furnaces are merely struc- 
 tures by which coal or other fuel heated to the degree at which it combines 
 rapidly with the oxygen of the atmospheric air, is placed in circumstances 
 favourable to the rapid renewal of the air, and a common blow-pipe is 
 merely a jet of air thrown from a minute opening into any flame, so as with 
 great decision to direct the point- of the flame upon the body to be heated. 
 The sand-bath and water-bath of the chemist are merely means of insuring 
 a^more uniform or steady temperature : a vessel imbedded in sand, so that 
 heat can reach it only through the sand, cannot be very suddenly heated or 
 cooled, because sand is a slow conductor , and a vessel immersed in boiling 
 water, can never have greater heat than 212, or the boiling heat of water. 
 For certain purposes, hotter baths, as of high-pressure steam, or of solutions 
 of certain salts, or of vapour of oil of turpentine, or of boiling whale oil, have 
 been used. On such subjects, readers may consult works on " chemistry 
 applied to the arts." 
 
 t( Condensation and friction as causes of heat" (Read Analysis, page 256.) 
 
 A soft iron-nail laid upon an anvil, and receiving in rapid succession three 
 or four powerful blows of a hammer becomes hot enough to light a match, 
 and if longer hammered, will become incandescent or red hot, parti v from 
 the diminished volume or condensation of the iron, on the principle already 
 explained, and partly from the percussion or friction, in a way not yet well 
 understood, but probably electrical 
 
 In the familiar case of the mutual percussion of flint and steel, small por- 
 tions of one or both are struck off by the violence of the collision, in a state 
 of white heat, and the particles of the iron burn in passing through the air j 
 in a vacuum, the heated particles are equally produced, but are scarcely 
 visible from this combustion not occurring. In both cases, they suffice to 
 inflame gunpowder, or to light tinder. When the materials are good the 
 shower or burst of sparks from the sudden blow is most brilliant. 
 
 The heat produced by friction alone without perceivable condensation of 
 the bodies concerned, is exemplified in many facts. Two dry branches of a 
 tree kept strongly rubbing against each other by the wind, have sometimes 
 set a wood on fire. Savages light their fires by analogous means. Men warm 
 their cold hands in winter, by rubbing them against each other, or against 
 their coat sleeves. Again, the axle-tree of a heavily laden wagon or other 
 carriage, if left without oil, is often heated so as to inflame wood near it. 
 The line attached to a whale-harpoon as it runs over the side of the boat 
 when the whale dives after the harpoon has entered his flesh, requires that 
 water be constantly thrown upon it to prevent its setting fire to the boat. 
 The cable of a ship drawn very rapidly through the hawse-hole by the falling 
 anchor produces there intense heat and smoke. When a great ship is 
 launched from the builder's yard into the deep, and glides along the 
 sloping beams, a dense smoke usually rises from the points of rubbing 
 contact. 
 
ANIMAL FUNCTIONS A SOURCE OF HEAT. 323 
 
 " The functions of animal life a source of heat." (Read the Analysis. 
 
 p. 256.) 
 
 It is one of the remarkable facts in nature, that living animal bodies, and 
 to a certain degree living vegetables also, have the property of maintaining 
 in themselves a peculiar temperature, whether surrounded by bodies that are 
 hotter or colder than they. Captain Parry's sailors, during the polar winter, 
 where they were breathing air that froze mercury, still had in them their 
 natural warmth of 98 of Fahrenheit : and the inhabitants of India, where 
 the same thermometer stands sometimes at 115 in the shade have their 
 blood at no higher temperature. 
 
 It was at one time the favourite explanation, of this, that animal heat was 
 produced in the lungs, during respiration, from the oxygen then admitted. 
 This oxygen combines with carbon from the blood, and becomes carbonic 
 acid as in combustion, and it was supposed to give out a portion of its latent 
 heat, as in actual combustion ; which heat, being then spread o\er the body 
 by the circulating blood, maintained the temperature. We now know that 
 if such a process assist, which it probably does, for the animal heat has 
 generally a relation to the quantity of oxygen required in any particular 
 case, and when an animal being already much heated needs no increase, 
 very little oxygen disappears, still much of the effect is dependent on 
 the influence of the nerves either directly or indirectly, through the vital 
 functions governed by them. Mr. Brodie, in his important experiments 
 upon the subject found that, although in animals apparently dead from injury 
 done to the nervous system, he could artificially continue the action of res- 
 piration, with the usual formation of carbonic acid, still the temperature fell 
 very quickly. Again, the maintenance of low temperature in an animal 
 immersed in air hotter than itself, is partly attributable to the copious per- 
 spiration and evaporation which then take place, and which absorb into the 
 latent form the excess of heat then existing. Perspiration, both from the 
 skin and internal surface of the lungs, occurs generaly in proportion to the 
 excess of heat. Dogs and other animals when much heated, as they cannot 
 throw oif or diminish their natural covering, increase the evaporating surface 
 by protruding a long, humid tongue. 
 
 The power in animals of preserving their peculiar temperature has its 
 limits. Intense ctld coming suddenly upon a man who has not sufficient 
 protection, first causes a sensation of pain, and then brings on an almost 
 irresistible sleepiness, which if indulged in proves fatal. Sir Joseph Banks 
 -having gone on shore one day near the cold Cape Horn, and being fatigued, 
 was so overcome by the feeling mentioned, that he entreated his companions 
 to let him sleep but for a little while. His prayer, if granted, might have 
 allowed to come upon him that sleep which ends not as, under similar 
 circumstances, it came upon so many thousands of the army which Buona- 
 parte led into Russia, and lost there during the disastrous retreat through 
 the snows. The celebrated bulletin which allowed that, in one night, when 
 the thermometer of Reaumur stood at 19 below zero, 30,000 horses died, 
 declared not the number of human victims tenderly loved husbands and 
 brothers, and children of thousands anxiously waiting their return, but doomed 
 never to see them more. Cold in inferior degrees, and longer continued, 
 acting on persons imperfectly protected by clothing, &c., induces a variety 
 of diseases which destroy more slowly ; as many of the winter diseases of 
 England. A great excess of heat ; again ; may at once excite a fatal appo- 
 
324 HEAT. 
 
 plexy, and heat in inferior degrees, "but long continued, may cause those 
 fevers, &c., which prevail in warm climates, and which are so destructive to 
 strangers. 
 
 Each species of animal "has a temperature natural to it, and in the diversity 
 are found creatures fitted to live in all parts of the earth, what is wanting in 
 internal bodily constitution being found in the admirable covering which has 
 been provided to protect them covering which grows from their bodies, 
 with form of fur or feather, in the exact degree required, and even so as in 
 the same animal to vary with climate and season. Such covering, however, 
 has been denied to man; but the denial is not one of unkindness, it is an 
 indication of his superior nature and destinies. God-like reason was be- 
 stowed on him, by which he subjects all nature to his use, and he was left 
 to clothe himself. 
 
 The human race is naturally inhabitant of a warm climate, and the para- 
 dise described as Adam's first abode, may be said still to exist over vast 
 regions about the equator. There the sun's influence is strong and uniform, 
 producing a*rich and warm garden, in which human beings, however igno- 
 rant of the world which they had come to inhabit, would have their necessi- 
 ties at once supplied. The ripe fruit is there always hanging from the 
 branches; of clothing there is required only what moral feelings may dictate, 
 or what may be supposed to add grace to the form ; and as shelter from the 
 weather, a few broad leaves spread on connected reeds, complete the Indian 
 hut. The human family, in multiplying and spreading in all directions from 
 such a centre, would find to the east and west, only the lengthened paradise, 
 with slightly varying features of beauty ; but to the north and south, the 
 changes of season, which make the bee of high lattitudes lay up its winter 
 store of honey, and send migrating birds from country to country in search 
 of warmth and food, would also rouse man's energies to protect himself. 
 His faculties of foresight and contrivance would come into play, awakening 
 industry : and as their fruits, he would soon possess the knowledge and the 
 arts which secure to him a happy existence in all climates, from the equator 
 almost to the pole. And it is chiefly because man has learned to produce at 
 will, and to control, the wonder-working principle of heat, that in the rude 
 winter, which seems the death of nature, he, and other tropical animals and 
 plants which he protects, do not in reality perish as exemplified when a 
 Canary bird escapes from its cage, or an infant is exposed among the snow- 
 hills. By producing heat from his fire, he then obtains a novel but most 
 pleasurable sort of existence : and in the night, while the dark and freezing 
 winds are howling over his roof, he basks in the presence of his mimic sun, 
 surrounded by his friends and all the delights of society ; while in his store- 
 rooms, or in those of merchants at his command, he has the treasured deli- 
 cacies of every season and clime. He soon becomes aware, too, that the 
 dreary winter, instead of being a curse, is really in many respects a bleseing, 
 by arousing from the apathy to which the eternal serenity of a tropical sky 
 so much disposes. He sees that in climates where labour and ingenuity 
 must precede enjoyment, every faculty of mind and body is invigorated ; and 
 that hence the sterner climates produce the perfect man ; that in them the 
 arts and sciences have reached their present advancement, and the brightest 
 examples have arisen of intellectual and moral excellence : while from them, 
 as centres, knowledge and example are spreading over all the earth and 
 promising soon to render the whole of human kind but one large and happy 
 family. 
 
LIGHT. 325 
 
 
 
 
 
 PART IV. 
 
 (CONTINUED.) 
 SECTION II. OX LIGHT, OR OPTICS. 
 
 ANALYSIS OF THE SECTION. 
 
 Light is an emanation from the sun and other self-luminous bodies, becom- 
 ing less intense as it spreads, and which, I y falling on other bodies, and 
 being reflected from them to the eye, renders them visible. Its absence is 
 called darkness. It moves with great velocity, and in straight lines 
 where there is no obstacle leaving shadows ichere it cannot fall. It 
 passes readily through some bodies which are therefore called transpa- 
 rent but when it enters or leaves their surfaces obliquely, it suffers, at 
 them a degree of bending or refraction proportioned to the obliquity. 
 And a beam of white light thus refracted or bent, does not all bend equally, 
 but is divided or resolved into beams of what are called the elementary 
 colours, which colours, on being again blended, become the white light as 
 before. 
 
 Transparent bodies, as glass, may be made of such form as, by the powers 
 of refraction thence received, to cause all the rays which pass through 
 them from any given point to bend and meet again in another point 
 beyond them ; the body, then, because usually in form somewhat resem- 
 bling a flat bean or lentil, being called a LENS. And when the light thus 
 proceeding from every point of an object placed before a lens is collected 
 at corresponding points behind it, a perfect image of the object is there 
 produced, which may be seen from any situation on a white screen placed 
 to receive it, or in the air, if viewed from behind. Now the most important 
 optical instruments, and even the living eye, are merely various arrange- 
 ments af parts for producing and examining such images as now described. 
 When this image is received upon a suitable white surface or screen in a 
 dark room, the arrangement is called, according to minor circumstances, 
 a CAMERA OBSCURA, a MAGIC LANTERN, or a SOLAR MICROSCOPE. And 
 the EYE itself is, in fact, but a small camera obscura, enabling the mind 
 to judge of external objects, by the size, brightness, colour, &c., of the very 
 minute but most perfect images or pictures formed at its back part, on the 
 smooth screen of nerve called the retina. The art of painting aims at 
 producing on a larger scale such a picture as is formed on the retina, and 
 which, when afterwards held before the eye, and reproducing itself in mini- 
 ature upon the retin-i, may excite the same impression as the original 
 objects. When the image beyond a lens, formed as above described, is 
 viewed in the air, by looking at it from behind, that is, from a situation 
 where the light continued from it passes, then there is exhibited the arrange- 
 ment of parts constituting the, TELESCOPE or COMMON MICROSCOPE. 
 
326 LIGHT. 
 
 
 
 Li gh falling on very smooth or polished surfaces, is reflected so nearly in 
 the order in which it falls, as to appear to the eye receiving it as 'if coming 
 directly from the objects originally emitting it and such surfaces are 
 called mirrors. Mirrors may be plane , convex, or concave ; and certain 
 forms will concentrate light, to produce images by reflection, just as 
 lenses produce them by refraction ; so that there are reflecting telescopes, 
 &c., a9 there are refracting instruments of the same name. Light, again, 
 falling on bodies of rougher or irregular surface, or which have other 
 peculiarities^ is so modified as to produce all those phenomena of colour 
 and varied brightness seen among natural bodies, and giving them their 
 distinctive characters and beauty. 
 
 " Light:' (See the Analysis.) 
 
 The phenomena of light and vision have always been held to constitute a 
 most interesting branch of natural science ; whether in regard to the beauty 
 of light, or its utility. The beauty is seen spread over a varied landscape 
 among the beds of the flower-gardens, on the spangled meads, in the plumage 
 of birds, in the clouds around the rising and setting sun, in the circles of the 
 rainbow. And the utility may be judged of by the reflection, that if man 
 had been compelled to supply his wants by groping in utter a.nd unchangeable 
 darkness, even if originally created with all the knowledge now existing in 
 the world, he could scarcely have secured his existence for one day. Indeed, 
 without light, the earth would have been an unfit abode, even for grubs, 
 generated and living always amidst their food. Eternal night would have 
 been universal death. Light, then, while the beauteous garb of nature, 
 clothing the garden and the meadow glowing in the ruby sparkling in the 
 diamond, is also the absolutely necessary medium of communication between 
 living creatures and the universe around them. The rising sun is what con- 
 verts the wilderness of darkness which night covered, and which, to the 
 young mind not yet aware of the regularity of nature's changes, is so full of 
 horror, into a visible and lovely paradise ] no wonder, then, if in early ages 
 of the world, man has often been seen bending the knee before the glorious 
 luminary, and worshipping it as the God of Nature. When a mariner, who 
 has been toiling in midnight gloom and tempest, at last perceives the dawn 
 of day, or even the rising of the moon, the waves seem to him less lofty, 
 the wind is only half as fierce, and hope and gladness beam on him with the 
 light of heaven. A man, wherever placed in light, receives by the eye from 
 every object around, nay, from every point in every object and at every 
 moment of time, a messenger of light to tell him what is there, and in what 
 condition. Were he omnipresent, or had he the power of flitting from place 
 to place with the speed of the wind, he could scarcely be more promptly 
 informed. Then, in many cases where distance intervenes not, light can 
 impart at once knowledge, which, by any other conceivable means, could 
 come only tediously, or not at all. For example, when the illuminated coun. 
 tenance is revealing the secret workings of the heart, the tongue would in 
 vain try to speak, even in long phrases, what one smile of friendship or 
 affection can in an instant convey; and had there been no light, man never 
 could have suspected the existence of the miniature worlds of life and activity 
 which, even in a drop of water, the microscope discovers to him ; nor could 
 he have formed any idea of the admirable structure of many minute objects. 
 
EMANATION FROM THE SUN, ETC. 327 
 
 It is light, again, which gives the telegraph, by which men readily converse 
 from hill to hill, or across an extent of raging sea and it is light which, 
 pouring upon the eye through the optic tube, brings intelligence of events 
 passing in the remotest regions of space. 
 
 " Emanation from the sun," &c. (See the Analysis, page 325.) 
 \ 
 
 The relation of the sun to light is most strikingly marked in the contrast 
 between night and day. In tropical countries where the sun rises and sets 
 almost perpendicularly, and allows not the long dawn and twilight of tem- 
 perate latitudes, the change from perfect darkness to the overpowering efful- 
 gence of day, and the contrary change, are so sudden as to be most impres- 
 sive. An eye turned in the morning to the east has scarcely noted a com- 
 mencing brightness there, when that brightness has already become a glow; 
 and the clouds floating near so as to meet the upward rays, appear like 
 masses of gold fleece suspended in the sky : a little after the whole atmos- 
 phere is bright, and as the stream of light reaches the lofty mountain-tops, 
 it makes them shine like burnished pinnacles ; then as that stream descends 
 to lower and lower levels, the inhabitants in succession see the radiant orb 
 first rising above the horizon like a tip of flame, and soon displaying all its 
 breadth and glory, too bright for the eye to dwell upon. With evening the 
 same appearances recur in a reversed order, ending, as in the morning they 
 began by complete darkness. 
 
 Light emanates also from the stars, but they are so distant as in that re- 
 spect to be of little importance to this earth. And all bodies in combustion 
 are self-luminious, as exemplified in our common fires and lamps. And 
 there are still other transient sources in animal and vegetable nature, and 
 among solar phosphori. 
 
 There have been two opinions respecting the nature of light; one, that it 
 consists of extremely minute particles darting all around from theluminious 
 body ; the other, that the phenomenon is altogether dependent on an undula- 
 tion among the particles of a very subtile elastic fluid diffused through space 
 as sound is dependent on an undulation among air-particles. To admit the 
 first opinion, the particles of light must be held to be most wonderfully 
 minute, for a common taper can fill with them during hours a space of four 
 miles in diameter; and with the extreme velocity of light, if its particle pos- 
 sessed at all the property of matter called inertia, their momentum should be 
 very remarkable; yet, even a large sunbeam collected by a burning-glass, 
 and with the precautions necessary in the case, thrown upon the scale of a 
 most delicate balance, has sot the slightest effect upon the equilibrium. 
 Such, and many other facts to be treated of in subsequent parts of this work, 
 lead to the opinion that there is an undulation of an elastic fluid concerned 
 in producing the phenomenon of light, although the fact of light spreading 
 so nearly in straight lines, as if only the crown of the wave had existence, 
 instead of being diffused like sound, is an important difference. 
 
 " Becoming less intense as it spreads" (See the Analysis, page 325.) 
 
 Any emanation from a central point in spreading through water space, 
 becomes proportionally thinner or less intense. Thus, if a taper be placed 
 in the centre of a box, each side of which is a foot square, the light falling 
 on the sides of the box will have a certain intensity there : if the taper be 
 then placed in a box with sides of two feet square, there will be only the 
 
328 LIGHT. 
 
 same quantity of light, but it will be spread over four times the surface, (a 
 square of two feet is made up of four squares of one foot,) and will, therefore, 
 on any part of that surface, be only one-fourth part as strong or intense as 
 in" the first box : and so for any other size of box or space, the intensity 
 will diminish as the square of the distance increases. 
 
 Hence four times as much light and heat fall upon a foot of this earth's 
 surface as upon a foot of the surface of the planet Mars, which is twice as 
 distant from the sun : as four times as much light and heat fall on a man 
 who is at one yard from the fire, as on another who is distant two yards. 
 
 " Falling on other bodies makes them visible." (Read the Analysis, 
 
 page 325.) 
 
 If the window-shutter of an apartment be perfectly closed, an eye there 
 turns upon an absolute blank : it perceives nothing. 
 
 If a ray of the sun be then admitted, and made to fall upon any object, that 
 object becomes bright, and affects the eye as if it were itself luminious. It 
 returns a part of the light which falls upon it, and it is visible in all directions, 
 proving that it scatters the received light all around. This scattered light, 
 again falling on other objects, and reflected from and among them until ab- 
 sorbed, like echo repeated many times and lost between perpendicular rocks, 
 makes all of them visible, although in a less degree, and the whole apartment 
 is said to be lighted. If the sun's ray be made to fall upon a thing which, 
 from its nature, reflects much of the light, as a sheet of white paper, the 
 apartment will be well lighted : if, on the contrary, it be received on black 
 velvet, which returns hardly any light, the apartment will remain dark ; 
 and, again, if received on a polished surface or mirror, which returns nearly 
 the whole light, but in one direction only, and therefore throws it upon some 
 other single object, the effect will be according to the nature of that object, 
 and nearly as if the ray had fcillen directly upon it. 
 
 Now all bodies on earth, and among these the constituent particles of the 
 mass of atmosphere surrounding the earth, retain and diffuse among them- 
 selves, for a time, the light received directly from the sun, and by so doing, 
 maintain every where that milder radiance so agreeable to the sight, which 
 renders objects visible when the sun's direct ray does not fall upon them. 
 But for this fact indeed, all bodies shadowed from the sun, whether by in- 
 tervening clouds or by any other more opaque masses on earth, would be 
 perfectly black or dark; that is, totally invisible. And without an atmos- 
 phere, the sun would appear a round luminious mass in a perfectly black sky. 
 On lofty mountain summits, where half the atmosphere is below the level, the 
 direct rays of the sun are painfully intense, and the sky is of darkest blue. 
 
 A shadow is the name given to the comparative darkness of places or 
 objects which are prevented by intervening things from receiving the direct 
 rays of some luminious body shining on the things around. The apparent 
 darkness 'of a shadow, however, is not proportioned to its real darkness, 
 but the intensity of the surrounding lights. A landscape may be very 
 bright, even when the sun is veiled by a cloud, and then little or no shadow 
 is perceived ; but as soon as the cloud passes away, deep shadows are cast 
 behind or beyond every projecting object. Yet the objects and places then 
 appearing so dark, are in reality more illuminated than before the shadow 
 existed, for they are receiving, and again scattering new light from all the 
 more intensely illuminated objects around them. A finger held between a 
 candle and the wall casts a shadow of a certain intensity; if another candle 
 
LIGHT MOVES WITH GREAT VELOCITY. 329 
 
 be then placed in the same line from the shadow, the shadow will appear doubly 
 dark, although in fact more light will be reaching it and reaching the eye from 
 it than before ; it will be more dark only by comparison. If the candles be 
 separated laterally, so as to produce two shadows of the finger, but which 
 coincide or overlap in one part, that part will be of double darkness, as 
 compared with the remainder, The most accurate mode of comparing lights 
 is to place them at such distances from a screen or wall, as to make them at 
 the same time throw equally dark shadows of the same object ; and then 
 according to the law of decreasing intensity explained above, to calculate the 
 intensities of the sources of light by the difference of their distances from 
 the wall. The eye judges very easily of the equal intensity of compared 
 shadows of the same object. 
 
 The real darkness of a shadow, then, depends on the number and nature 
 of the light reflecting objects around it. Thus shadows are less remarkable 
 opposite to any white surface, as that of a recently painted wall, than in other 
 situations. The reason why the moon, when eclipsed, that is, as will be 
 afterwards explained, when passing behind the earth, or through the shadow 
 cast by the earth in a direction away from the sun, becomes almost, if not 
 quite invisible, is that there are no other moons or bodies bearing laterally on 
 the moon to share their light with it. And the reason why our nights on 
 earth are darker than the shadows behind a house or rock in the sunshine of 
 day, is merely that there are not other earths near us to reflect light into the 
 great night-shadow of the earth, as there are other houses and rocks to illu- 
 minate the day-shadow of these. The moon is the only light-reflecting body 
 which the earth has near it ; and we perceive how much less dark the earth's 
 night-shadow is when the moon is so placed as to bear upon it. The eclipsed 
 moon, again, is invisible to men on earth, because it receives neither sunshine 
 nor reflected light from this earth, for the side turned towards us faces the 
 shadowed part of the earth ; but when the moon is near the situation in 
 which it is called new moon, or between us and the sun, the shaded side of 
 the moon is then, in a degree, visible to us because facing the enlightened 
 side of the earth; the bright crescent, or part of the moon illuminated by 
 the sun, appearing to embrace the non-illuminated part, and giving occasion 
 to the popular saying, that the new moon holds the old moon in its arms. 
 
 Many persons have doubted whether the light of the moon could be 
 altogether reflected light of the sun ; the moon appearing to them more 
 luminous than any body on earth merely exposed to the sunshine. Their 
 error has arisen from contrasting the moon while returning direct sunshine 
 with the shadows of night on the earth around them. But could they at 
 night see on a hill near them, a white tower or other object scattering light 
 as when it receives the rays of the sun, that object being nearer than the 
 moon, would appear to them almost to be on fire, and much brighter than 
 the moon. The moon when above the horizon in the day time, is perfectly 
 visible on earth, and is then throwing towards the earth just as much light 
 as during the night; but the day moon does not appear more luminous than 
 any small white cloud : and although visible every day, except near the 
 change, many persons have passed their lives without ever observing it. The 
 full moon gives to the earth only about a one-hundred-thousandth part as 
 much light as the sun. 
 
 " Light moves with great velocity" (See Analysis, page 325.) 
 
 The extraordinary precision with which the astronomical skill of modern 
 days enables men to foretell the times of remarkable appearances or changes 
 
330 LIGHT. 
 
 among the heavenly bodies has served for the detection of the fact, that 
 light is not an instantaneous communication between distant objects and the 
 eye, as was formerly believed, but is a messenger which requires time to 
 travel : and the rate of travelling has been ascertained. 
 
 The eclipses of the satellites or moons of the planet Jupiter had been 
 carefully observed for some time, and a rule was obtained which foretold the 
 instants in all future time when the satellites were to glide into the shadow 
 of the planet and disappear, or when again to immerge into view. Now it 
 was found, that these appearances took place 16J minutes sooner when Ju- 
 piter was near the earth, or on the same side of the sun with the earth than 
 when it was on the other side, that is to say more distant from the earth 
 by one diameter on the earth's orbit and at all intermediate stations, the 
 difference diminished from the 16 2 minutes, in exact proportion to the less 
 distance from the earth. This proves, then, that light takes 16 minutes 
 to travel across the earth's orbit, and 81 minutes for half that distance, or 
 to come to us from the sun. 
 
 The velocity of light, ascertained in this way, is such, that in one second 
 of time, viz : during $ single vibration of a common clock pendulum, it 
 would go and come from London to Edinburgh 200 times, the distance 
 between these being 400 miles. This velocity is so surprising that the 
 philosophic Dr. Hooke, when it was first asserted that light was thus pro- 
 * gressive, said he could more easily believe the passage to be absolutely in- 
 stantaneous, even for any distance, than that there should be a progressive 
 moment so prodigiously rapid. The truth, however, is now put quite beyond 
 a doubt by any collateral facts bearing upon it. 
 
 As regards all phenomena upon earth, they may be considered as happen- 
 ing at the very instant when the eye perceives them ; the difference of time 
 being too small to be appreciated ; for, as shown in the preceding paragraph, 
 if our sight could reach from London to Edinburgh, we should perceive a 
 phenomenon there in the four-hundredth part of a second after its occurrence. 
 
 It is hence usual and not sensibly incorrect, when we are measuring the 
 velocity of sound, as when a cannon is fired, by observing the time between 
 the flash and the report, to suppose that the event takes places at the very 
 moment when it is perceived by the eye. 
 
 In using a telegraph, no sensible time is lost on account of light requiring 
 time to travel. A message can be sent from London to Portsmouth in a 
 minute and a half; and at the same rate a communication might pass to Rome 
 in about half an hour, to Constantinople in forty minutes, to Calcutta in a 
 few hours, and so on. A telegraph is any object that can be made to as- 
 sume different forms or appearances at the will of an attendant, and so that 
 the changes may be distinguished at a distance. A pole with movable arms 
 is the common construction, each position of the arms standing for a letter, 
 or cypher, or word, or sentence, as may be agreed upon. Telegraphic signals 
 between ships at sea are generally made by a few flags, the meanings of each 
 being varied by the mast on which it is hoisted, and by its combination with 
 others. 
 
 " Light proceeds in straight lines" &c. (Read the Analysis, page 325.) 
 
 "We have scarcely a clearer notion of a straight line than that received from 
 the direction in which light moves : but we can verify a line so obtained by 
 other means, as by stretching a cord between the two extremes, or by sus- 
 pending a weight by a cord, and making a moveable solid measure to corres- 
 pond with the cord, which standard may be used in any other case. 
 
LEAVING SHADOWS WHERE IT CANNOT FALL. 331 
 
 We can see through a straight tube, but not through a crooked one. The 
 vista through a long straight tunnel is striking as an illustration of this fact 
 and of the diminution of the apparent size of objects as they are more distant. 
 If a person enter one end of the canal tunnel two miles long, cut through the 
 chalk-hills near Rochester as part of the canal which joins the Thames and 
 Medway rivers, the opening at the distant end is seen as a minute luminous 
 speck, having the form of a general arch ; and a person who has advanced 
 half way through the tunnel may see the luminous speck at each end, then 
 appearing a little larger than in the former case. 
 
 In taking aim with gun or arrow, we are merely trying to make the pro- 
 jectile go to the desired object nearly by the path along which the light comes 
 from the object to the eye. 
 
 A carpenter looks along the edge of a plank, &c., to see whether it be 
 straight. 
 
 Because light moves in straight lines, if a number of similar objects be 
 placed in a row from the eye, the nearest one hides the others. In a wood 
 or city, a person sees only the trees or houses that are next him. 
 
 He who believes that a squinting person can see round a corner, may also 
 believe that a crooked gun can shoot round a corner. 
 
 All astronomical and trigonometrical observations are made on the faith of 
 this property of light, the observer holding that any object is situated from 
 him in the direction in which the light comes to him from it. When the 
 mariner, after watching for hours in cloudy weather has caught a glimpse of 
 the sun or star through his sextant-glass, he has ascertained his place among 
 the trackless waves, and boldly advances through the mist of hidden dan- 
 gers. And the beam darting from the light-house across the stormy sea, 
 would be useless if the light moved not in a straight line. 
 
 "Leaving shadows where it cannot fall." (See the Analysis, page 325.) 
 
 The form of shadows proves that light moves in straight lines, for the out- 
 line of the shadow is always correctly that of the object as seen from the 
 luminous body. If the light bent round the body, this could not be. 
 
 The shadow of a face on the wall is a correct profile. 
 
 As a wheel presented edgeways to the eye appears only like a broad line, 
 becomes oval or round as it is more turned, so a wheel presented edgeways 
 to the sun or other light casts a linear shadow on the wall behind it, the 
 shadow becoming oval or round as the position is changed. 
 
 A globe, a cylinder, a cone, and a flat circle, will all throw the same round 
 shadow if held with their axes pointing to the luminous body, and, there- 
 fore, by the shadow only, these objects could not be distinguished. 
 
 The figure of a rabbit cut in paste-board, will throw the same shadow on 
 the wall as the animal itself; and, again, that shadow may be well imitated 
 by a certain position of the two hands joined, as is known to those who find 
 pleasure in witnessing the surprise and delight of a child who beholds such 
 a shadow made to mimic the actions of life. 
 
 A man under the vertical sun stands upon his little round shadow ; but 
 as the sun declines in the afternoon, the shadow juts out on the opposite side, 
 and at last may extend across a whole field. 
 
 A distant cloud which appears to the eye of an observer only as a streak 
 along the sky, may yet be broad enough to shadow a whole region ; for 
 clouds generally form in level strata, and when viewed by a spectator on 
 earth at a distance are seen nearly edgeways. 
 
332 LIGHT. 
 
 The velocity of the wind may be ascertained by marking the time which 
 the shadow of a cloud takes to pass over a plain or other space of known 
 dimension. 
 
 A body held between a candle and the wall darkens a portion of the wall, 
 or casts its shadow there ; and the whole space between it and the wall 
 is a shadowed space, for anything introduced there is as much shadowed as 
 the portion of the wall. Thus, also, all the heavenly bodies which revolve 
 about the sun cast a shadow beyond them or away from the sun, as is seen 
 when one of them, before brightly visible, passes where the shadow of an- 
 other is. The satellites or moons of Jupiter, when they suddenly disappear 
 to our glasses, or are eclipsed as we term it, have generally only plunged 
 into the shadow of the planet, and are not hidden by being then on the 
 other side of the planet, as many suppose. When our own moon is eclipsed, 
 that phenomenon so awful in the early ages of the world, she is only passing 
 through the long shadow which the earth casts beyond it. 
 
 When in the case of a luminous centre and a body casting a shadow, the 
 centre is larger than the body, then the cross section of the shadowed space, 
 or the shadow as thrown on a plane surface, will be less than the body, and 
 less, moreover, the farther the surface is from the body, for the shadowed 
 space terminates in a point. This is true of the shadows of all the planets 
 and of the earth, because they are less than the sun. On the contrary, if the 
 light-giving surface is smaller than the opaque body, the shadow will be larger 
 than the body. The shadow of a hand held between a candle and the wall 
 is gigantic ; and a small pasteboard figure of a man placed near a narrow 
 centre of light, throws a shadow as big as a real man. The latter fact has 
 been amusingly illustrated by the art of making phantasmagoric shadows. 
 
 When the surface which receives a shadow is not directly exposed to the 
 light, the shadow may be much larger than the object, even although the 
 sun himself be throwing the light; as is seen when a slightly projecting 
 roof, or a veranda, shadows from the high sun of summer noon the whole 
 front of a house ; or, as is proved by the long evening shadows of all coun- 
 tries, a low wall will shadow from the setting sun a whole field. 
 
 "Light posses readily through some bodies which are, therefore, called 
 transparent , but when it enters, or leaves their surfaces obliquely, its 
 course is bent." (Read the Analysis, page 325.) 
 
 It may well excite the surprise of inquirers that light, of which the con- 
 stitution is so fine or flimsy, should still be able to dart readily and in every 
 direction through great masses of solid matter, but such is the truth. Thick 
 plates of solid glass, blocks of rock crystal, mountains of ice, &c., are in- 
 stantly pervaded by the beam of the sun. 
 
 What it is in the constitution of ono mass as compared with another, which 
 fits the one to transmit light, and the other to obstruct it, we cannot clearly 
 explain, but we perceive that the arrangement: of the particles has more in- 
 fluence than their peculiar nature. Nothing is more opaque than thck masses 
 of the metals, but nothing is more transparent than equally thick masses of 
 the same metals in solution, nor than the glasses of which a metal forms a 
 large proportion. The thousand salts formed by the union of the metals or 
 earths with the diluted acids, are all transparent, when in cooling from the 
 fluid to the solid state, their particles have been allowed to arrange themselves 
 according to the laws of their mutual attraction, that is to say, to form crys- 
 tals; but the same substances in other states, as when reduced to powder, 
 
REFRACTION. 
 
 333 
 
 are opaque. Even the pure metals themselves, when reduced to leaves of 
 great thinness, are transparent, as may be perceived by looking at a lamp 
 through a fine gold leaf. It is to be remarked, however, that even the most 
 transparent bodies intercept a considerable part of the light which enters 
 them : a depth of seven feet of pure water intercepts about one-half, so that 
 the bottom of the sea is very dark. And of the sun's light, when passing 
 obliquely through the atmosphere towards the earth, as when the sun has 
 lately risen or is about to set, only a small part arrives. 
 ' Light having once entered a transparent mass of uniform nature passes 
 forward in it as straightly as in a vacuum ; but at the surface, whether on 
 entering or leaving it, if the passage be oblique, and- if the mafs.be of a 
 different density from the transparent medium around it, a very curious and 
 most important phenomenon occurs, namely, the light suffers a degree of 
 bending from its antecedent direction, or a refraction, proportioned to the 
 obliquity. 
 
 But for this fact, which to many persons might at first appear a subject of 
 regret, as preventing the distinct vision of objects through all transparent 
 media ; light could have been of little utility to man. There could have 
 been neither converging lenses as now, nor any optical instruments, of which 
 lenses form a part, as telescopes and microscopes ; nor even the eye itself, 
 which has its crystalline lens. 
 
 Light falling from the air directly or per- 
 pendicularly upon a surface of water, glass, Fig. 137. 
 or any such transparent body, passes through 
 
 without suffering the least bending; a ray ^i -^ d> 
 
 for instance, shot from a to the point c, in the 
 surface of a piece of glass g li, would reach 
 
 directly across to o and b ; but if the ray fell I/ r cy r 
 
 obliquely, as from d to c, then, instead of 
 continuing in its first direction to* and, 7c } it 
 would at the moment of its entrance be bent 
 downwards in the path c e, nearer to a line c 
 o, called the perpendicular to the surface at 
 the point of entrance, and the moving 
 straightly while in the substance of the glass 
 it would, when it passed out again at e, in the 
 opposite surface, he bent just as much as at first, but in the contrary direc- 
 tion, or away from a similar perpendicular at that surface, viz., into the line 
 e f, instead of e n. A ray, therefore, passing obliquely through a transparent 
 body with parallel surfaces, has its course shifted a little to one side of the 
 original course, but still proceeds in the same direction, or in a line parallel 
 to the first as here shown in the line ef, parallel and near to the line^& ; if 
 the surfaces of the body are not parallel, the ray is ultimately bent as will be 
 explained some pages hence. 
 
 The degree of bending or refraction of light in traversing a single trans- 
 parent surface is measured by comparing the obliquity of its approach to the 
 surface with the obliquity of its departure after passing ; and for this purpose 
 a line is supposed to be drawn perpendicularly through the surface of the 
 point where the ray passes (as a b in the above figure drawn through c, where 
 the ray d c passes) and the relative positions of the ray to this line on both 
 sides of the surface, are easily ascertained. Thus the line a d, drawn from 
 any point of the ray before passing to such perpendicular, is a measure of the 
 original obliquity or angular distance of the ray, and is called the sine of the 
 
 7, 
 
334 LIGHT. 
 
 angle of incidence, and the other line o c drawn from a corresponding point 
 of the ray after passing to the perpendicular, is a measure of the jobliquity 
 after refraction, and is called the sine of the angle of the refraction : by 
 comparing these two lines in any case, the problem is solved. 
 
 When light passes obliquely from air into water, the refraction or bending 
 produced is such, that the line a d measuring the obliquity before refraction, 
 is always longer than the line o e measuring it after refraction, by nearly one- 
 third of the latter, and the refractive power of water is, therefore, signified by 
 the index 1J or 1,33 ; in like manner the greater refractive power of common 
 glass has the index , of diamond the index 1, and so on. And it is im- 
 portant to remark, that for the same substance the same relation holds, 
 whatever the obliquity of the incidence ray may be. If, for instance, where 
 the obliquity, as measured by its sine, is 40, and the refraction, is half or 20, 
 then in the same substance an obliquity of 10 will occasion a refraction 
 of 5, and obliquity of 4 will occasion a refraction of 2, and so on. 
 
 As a general rule, the refractive power of transparent substances of media 
 is proportioned to their densities. It increases, for instance, through the 
 list of air, water, salt, glass, &c. But Newton, while engaged in his experi- 
 ments upon this subject, observed that inflammable bodies had greater re- 
 fractive powers than others, and he then hazarded the conjecture, almost of 
 inspired sagacity, which chemistry has since so remarkably verified, that 
 diamond and water contained inflammable ingredients. We now kncrw that 
 diamond is merely crystallized carbon, and that water consists of hydrogen 
 or inflammable air and oxygen. Diamond has nearly the greatest light- 
 bending power of any known substances, and hence comes in part its 
 brilliancy as a jewel. 
 
 No good explanation has been given of the -singular fact of refraction; but 
 to facilitate the conception and remembrance of it, we say that jit happens 
 as if it were owing to an attraction between the light and the refracting body 
 or medium. The light approaching from d to c, for instance (in the last 
 figure,) may be supposed to be attracted by the solid body below it, so as at 
 the surface to be bent into the direction c e ; and, again, on leaving the body 
 to be still equally attracted and bent back, so as to take the direction ef, 
 instead of e n ; and we see why the attraction and bendingjjshould be greater, 
 the greater the obliquity. 
 
 The following are familiar examples of this bending of light in passing from 
 one medium to another, 
 
 If an empty basin or other vessel 6 c/e, be in the sun's light, so that the 
 rays falling within it may reach low on the side as to <f, but not to the bot- 
 tom, then, on filling the vessel with water, the sun will be found to be shin- 
 ing on the bottom or down to e, as well as on 
 Fig. 138. the side. The reason of this phenomenon is, 
 
 that water being a denser medium than air, 
 # f the light, on entering it at c, is bent towards 
 the perpendicular (c /,) at the point of inci- 
 dence, and so reaches the bottom. Again, if 
 a coin or metal, were laid on the bottom of 
 such a vessel at e, it would not, while, the ves- 
 sel were empty, be seen by an eye at a, but 
 would be visible there immediately on the 
 vessel being filled with water ; because then, 
 the light leaving the coin in the direction e c } 
 
REFRACTION. 335 
 
 towards the edge of the vessel, would at c, on passing from the water into 
 air, be bent away from the perpendicular, and instead of going to g, would 
 reach the eye at a. The coin, moreover, would appear to the eye to be in 
 the direction of c d, instead of the true direction c e : for the eye not being 
 able to discover that the light had been bent in its course, would judge the 
 object to be in the line by which the light came from it. 
 
 It is thus because objects at the bottom of water, when viewed obliquely, 
 do not appear so low as they really are, tfrat a person examining a river or 
 pond, or any clear water, from its bank, naturally judges its depth to be less 
 than it is. Many a young life has been sacrificed to this error. A person 
 looking from a boat directly down upon the objects at the bottom of water, 
 sees them in their true directions, but even then not in their true distances, 
 as will be afterwards explained ; and if he view them more and more ob- 
 liquely, the appearance becomes more and more deceiving, until at last it 
 represents them as at much less than half of their true depth. 
 
 The ship in which the author sailed, once in the middle of the China Sea, 
 where no danger was apprehended, entered by a narrow passage a large 
 hore-shoe enclosure of coral rocks. When the looker-out gave the alarm, 
 the predicament had become truly terrific. On every side, in water most 
 singularly transparent, the rocks appeared to be almost at the surface of the 
 water, and the anchor, which in the first moment had been let go to arrest 
 the ship, appeared to have been dragged to a shallow place. It seemed that 
 if the ship, then 'drawing 24 feet, or the depth of a two-storied house, moved 
 but a little way in almost any direction, she must inevitably meet her 
 destruction On sending boats around to sound and to search, the place of 
 entrance was again discovered, and was safely traversed a second time as an 
 outlet from that terrible prison. 
 
 On account of this bending of light from objects under water, there is 
 more difficulty in hitting them with a bullet or spear. The aim by a person 
 not directly over a fish, must be made at a point apparently below it, other- 
 wise the weapon will miss by flying too high. The spear, sometimes used 
 in this country for killing salmon, is a common weapon among the islanders 
 of the Atlantic and Pacific Oceans for killing the albacore; the use of it, like 
 that of the fly-hock in England, affording to the fishermen, sport as well as 
 profit. The author once witnessed at St. Helena, this employment of the 
 spear. A small fish previously stunned, that it might notrtry to escape, was 
 every minute or two thrown upon the water as a bait, in the sight of perhaps 
 a hundred great albacores, greedily waiting for it at one side below, and 
 knowing the danger to which they exposed themselves by darting across to 
 seize it. Some albacore bold enough, soon made at the mouthful, apparently 
 with the speed of lightning, but yet with speed which did not save him, for 
 every now and then the thrown spear met him, and held him writhing there 
 in a cloud of his death-blood After a victim so destroyed, the scene of 
 action was changed. 
 
 The bending of light when passing obliquely from water, is also the reason 
 of the following- facts. A straight rod or stick, of which a portion is immersed 
 in water, appears crooked or broken at the surface of the water, the portion 
 immersed seeming to be bent upwards. That part of a ship or boat visible 
 under water, appears much flatter and shallower than it really is. A deep- 
 bodied fish seen near the surface of water, appears almost a flat fish. A 
 round body there appears oval. A gold fish in a vase may appear as two 
 fishes, being seen as well by light bent through the upper surface of the 
 water, as by straight rays passing through the side of the glass. To see 
 
336 
 
 LIGHT. 
 
 OB 
 
 bodies under water, in their true directions and nearly of their true propor- 
 tions, the eye must view them through a tube, of which the lower end closed 
 with a plate glass, is held in the water. 
 
 . As light is bent on entering from air into water, glass, or other substance 
 denser than air, so it is also bent on coming from void space into the ocean 
 of our atmosphere. Hence none of the heavenly bodies, except when directly 
 over our heads, are seen by us in their true situations. They all appear a 
 little higher than they really are, and more so the nearer they are to the hori- 
 zon ; as when to a spectator at d, 
 
 Fig. 139. suppose on the surface of the earth, 
 
 a star really at A appears to be at 
 a, because its ray, on reaching the 
 atmosphere ate, is bent downwards. 
 In astronomical books there is al- 
 ways introduced a table of refraction 
 as it is called, showing what correc- 
 tion must be made on this account 
 for different apparent altitudes. 
 This effect of our atmosphere so 
 bends the rays of the sun, that we 
 see him in the morning before he 
 is really above the horizon, and we 
 
 see him in the evening after he is really below it for the ray coining hori- 
 zontally from e to rf, appears to come from 6, although in truth it really 
 comes from the lower situation B, and is bent into the level line only at e. 
 Our atmosphere thus, by the bending of light as well as by itself becoming 
 luminous, lengthens at dawn and twilight the durations of the precious day. 
 As the atmosphere is denser near the surface of the earth than higher up, 
 the light is more and more bent as it descends, and hence describes a course 
 which is sensibly curved, and therefore unlike the course of light in water. 
 Certain states of the atmosphere, depending chiefly on its humidity and 
 warmth, change very considerably its ordinary refractive power ; hence, in 
 one state, a certain hill or island may appear low and scarcely rising above 
 the intervening heights or ocean, while in another state, the same object will 
 be seen towering above ; and from a certain station, a city in a neighbouring 
 valley may be either entirely visible, or it may show only the tops of its 
 steeples, as if the bed on which it rested had sunk deeper into the earth. 
 In days of ignorance and superstition, such appearances occasionally excited 
 a strange interest. 
 
 Owing to the bending of light in passing through the media of different 
 densities, a beautiful phenomena is often observable in a day of warm sun- 
 shine. Black or dark-colored substances, by absorbing much light and 
 heat from the sun's rays, and warming the air in contact with them, until 
 it dilates and rises in the surrounding air, as oil rises in water, cause the 
 light from more distant objects, reaching the eye through the rarefied me- 
 dium, to be bent a little ; and owing to the heated air rising irregularly 
 under the influence of the wind and other causes, these objects acquire the 
 appearance of having a tremulous or a dancing motion. In a warm, clear 
 day, the whole landscape at last appears to be thus dancing. 
 f The same phenomenon is to be observed at any time, by looking at an 
 object beyond the top of a chimney from which hot air is rising. An illicit 
 distillery has been discovered by the exciseman happening thus to look across 
 
REFRACTION. 
 
 33T 
 
 a hole used as the chimney, although charcoal was the fuel, and there was 
 no vestige of smoke. 
 
 This bending of light by the varying states of the atmosphere renders 
 precaution necessary in making very nice geometrical observations : as in 
 measuring base lines for the construction of maps or charts. 
 
 As it is the obliquity with which a ray traverses the surface, which, in any 
 case of refraction, determines the degree of bending, a body seen through a 
 medium of irregular surface appears distorted according to the nature of that 
 surface. It is because the two surfaces of common window-glass are not as 
 in the case of plate-glass perfect planes, and perfectly parallel to each other, 
 that objects seen through a common window appear generally more or less 
 out of a shape; and hence come the elegance and beauty of plate-glass win- 
 dow: and hence the singular distortion of things viewed through that swell- 
 ing or lump of glass, which appears at the centre of certain very coarse 
 panes and which remains where the glass-blower's instrument was attached. 
 
 The refraction or bending of light is interestingly exemplified in the effect 
 of the glass called a prism, viz., a 
 
 wedge or three-sided rod of glass Fig. 140. 
 
 such as that of which the end is 
 here represented at b c. A ray 
 from a falling on the surface at b is 
 bent towards the internal perpen- 
 dicular, and therefore reaches c, but 
 on escaping again at c, it is bent 
 away from the external perpendicu- 
 lar and thus with its original deviation doubled, goes on to d. 
 
 The law of lights bending, according to the obliquity 
 with which it traverses the surfaces of a transparent body, 
 is well elucidated by the effect of what is called a multi- 
 plying glass \ that is to say, a piece of glass like a b c e, 
 having many distinct faces cut upon it at angles with each 
 other. If a small object, a coloured bead for instance, be 
 placed at d, an eye at e will see as many beads as there 
 are distinct surfaces or faces at the glass ; for first, the 
 ray d a, passing perpendicularly, and therefore straight 
 through, will form an image as if no glass intervened ; 
 then, the rays from d to the surface b, will be bent by the 
 oblique surface, and will show the object as if it were in 
 the direction e b; and the light falling on the still more 
 oblique surface, c, will be still more bent, and will reach 
 the eye in the direction c e, exhibiting a similar object 
 also in that direction and so of all the other surfaces. 
 If the eye were at d, and the object at e, the result would 
 still be the same. A plate of glass roughened, or cut into 
 cross furrows, becomes a very good screen or window- 
 blind, by disturbing the passage of light through it so that 
 objects beyond it are not distinguishable. 
 
 t( And a beam of while light thus made to bend, is resolved into beams of the 
 various primary colours ; which beams, however, on being again blended, 
 become white light as before." (Read the Analysis, page 325.) 
 
 The most extraordinary fact connected with the bending of light is that a 
 pure ray of white light from the sun admitted into a darkened room by a hole 
 
 22 
 
 Fig. 141. 
 
338 LIGHT. 
 
 in the window-shutter, and made to 
 
 Fig. 142. bend by passing through transparent 
 
 surfaces which it meets very oblique- 
 ly (as the ray a, admitted and made 
 to bend by passing through the prism 
 of glass b c, to fall upon the wall at 
 d,] instead of bending altogether and 
 appearing still as the same white 
 ray, is divided into several rays, 
 
 which falling on the white wall, are seen to be of different most vivid 
 colours. The original white ray is said thus to be analyzed, or divided into 
 its elements. 
 
 This solar spectrum, as it is called, formed upon the wall, consists, when 
 the light is admitted by a narrow horizontal slit, of four coloured patches 
 corresponding to the slit, and appearing in the order, from the bottom of red, 
 green, blue, and violet. If the slit be then made a little wider, the patches 
 at their edges overlap each other, and produce by the mixture of their ele- 
 mentary colours, certain new tints. Then the spectrum consists of the seven 
 colours commonly enumerated and seen in the rainbow, viz., red, orange, 
 yellow, green, blue, indigo, and violet. Had red, yellow, blue, and violet 
 been the four colours obtained in the first experiment, the occurrence of the 
 others, viz., of the orange, from the mixing edges of the red and yellow of 
 the green, from the mixture of the yellow and blue, and of the indigo, 
 from the mixture of blue and violet, would have been anticipated. But the 
 facts of the case not being such, we see that they are not yet well under- 
 stood. When Newton first made known the phenomenon of the many- 
 coloured spectrum, and the extraordinary conclusions to which it led, he ex- 
 cited universal astonishment ; for the common idea of purity, the most un- 
 mixed was that of white light. In farther corroboration of the notion of 
 the compound nature of light, he mentioned, that if the colours which ap- 
 pear on the spectrum be painted separately around the rim of a wheel, and 
 the wheel be then turned rapidly, the individual colours cease to be distin- 
 guished and a white band only appears where they are whirling ; also, that 
 if the rays of the spectrum, produced by a prism, be again gathered together 
 by a lens, they reproduce white light. The red is the kind of light which 
 is least bent in refraction, and the violet that which is most bent. It was at 
 one time said, as an explanation, that the differently coloured particles in 
 light had different degrees of gravity or inertia, and were therefore, not all 
 equally bent. It is farther remarkable, with respect to the solar spectrum, 
 that much of the heat in the ray is still less refracted than even the red light, 
 for a thermometer held below the red light rises higher than in any part of 
 the visible spectrum ', and there is an influence or something in the beam 
 more refrangible than even the violet rays, and capable of producing power- 
 ful chemical and magnetical effects. The different spots of colour in the 
 spectrum are not all the same size, and there is a difference in this respect 
 according to the refracting substance. 
 
 All transparent substances in bending light produce more or less of the 
 separation of colour ; but it is an important fact, that the quality of merely 
 bending a beam, or of refraction, and that of dividing it into coloured beams, 
 or of dispersion, are distinct qualities, and .not having the same proportions 
 to each other in different substances. Newton, from not discovering this, 
 concluded that a perfect telescope of refraction could never be made ; he sup- 
 posed that the bent light would always become coloured, and so render the 
 
REFRACTION BY LENSES. 339 
 
 objects indistinct. We now know, however, that by combining two or more 
 media, we may obtain bending of light without dispersion, thus, by oppos- 
 ing a glass which bends five degrees and disperses one degree, to another 
 glass which bends three degrees and disperses one, the opposing dispersions 
 will just counterbalance or neutralize . each other, while the two degrees of 
 excess of bending will remain to be applied to use. 
 
 The diversified colours of the substances around us depend merely upon 
 their fitness, from texture or other cause, to reflect or transmit other modi- 
 fications of common light, and the colour is not a part or property of the 
 body itself. We shall soon find that the vivid colours of the rainbow are 
 merely the white light of the sun, reflected to us after being bent and 
 modified by the colourless drops of falling rain ; and that the sparkling with 
 appearance of rubies and emeralds, which we see in cut-glass lustre, is a 
 phenomenon of the same kind : and that by scratching the surface of a 
 piece of metal so as to have a given number of lines in a given space, we 
 can cause the same substance to appear of any colour we please. 
 
 lt Transparent bodies, as glasSj 'may be made of such form as to cause all 
 the raijs of light which pass through them from any one point, to bend so 
 as to meet again in another corresponding point beyond them, the body 
 itself, from the required form generally resembling that of a bean or 
 lentil, being then called a LENS." (Read the Analysis, page 325.) 
 
 The innumerable rays of light (of which five only are here represented,) 
 issuing from any point as c, towards any surface in the situation a b } are said 
 to form a cone or pencil of diverging light. Now it is evident that to make 
 
 Fig. 143. 
 
 z >::- 
 
 all such rays converge or meet again in one place as/, beyond a transparent 
 body place at a 6, it would be necessary, while the middle ray or axis of 
 the pencil c df did not bend at all, for the others to be bent more and more, in 
 proportion as they fell upon the body farther and farther from the centre d. 
 Recollecting, then the law of refraction, that light entering from air through 
 the surface of any denser medium, as glass, is bent there towards the perpen- 
 dicular at the internal surface, in proportion to the obliquity of incidence, 
 and on leaving the opposite surface, is correspondingly bent away from its 
 external perpendicular, (see the case of the prism at p. 337,) we see that if 
 a piece of glass were placed at a b, of such form that the rays falling upon 
 it from c should meet and leave its surfaces with greater and greater obliquity 
 in some regular proportion, as the points of incidence were more distant from 
 
340 LIGHT. 
 
 the centre d, the purpose would "be obtained. And we have the satisfaction 
 of knowing that a glass, of which the surface is ground which it easily may 
 be to have a regular convexity or bulging, as if it were a portion cut off 
 from the surface of the globe, can be shown to answer very correctly the re- 
 quired condition. Such a glass similarly ground on both sides, is here repre- 
 sented edgeways between a and b, where the ray c d falling on its middle, or 
 perpendicularly, and similarly leaving it, is seen going straight through to/, 
 but the ray c e meeting the surface with a certain degree of obliquity is bent 
 down a little, first on entering the surface at e, and then as much more on 
 leaving the opposite surface with equal obliquity, and so arrives at/; then 
 the rayc or, for corresponding reasons is still more bent, and equally arrives 
 at/; and the case would be similar of any other rays that might be exam- 
 ined. The point /is usually called a, focus (meaning a fire-place,) because 
 when the light of the sun is thus gathered, the heat concentrated with it is 
 powerful enough to make combustibles inflame. We have here to remark 
 farther, that in accordance both with calculation and experiment, the direction 
 in which a pencil of rays falls upon a lens does not affect the result of the 
 covergence to a focus, only the focus is always in the direction of the cen- 
 tral ray of the pencil or beam ; it will be atp, for instance, for light issuing 
 from o, and at z for light issuing from x. 
 
 The lens represented at a b above or in the annexed diagram, at fig. 1, 
 having both sides convex, is called a double convex lens. A glass convex 
 only on one side ; and plane or flat on the other, as shown at fig 2, would 
 
 Fig.l 
 
 as effectually gather the rays, but with half the power, and the point of 
 meeting or focus would be therefore proportionably more distant. Such a 
 glass is called a plano-convex lens. Then the gathering or converging 
 power of any glass, whether doubly or singly convex, is in proportion to 
 the degree of its convexity or bulging of surfaces, for the less it bulges, 
 the more nearly does it approach to a plane glass, and the more it bulges, 
 the more obliquely will the rays, at any distance from the centre, fall upon 
 its surface, and the sooner, therefore, in consequence of their being more 
 bent, will they all meet the axis ray ; hence, fig. 1 would converge much 
 more quickly than fig. 3, which represents nearly a common spectacle glass; 
 and a very minute globe is the form most powerfully converging of all. 
 The surfaces of fig. 1 are proportions of a small globe; those of fig. 3, are 
 smaller portions", but of a globe much larger. Concave lens as fig. 4, 
 a double concave, and at fig. 5, a plano-concave lens, in obedience to the 
 same law of refraction, spread rays, or bend them away from the axis of 
 the pencil, in the same degree that similarly convex lenses gather them. 
 A concave lens, therefore, receiving the converging pencil of rays from a 
 convex lens, might restore them to their former direction. Very useful 
 purposes, as will be afterwards explained, are served in optics, by certain 
 
CAMERA OBSCURA. 341 
 
 combinations of differently formed lenses. A lens may be convex on one 
 side and concave on the other, as at fig. 6, called a meniscus lens, because 
 it resembles the crescent moon, and its effect will be according to the curve 
 which predominates. 
 
 A person collecting the case of the " multiplying glass," described a few 
 pages back, might say, but is not a convex lens merely a multiplying glass 
 of a much greater number of faces, and if so, why instead of one image, 
 does it not make thousands ? The answer is, that the multiplying glass, by 
 every face, bends a set of rays, capable of forming a distinct and complete 
 image j but the lens has no surface large enough to bend more than a 
 single ray and it concentrates all the single rays into one place, to form 
 there one image of great vividness and beauty. 
 
 " And ivhen the light proceeding from every point of an object placed before 
 a lens is collected in corresponding points behind it, a perfect image of 
 the object is there produced. When the image is received upon a suitable 
 white surface in a dark place, the arrangement is called, according to 
 minor circumstances, the CAMERA OBSCURA, SOLAR MICROSCOPE or MAGIC 
 LANTERN." (Read the Analysis, page 326.) 
 
 Words are wanting to express the admirable consequences to man of the 
 curious property of a lens that it can bring together to focal points behind 
 all the rays of light which traverse it from any points of an object placed 
 before it. The following instance will lead to the understanding of others. 
 If a lens as a, be placed so as to fill 
 
 up an opening made in the window- Fig. 145. 
 
 shutter of a darkened room, then, from 
 any object before that opening as the 
 cross here presented, all the light 
 which each point emits towards the 
 lens will be concentrated or gathered 
 together in a corresponding focal point 
 behind the lens or within the room 
 and if a sheet of paper be held there at the distance of the focal points, a 
 beautiful image of the object will be seen upon the paper. 
 
 In these few words, we have described the interesting contrivance called 
 the camera obscura or dark chamber ; and when a glass is chosen of 
 proper size and focal distance, and a screen or the wall of the chamber (if at 
 the required distance,) is properly prepared to receive the light, the most en- 
 chanting portraiture is instantly produced of the whole scene which the win- 
 dow commands. With what rapture does the school-boy first view this 
 lovely picture drawn by nature's own pencil, and with colors taken directly 
 from the sun's bright ray with what rapture, as his eyes search over it, 
 does he recognize, perhaps, his playmates there, and the river in which he 
 bathes, and where he sails his boat, and the wood in whose solitudes he 
 loves to wander, and the mountain heights which he climbs to meet the fresh 
 breeze, and at a distance from the world, to allow his young fancy to work, 
 beginning to shoot far into time and space. The great peculiarity of such 
 a picture is, that it does not, like others, portray still-nature, but every thing 
 with appropriate motion or changes : the playmates are all in action : the 
 leafy trees wave in the wind, the clouds sail along, the sun may rise or may 
 set, and even the lightning's gleam may dart across : or, again commenced 
 enterprize may be brought to a close ; the traveller may climb the distant 
 
342 LIGHT. 
 
 hill and disappear; the fisherman may draw his net and secure his prize; 
 the contested race may be won or lost. A Maylan chief in the island of 
 Sumatra, was so surprised and pleased by a small portable camera obscura 
 which the author happened to have among his apparatus, that he seemed 
 disposed to give for it almost any thing he possessed. 
 
 It appears in the last diagram that the image formed beyond a lens by the 
 gathered light, is in a contrary position to the object itself, that is, inver- 
 ted, because the light from the top of the object darts through the opening 
 or glass in a descending direction, and that from the bottom rises to the 
 opening, and in the same direction passes beyond it. It is usual, therefore 
 in a camera obscura, to place a small mirror immediately behind the lens, 
 so as to throw all the light which enters downwards, to a whitened table, 
 upon which the picture may be conveniently contemplated. 
 
 The camera obscura often gives very useful assistance to young painters, 
 by enabling them to trace correctly the outlines of the objects placed before 
 it, and also to study effects of light, shade and colour, more profitably than 
 they at first can, by looking at the objects themselves. The laws of per- 
 spective are most intelligibly illustrated in this most true picture 
 
 An effect, approaching in a degree to that of the complete camera obscura 
 now described, is produced by merely making a small hole in the shutter of 
 a dark room, and letting the light which enters by it fall on any white 
 surface beyond. The whole landscape is then dimly portrayed upon the 
 surface. Barry, the painter, while lying on a sick bed, mistook such a 
 scene appearing on the ceiling of his room for a supernatural vision. If a 
 cross be held before the opening as in the last figure, it is evident that from 
 every point of the cross light will enter by the opening, and will foil on 
 corresponding parts of a sheet of paper held behind, but as the light from 
 each point is not a single ray, but a spreading pencil or cone of light, it will 
 fall on the paper, not on one point, but on a surface at least as large as the 
 opening, and thus the light from adjoining points will mix at the edges, and 
 will render the images misty and indistinct, somewhat like those on the 
 back of tapestry. If the opening be very small, the picture will be well 
 defined, but very feebly illuminated ; and if the opening be of considerable 
 size, the mixing of the pencils will be so great as to leave no particular 
 object distinguishable. But, in the latter case, and however large the 
 opening be, if a lens be introduced, it will converge every pencil of light to 
 an exact point, and the picture will instantly be rendered perfectly clear. 
 A lens is never held up in the light without forming beyond it pictures such 
 as now described, of every visible object about it, and the pictures are not 
 seen, only because there are no screens placed to receive them, and because 
 they are so numerous as to confuse one another, in other words, because 
 they are not admitted singly into a dark chamber. 
 
 The distance from a lens at which an image is formed or the rays of tbe 
 light meet, depends first upon the refractive or bending power of the lens, 
 and therefore, on its form and on the nature of its substance ; and, secondly, 
 upon the direction of the rays of light when they reach the lens, as to whether 
 they are divergent, parallel or convergent. We have already explained that 
 glass refracts about twice as much as water, and that diamond refracts about 
 twice as much as glass : and we have considered the effect of different degrees 
 of convexity in lenses arising equally whether the lens be of water 
 enclosed between glasses like watch-glasses, or of solid glass, or of rock- 
 crystal or of diamond itself. We now proceed to consider the joint effect 
 of the refractive power, and of the direction.of the incident rays. 
 
CAMERA OBSCURA. 343 
 
 Rays falling from a on a comparatively flat or weak lens at L, might meet 
 
 Fig. 146. 
 
 i* 
 
 only at d, or even farther off: while, with a stronger or more convex lens, 
 they might meet at c or at b ; a lens weaker still might only destroy the diver- 
 gence of the rays, without being able to give them any convergence or to bend 
 them enough to bring them to a point at all and then they would proceed all 
 parallel to each other, as seen at e and/; and if the lens were yet weaker, 
 it might only destroy a part of the divergence, causing the rays from a to go 
 to g and h, after passing through, instead of to i and k, in their original direction. 
 
 In an analogous manner, light coming to the lens in contrary directions 
 from bed, &c., might, according to the strength of the lens, be all made 
 to come to a focus at a or at /, or in some more distant point ; or the rays 
 might become parallel, as m and n, and, therefore, never come to a focus, 
 or they might remain divergent. 
 
 It may be observed in the figure above, that the farther an object is from 
 the lens, the less divergent are the rays which fall from it upon the lens ; 
 or the more nearly do they approach to being parallel. From b there is 
 much divergence in the exterior rays, from c less, from d less still, and rays 
 from a great distance, as those represented by e and/, appear quite parallel. 
 If the distance of the radiant point be very great, they really are so nearly 
 parallel, that a very nice test is required to detect the non-accordance. Rays 
 for instance, coming from the earth to the sun, do not diverge the millionth 
 of an inch in a thousand miles. Hence, where we wish to make experiments 
 with parallel rays, we take those of the sun. 
 
 Any two points so situated on the opposite sides of a lens, as that when 
 either becomes the radiant point of light, the other is the focus of such light, 
 are called conjugate foci. An object and the image of it formed by a lens, 
 must always be in conjugate foci, and as the one is nearer the lens, the other 
 will be in a certain proportion more distant. 
 
 What is called the principal focus of a lens, and by the distance of which 
 from the glass we compare or classify lenses among themselves, is the point 
 at which the sun's^rays, or any parallel rays, are made by it to meet; and 
 thus, by holding the glass in the sun, and noting at what distance behind it 
 the little luminous spot or image of the sun is formed, we can at once ascer- 
 tain the focus of a glass as at a for the rays e and / 
 
 It is a remarkable coincidence that the bending power of the common glass 
 used for lenses should be such, that the focus of a double lens is just where 
 the centre of the sphere would be, of which the surface of the lens is a por- 
 tion. This gives us another, fact with which to associate the recollection 
 that the focus is nearer as the convexity of the lens is greater, that is to say, 
 as the surface is a portion of a smaller sphere, And such being the law, it 
 may be proved by calculation as well as by the fact, that if a candle be held 
 in relation to a lens at twice the principal focal distance, suppose at c for a 
 
344 LIGHT. 
 
 lens with the focus at cr, the image of the candle will be formed at I just as 
 far on the other side Thus, then, by trying with the lens until the image 
 of a candle is formed at the same distance from it as the object is, we have 
 a second mode of ascertaining the focal distance of a lens. Other kinds of 
 glass and other substances refract with different power ; but the facts now 
 stated should be retained in the memory as standards of comparison. 
 
 Because the focal point of light passing through a lens is at the same dis- 
 tance from the centre of the lens, in whatever direction the light passes 
 through, a surface placed to receive the image of any broad object should 
 really be concave, that is to say, all parts of it should be at the same distance 
 from the centre of the lens, otherwise the image will be more perfect either 
 at its middle than towards its edges, or vice versa but it is not found neces- 
 sary to attend to this in common practice, where the object and its image 
 are not of great extent. 
 
 The size of an image formed behind a lens is always proportioned to the 
 distance of the image from the lens, and the image is much larger or smaller 
 than the object as it is farther from or nearer to the lens than the object. This 
 
 will be evident from considering the an- 
 
 Fig. 147. nexed figure c represents the place of a 
 
 lens, and the lens, according to its power, 
 will form an image of the cross a b, in some 
 situation, as at d, e, g, &c. Now when- 
 ever the image is formed and by whatever 
 lens, one end of it must be in contact with 
 the line a g, and the other end with the 
 line b h ; and as these lines cross each 
 other at c, and widen regularly afterwards, 
 a line adjoining them (and the image is such a line,) must always be shorter 
 the nearer it is to c, that is to say, shorter in proportion to the converging 
 power of the lens. 
 
 Many persons may not have reflected, that the luminous circle called the 
 focus of a burning glass, is really but the image or picture of the sun formed 
 by the glass or lens. The intensity of the heat and of the light is of course 
 in proportion as the image is smaller than the glass which forms it, and the 
 nearer that the image is formed to the lens, or the more powerfully convergent 
 that the lens is, the smaller will the image be. Mr. Parker's famous burning 
 lens, which cost 700, and is now the property of the Emperor of China, 
 was three feet in diameter, and the diameter of the sun's image formed by it 
 was one inch : it concentrated the light and heat, therefore, about 1,800 times. 
 To render the effect still more powerful, a smaller lens was placed behind 
 the larger, farther reducing the size of the image to one-sixth. Very sur- 
 prising effects were produced by this lens, in the melting of metals, inflaming 
 of combustibles, &c. The size of burning lenses, until lately, was limited 
 by the difficulty of obtaining the great pieces of glass required to form them : 
 but they are now built up of many pieces suitably united together. Some 
 large lenses have been made of water, that is, of water enclosed between 
 rnenicus glasses, like watch-glasses. A common goblet of water, or a vase 
 holding gold-fishes, has in some cases acted as a burning glass, setting fire to 
 the curtains, near which it had been left in the sunshine. 
 
 And the nearer that an object is brought to a lens, the more distant, and 
 therefore the larger will its image be ; for, as the rays falling upon a lens are 
 divergent in proportion to the nearness of the object, and therefore with the 
 same power of lens, must meet farther behind (as seen in the figure at page 
 
MAGIC LANTERN. 345 
 
 343,) then the axis of the rays, as the lines c a and c I in the last figure, 
 will have separated far before the rays meet, and will have made the image 
 proportionally larger. If we suppose little d in the same diagram to be the 
 object, its image would be a b. The sun is exactly as much larger than his 
 image formed by a burning-glass, as he is more distant from it than the 
 image ; and if we had a screen of sufficient size hung up in a distant space, 
 a very bright object of a quarter of an inch in diameter might be made by a 
 lens to form an image as broad as the sun. 
 
 From all these considerations, we see that, in a camera obscura, the screen 
 should be from the lens, at the distance of its principal focus for distant ob- 
 jects, and a little farther than this for near objects Accordingly the lens is 
 generally fixed in a sliding piece, which allows the distance from the screen 
 to be adjusted to circumstances. If the representation be desired large, the 
 lens must be of a long focus ; if small the lens must be of a short focus. 
 Again, when the reversed use of the lens, a small object as d is to be mag- 
 nified to such a size as a b, then the object must be placed a little beyond 
 the focus of the glass ; for if placed nearer, the pencils of rays from it would 
 never be gathered to focal points at all, and no image would be formed at 
 any distance. 
 
 When, as alluded to in the last sentence, a small object is placed very near 
 a lens, and the image of it is thrown upon the wall of a dark room, perhaps 
 a hundred times farther from the lens than the object is, the image is a greatly 
 magnified representation of the object, viz. t it is a hundred times longer and 
 a hundred times broader, and therefore has ten -thousand times as much sur- 
 face as the object; but if in this experiment the object be illuminated only 
 in an ordinary degree, the light from it is so scattered as not to suffice for 
 distinct division. Hence, to attain fully in this manner the purpose of a 
 microscope, a very strong light, concentrated by a suitable mirror or glass, 
 must be directed upon the object. When the light of the sun is used in 
 such a case, the complete apparatus is called a solar microscope, and serves 
 beautifully to display the structure of any minute objects. When artificial 
 light is used, as of a lamp, the apparatus is called the lucernal microscope 
 or magic lantern. 
 
 A good solar microscope becomes one of the most interesting presents 
 which science has made to man, for aiding him in his researches into the secrets 
 of nature. With the late improvements in the construction of lenses by which 
 the dispersion of light or the rainbow-fringe, is prevented, (as will be ex- 
 plained under the head of Telescopes,) objects may be magnified two or 
 three hundred thousand times, and still be so luminous as to be beautifully 
 distinct; thus a cheese-mite will appear of the dimensions of a hog, and crea- 
 tures altogether invisible to the naked eye. or perceived by it only as minute 
 white points are discovered to be animated beings, having the perfect pro- 
 portions, and often the beauty of larger animals, and endowed with similar 
 appetites, passions, and apparent ingenuity, but with an activity far surpass- 
 ing that met with in the more bulky creation. A judicious selection of objects 
 for the solar microscope is calculated exceedingly to surprise the mind on its 
 first attending to them, and to fill it with high conceptions of the infinity of 
 creation. With the common microscope only one person at a time can feast 
 his wonder; but with a solar, a whole roomfull of company may at once con- 
 template the same objects and witness the same actions, and thus have their 
 admiration increased by the consciousness of sympathy. 
 
 The magic lantern, we have said, consists of a powerful lens, with objects, 
 highly illuminated by lamp-light, placed so near it that their images are 
 
346 LIGHT. 
 
 formed far off, and arc therefore proportionally larger. For the magic lan- 
 tern the objects are generally paintings made on thin plates of glass with 
 transparent colours ; and each plate is formed to slide through a slit or pas- 
 sage behind the lens. The lens itself, or what may be called half of it, (for 
 there are often two lenses joined to give greater power,) is moveable with 
 the tube which is seen projecting from the lantern, so that its distance from 
 the object may be varied, and thus a corresponding approach to or receding 
 from the screen may be allowed, which will produce an increase or lessening 
 of the magnitude of the visible picture on the wall. 
 
 Some public lectures on astronomy and other branches of natural history, 
 have had the drawings and paintings required for the elucidation of their 
 subjects made in miniature upon glass, to be magnified afterwards to the 
 degree desired, and shown upon any part of the lecture-room by the magic 
 lantern. 
 
 A thick fog or smoke at night will sometimes reflect the images of a 
 magic lantern so as to make them distinctly visible ; and there are several 
 cases on record, where persons, wickedly ingenious- in this way, have terri- 
 fied ignorant individuals almost to death, by throwing spectres from a con- 
 cealed lantern. Some years ago a sentinel in St. James' Park was thus 
 persuaded that he had seen supernatural beings near him among the trees. 
 
 A very charming illusion is produced by a magic lantern manoeuvred on 
 one side of a thin screen, while the spectators not aware of the existence of 
 a screen, are sitting on the other side. The image let us suppose it that 
 of a genius flying in the air may be first thrown upon the screen from the 
 lantern while very near, and then it will be small, and, if desired, exceed- 
 ingly bright because the light is much concentrated. If the exhibitor then 
 gradually recede from the screen, adjusting at the same time the distance of 
 the lens from the picture, the image will become progressively larger, and 
 to the spectators will appear to be soaring and approaching, until at last the 
 expanded wings and limbs seem hovering almost over their heads. An end- 
 less variety of most ingenious and beautiful exhibitions of this kind have 
 been made under the name of phantasmagoria or raising of spectres. 
 
 " The EYE itself is, in fact, but a small camera obscura." (Read the 
 Analysis, page 325.) 
 
 Who could at first believe that in describing the camera obscura, as we 
 have now done, we had in reality been describing only a large model of that 
 most interesting of the objects of creation, the living eye itself, the inlet of 
 man's knowledge, or what may be called the visible dwelling of the soul 
 that from which the fire of passion darts, through which the languor of ex- 
 haustion is perceived, and in which life and thought seem concentrated ! Yet 
 the eye is nothing but a simple camera obscura : formed of the parts des- 
 cribed above as essential to the camera obscura : but in its simplicity so 
 perfect that they who delight to find around them tangible evidences of the 
 existence of an all- wise and good Creator, point to this in the midst of thou- 
 sands, as one of the most undeniable and triumphant proofs. We shall now 
 describe the eye and its actions : and keeping present to us the idea of the 
 camera obscura, as already treated of, we shall find that the use of the various 
 parts will be declared by merely enumerating them. This paragraph should 
 be perused while the reader has the opportunity of observing either his own 
 eye reflected in a mirror, or the eye of some companion near him. 
 
 The human eye, then is a globular chamber of the size of a large walnut, 
 having for its outer wall a very tough membrane called, from its hardness, 
 
THE EYE STRUCTURE. 
 
 347 
 
 the sclerotic coat, in the front of which there is one round opening or win- 
 dow, named, because of its horny texture, the cornea. The chamber is 
 lined with a finer membrane or web, the choroid (having relation to colour,) 
 which, to insure the internal darkness of the place, *is covered with a black 
 paint, the pigmentum niarum. This lining is bordered at the edge of the 
 round window by a folded drapery, the ciliary processes, hidden from with- 
 out by being behind the curious contractile window-curtain the iris, (so named 
 for its rainbow variety of colour in different persons,) through the central 
 opening of which, called the pupil j the light enters. Immediately behind 
 the pupil is suspended, by attachments among the ciliary processes, the crys- 
 talline lens, a double convex, most transparent body of considerable hard- 
 ness, which so influences the light passing through it from external objects, 
 as to form perfect images of these objects, in the way already described, on 
 the back wall of the eye, over which the optic nerve, there called the retina, is 
 spread as a second lining. The eye is maintained in its globular condition 
 by a watery liquid which distends its external coverings, and which, in the 
 space before the lens, or the anterior chamber of the eye, being perfectly 
 limpid, is called the aquous humour, and in the remainder or larger poste- 
 rior chamber, being enclosed in a pellucid spongy structure, so as to acquire 
 somewhat of the appearance of melted glass, is called the vitreous humour. 
 
 The annexed figure represents an eye of the common dimensions, supposed 
 to be cut through its middle, from above downwards, so as to show the edges 
 of the coats, &c. C is the outer or sclerotic coat, known popularly, where 
 most exposed in front, as the white of the eye. A is the transparent cornea 
 joined to the edge of the round opening of the sclerotic; it is more bulging 
 than the sclerotic, or forms a portion of a smaller sphere than the general 
 eyeball, so that, while it may be truly called ^.'bow-window, it, or rather the 
 convex surface of its contained water, is also a powerful lens for acting on the 
 pencils of entering light. At B, the similarity all round the edge of the cor- 
 nea, is attached to the window-curtain or iris, shown here edgeways immersed 
 in the aqueous humour, and hanging inwards from above and below towards 
 its central opening or pupil, through which the rays of light are passing to 
 the lens. The iris has in its structure two sets of fibres, the circular and the 
 radiating, which cross and act in opposition to each other ; when the circular 
 fibres contract, the pupil is lessened, when the radiating contract, it is en- 
 larged ; and the changes happen according to the intensity of light and the 
 state of sensibility of the retina, as may at any time be proved by closing the 
 eyelids for a moment to make the pupil dilate, and then opening them to- 
 wards a strong light, to make it contract. Behind the pupil is seen the lens D 
 
 Fig. 148. 
 
348 LIGHT. 
 
 with its circumference attached to the ciliary processes E : it is more convex 
 behind than before. The disease of the eye called cataract (from the Greek 
 word implying obstruction,*) is the circumstance of the lens become opaque, 
 and the cure is to extract the lens entirely, or to depress it to the bottom of 
 the eye, and then to substitute for it externally a powerful artifical lens or 
 spectacle-glass. The three lines marking here the boundary of the eye stand 
 for its three coats as they have been called, the strong sclerotic, and the double 
 lining of the choroid and retina. The figure of a cross is represented upon 
 the retina as formed by the light entering from the cross without (which cross 
 has to appear here small aud near, although supposed to be large and dis- 
 tant.) The image of the cross is inverted, as explained for the camera ob- 
 scura : but we shall learn below that the perception of an object may be 
 equally distinct in whatever position the image fall on the retina. It has been 
 explained above, that a lens can form a perfect image of considerable ex- 
 tent only on a concave surface, and the retina is such a surface. The 
 present diagram further explains what is meant by the anterior and poste- 
 rior chambers of the eye, namely, the compartments which are before and 
 behind the crystalline lens D. 
 
 The nature of the eye as a camera ,obscura is beautifully exhibited by 
 taking the eye of a recently killed bullock, and after carefully cutting away 
 the back part of the two outer coats, by going with it to a dark place and 
 directing the pupil towards any brightly illuminated objects ; there may 
 then be seen though the semi-transparent retina, left as a screen at the 
 back of the eye, a minute but perfect picture of all such objects a picture, 
 therefore, formed on the back of the little apartment or camera obscura by 
 the agency of the convex cornea and lens in front. The picture is inverted, 
 for reasons explained above. 
 
 Understanding from all this, that when a man is said to be looking at an 
 object, his mind is in truth only taking cognizance of the picture or impres- 
 sion made on his retina, it excites admiration in us to think of the exquisite 
 delicacy of texture and of sensibility which the retina must possess, that there 
 may be the perfect perception which really occurs of even the separate parts 
 of the minute images there formed. A whole printed sheet of newspaper, 
 for instance, may be portrayed on the retina on less space than the surface 
 of a finger-nail, and yet not only shall every word and letter be separately 
 perceivable, but in the centre of the picture at least, even an imperfection of 
 a single letter. Or, more wonderful still, when at night an eye is turned up 
 to the blue vault of heaven, there is portrayed on the little concave of the 
 retina the boundless concave of the sky, with every object in its just propor- 
 tions. There a moon in beautiful miniature may be sailing among white- 
 edged clouds, and surrounded by a thousand twinkling stars, all in just pro- 
 portion, so that to an animalcule within and near the pupil, the retina might 
 appear another starry firmanent decked in its glory. If the image in the 
 human eye be thus minute, what must they be in the little eye of a wren, or 
 of other animals smaller still ! How wonderful are the works of nature ! 
 
 Because the images formed on the retina are always inverted as respects 
 the true position of the objects producing them just as happens in a simple 
 camera obscura persons have wondered that things should appear upright, 
 or in their true situations. The explanation is not difficult. It is known 
 that a man with wry neck judges as correctly of the position of the objects 
 around him as any other person never deeming them to be inclined or 
 crooked, because their images are inclined in relation to the natural perpen- 
 dicularity of his retina; and that a bed-ridden person, obliged to keep his head 
 
THE EYE DOUBLE SIGHT. 349 
 
 upon his pillow, soon acquires the faculty of the person with wry neck ; and 
 that an affected girl inclining her head while trying her various attitudes, 
 learns from much practice, to judge of the manoeuvres of a beau as conveni- 
 ently in that way as in any other ; and that boys who at play bend themselves 
 down to look backwards through their legs, although a little puzzled at first, 
 because the usual position of the images on the retina is reversed, soon see 
 as well in that way as in any other. It appears, therefore, that while the 
 mind studies the form, colour, &c., of external objects in their images pro- 
 jected on the retina, it judges of their position, not by the accidental position 
 of the images on the retina, but by the direction in which the light comes 
 from the object towards the eye no more deeming an object to be placed 
 low because its image is low in the eye, than a man in a room into which 
 a sunbeam enters by a hole in the window-shutter, deems the sun low be- 
 cause its image is on the floor. A candle carried past a key-hole, throws its 
 light on the opposite wall, so as to cause the luminous spot there to move in 
 a direction the opposite of that in which the candle is carried; but a child is 
 very young, indeed who has not learned to judge at once of the true motion 
 of the candle by the contrary apparent motion of the image. A boatman, 
 who, being accustomed to his oar, can direct its point against any object with 
 great certainty, has long ceased to reflect, that to move the point of the oar 
 in some one direction, his hand must move in the contrary direction. Now 
 the seeing things upright by images which are inverted, is a phenomena akin 
 to those which we have here reviewed. 
 
 Another question somewhat allied to the last is, why, as we have two eyes, 
 and an image of any object placed before them is formed in each why the 
 object does not appear to us to be double. In answer to this, again, we shall 
 only state the simple facts of the case. As in two chess-boards there are 
 corresponding squares, so in the two eyes there must be corresponding points, 
 and when on those points a similar impression is made at the same time, the 
 sensation or vision is single; but if the impression be made on points which 
 do not correspond, owing to some disturbance of the natural position of the 
 eyes, the vision becomes double. Healthy eyes are so wonderfully associated, 
 that from earliest infancy they constantly move in perfect unison. By slightly 
 pressing a finger on the ball of either eye, so as to prevents its following the 
 motion of the other, there is immediately produced the double vision ; and 
 tumours about the eye often have the same effect. Person&who squint have 
 always double vision : but they acquire the power of attending to the sensa- 
 tion in one eye at a time. Animals which have the eyes placed on opposite 
 sides of the head, so that the two can never be directed to the same point, 
 must have in a more remarkable degree the faculty of thus attending to one 
 eye at a time. 
 
 The corresponding points in the two eyes are equidistant and in similar 
 directions from the centres of the retinae, which centres are called the points 
 of distinct vision, and at them the imaginary lines named the axes of the eye 
 terminate but it is worthy of remark that these points, in being both to 
 the right or both to the left of the centres, must be one of them on the inside 
 of the centre as regards the nose, and the other on the outside that is to 
 say, a point of the left eye between the centre and nose, has its correspond- 
 ing point in the right eye between the centre and the cheek and from this 
 fact arises consequences meriting attention. When the two eyes were directed 
 to any object, their axes meet at it, and the centres of the two retinae are oppo- 
 site to it, and all the other points of the eyes have perfect mutual correspond- 
 ence as regards that object, giving the sensation of single vision ; but the 
 
350 LIGHT. 
 
 images formed at the same time, of an object nearer to or farther from the 
 eye than the first supposed, cannot fall on corresponding points for an object 
 nearer than where the axes meet would have, both its images on the outsides 
 of the centres, and an object more distant would have both its images on the 
 insides of the centres, and in either case the vision would be double. Thus 
 if a person hold up one thumb before his nose, and the other in the same 
 direction, but farther off, by then looking at the nearest, the more distant 
 will appear double, and by lo'oking at the more distant, the nearest will 
 appear double. 
 
 The reason for applying the term " point of distinct vision" to the centre 
 of the retina, is felt at once by looking at a printed page, and observing that 
 only the one letter to which the axes of the eye is directed, is distinctly seen; 
 and, consequently, that although the whole page be .depicted on the retina 
 at once, the eye, in reading has to direct its centre successively to every part. 
 
 On examining a dead eye, the point of distinct vision is distinguishable 
 from the retina around by being more transparent. It might have been 
 expected that this point would have been where the optic nerve enters the 
 eye : but, in fact, the optic nerve enters considerably nearer to the nose than 
 the point of distinct vision is ; and singularly, where it enters, the part is 
 altogether blind or insensible. Had the two optic nerves, therefore, entered, 
 at corresponding points of the retina, (in the sense explained above,) there 
 would have appeared a black spot on every object opposite to the insensible 
 points; but as the case really stands, the part of any object from which the 
 light passes to the insensible or blind part of one eye must be opposite to a 
 sensible part of the other. The existence of the blind spot, where the nerve 
 of the eye enters, is discoverable by placing in a row three objects wafers, 
 for instance across a table, with intervals of about two inches between them, 
 and then looking with one eye, (the other being shut) from a distance of about 
 eight inches, at the wafer which is on the side of the nose; the middle 
 wafer will be invisible, although the eye will see that on each side of it ; and 
 if the eye be then directed to the middle wafer, the external one will disap- 
 pear. Another proof is obtained by shutting one eye and looking with the 
 other at the points of two fingers held together before it; if one of the fin- 
 gers be then gradually moved away laterally, its point when at a certain dis- 
 tance from the other will disappear, but will be seen again when its distance 
 is still increased. 
 
 It appearing, from the explanations now given, that there canot be perfect 
 sight unless where a perfect image is formed on the retina, and the truth 
 having been formerly explained, that images behind any lens will be at dif- 
 ferent distances from it, according to the various distances of the objects in 
 front, that is to say, according as the pencils of light which fall upon it have 
 more or less of divergence in them, it follows, that the eye in being able, as 
 it is, to see distinctly objects at different distances, (the nearest is about five 
 inches,) possesses a power of altering the relation of its parts to accommodate 
 itself to the circumstances. We do not yet perfectly know whether it does 
 this by lengthening or changing the form of the ball through the action of 
 the surrounding muscles, or by changing the place or the form of the lens, 
 but that one or more of these events occurs there can be no doubt. 
 
 Among the eyes of the myriads of mankind, however, it happens that all 
 do not originally possess these powers exactly in the requisite degree, and 
 that many lose them, as life advances, from a natural and usual decay. 
 
 Persons are called short-sighted whose eyes from too great convexity of 
 the cornea or lens, have so strong a bending or converging power, that the 
 
THE EYE SHORT SIGHT LONG SIGHT. 
 
 351 
 
 rays of light entering them are brought 
 
 to a focus before reaching the retina Fig. 149. 
 
 at a, for instance, instead of at b : so 
 
 that the rays, by spreading again beyond 
 
 the focus, produce on the retina that 
 
 sort of indistinct image which is seen in 
 
 the camera obscura, of which the screen 
 
 is too distant from the lens. This 
 
 defect of sight obliges the individual 
 
 when using the naked eye to hold ob- 
 
 jects very near it, that the consequent greater divergence of the rays may 
 
 be proportioned to the unusual refracting power of the eye or the person 
 
 may find a remedy in placing concave lenses between the object and the 
 
 eyes, which lenses, by rendering light from objects at a usual distance 
 
 more divergent, (as explained page 340,) cause the perfect images in the 
 
 eye to be formed farther from the lens, and thereby on the retina itself. 
 
 Without concave spectacles as the lenses are called when fixed together in 
 
 a frame persons with the defect now under consideration cannot see dis- 
 
 tinctly any object that is distant, for the rays, coming nearly parallel, are 
 
 quickly gathered to a focus. This defect often diminishes with years, and 
 
 the person who in youth needed spectacles, in old age sees well without 
 
 them. 
 
 There is an opposite defect of deficient converging power in the eye, 
 dependent on a too great flatness of the cornea or lens, and which is much 
 more common than the last-mentioned defect ; indeed, the great majority of 
 persons after middle age sooner or later begin to experience it. In this case, 
 the rays of light are not yet collected into a focus when they reach the 
 retina : they would only meet at b, 
 
 for instance, instead of as they should Fig. 150. 
 
 do ate, and hence the image is indis- 
 tinct, in the same* manner as in a 
 camera obscura, when the screen is 
 held too near the lens. Persons 
 suifering this defect cannot, when 
 using the naked eye, see distinctly 
 to it, because the gathering or converging power of the eye cannot conquer 
 the great divergence of rays coming from a near point ; and hence such per- 
 sons always remove objects under consideration to a considerable distance, 
 often to that of arm's length, so as to receive from them only the rays 
 nearly parallel. These persons in contradistinction to the last described, 
 are called long -sighted persons } and after middle age, most persons become 
 more or less long-sighted. Their defect is remedied by the common convex 
 spectacles, which do part of t*he converging work, so to express ourselves, 
 before the light enters the eye, leaving undone only that which the eye can 
 easily accomplish. As this defect, like the last, is met with in all degrees, 
 spectacles must be chosen accordingly. Certain curvatures or strengths of 
 these have been particularized and numbered as naturally belong to dif- 
 ferent ages or periods of life, but each person should choose under the 
 direction of an experienced judge, until that strength be found which ena- 
 bles him to read, without any straining of the eyes, at the common dis- 
 tance of from twelve to eighteen inches. We cannot apply the mind to this 
 part of our subject without feeling admiration at what science has accom- 
 plished for man in assisting and restoring his sight. Now that in civilized 
 
352 LIGHT. 
 
 society, the common employments and enjoyments of life require a visual 
 power capable of distinguishing such minute objects as written or printed 
 characters, to deprive old men of their spectacles, would be to condemn 
 many of them to useless inactivity and a listless blank of mind for the 
 remainder of their lives. 
 
 An eye much accustomed to examine near and minute objects, often loses 
 something of its pliancy, and becomes defective when tried at a distant 
 thing, as the watchmaker's eye, the engraver's, &c. On the other hand, 
 the old seamen's, which has so often and uninterruptedly been bent on the 
 distant horizon, straining to catch the view of an expected sail, or of land, 
 has a power- of discovering distant things which is wonderful ; but it often 
 experiences deficiency in regard to near things. 
 
 A man who uses his eyes under water sees very indistinctly, because the 
 difference of density between water and the eye not being so great as be- 
 tween air and the eye, the bending or refraction of light entering from the 
 water is not so great as of light entering from air, and the internal structure 
 of the human eye being adapted to the greater refraction, perfect images are 
 not formed on the retina. A man to see well under water, therefore, requires 
 to aid the usual power of his eyes by strong convex spectacles. It is to 
 meet the necessity now explained, that the lens of a fish's eye is extremely 
 convex, or almost round, as is everyday seen in the white round bead which 
 issues from the eye of a boiled fish that little globe being the crystalline 
 lens of the fish coagulated or hardened like the white of an egg during 
 cooking. 
 
 There are many important considerations connected with the sensibility of 
 the retina, which regard rather the laws of life than of light, but we must 
 here glance at a few of them. 
 
 Any impression of light made upon the retina lasts for about the sixth 
 of a second. Hence when the burning end of a stick is made to sweep 
 rapidly across the view, its path appears to the eye a Irne of light : ^nd if 
 it be made to revolve in a circle six times in a second, as when moved by the 
 hand or fixed to a turning wheel, that circle will appear to the eye a com- 
 plete ring of fire. The polished end of an elastic wire, of which the other 
 end is fixed in a block of wood, when caused to vibrate, similarly forms a 
 line or a curve of light. A harp-string, while vibrating as it sounds, ap- 
 pears like a flat transparent riband. Lightning or other meteor darting 
 across the sky, although in fact but a moving luminous point, is generally 
 thought of as a long line of light : the term forked-lightning has reference 
 to this prejudice. The same remark applies in a degree to a sky-rocket in 
 its rapid ascent. Two or more colours painted separately on the rim of a 
 wheel which is made to turn rapidly, appear to a spectator to be as com- 
 pletely united as if they were really mixed ; it has been already explained 
 how patches of all the colours of the rainbow, when mixed in this way, form 
 white light. If on one side of a card a little bird be painted, and on a 
 corresponding part of the other side a cage, then, on making the card turn 
 rapidly by twisting between the fingers and thumbs two threads fixed to its 
 opposite edges, the bird and cage will be seen at once, and the bird will 
 appear to be imprisoned in the cage ; or, if a pensive Juliet, sitting in her 
 bower, occupy one side of the card, and a longing Romeo the other, by the 
 magic turn of the threads the lovers may instantly be brought together. 
 Dr. Paris displayed taste and an amiable ingenuity in designing this toy 
 with great variety of subjects. 
 
THE EYE JUDGING BY APPEARANCES. 353 
 
 A certain intensity of light is necessary to distinct vision, but the degree 
 varies with the previous state of the organ. A person passing from the bright 
 day into a shaded room, might for a time fancy himself in totftl darkness, and 
 to persons sitting in the room, and become so accustomed to the less light as 
 to see well with it, he might appear to be almost blind. The dawn of morning 
 after the darkness of night appears much brighter than an equal degree of 
 light in the evening. When, as the night falls, our lamps or candles are first 
 introduced, the glare is often for a time offensive to the eye; and a similar 
 feeling but still stronger is experienced, when in the morning, bed-room, 
 window shutters or close drawn curtains are suddenly opened After the 
 repose of night, the sensibility of the eye, when first opened, is often such 
 that the globules of blood moving in the capillary vessels of the retina pro- 
 duce the impression there of little balls of light pursuing one another along 
 the tortuous vessels. To a prisoner after long confinement in a dark dun- 
 geon, the light of the sun is almost insupportable. And a dungeon, which to 
 common eyes is utterly dark, still to its long-held inmate has ceased to be 
 so ; thtre are various instances in the records of the barbarous ages of 
 prisoners confined for years in darkness, deemed absolute, but who after a, 
 time could see in it, and make entertaining companions of the mice and 
 spiders which frequented their cells. The darkness of a total eclipse after 
 bright sunshine, appears deeper than that of midnight, because of the sudden 
 contrast. The long polar night of months ceases to appear very dark to the 
 polar inhabitants If an eye be directed for a time to a black wafer laid on a 
 sheet of white paper, and be then turned to another part of the sheet, a por- 
 tion of the paper at that other part, of the size of the wafer, will appear 
 brilliantly illuminated } for the ordinary degree of light from it appears intense 
 to the part of the retina lately receiving almost none. An eye directed long 
 and intensely upon any minute object as when a sailor watches a speck in 
 the distant horizon, supposed to be a ship, or when a hunter on the brown 
 heath keeps his eye, fixed on a bird nearly of the colour of the heath or 
 when an astronomer gazes long at a little star has the sensibility of its 
 centre at last exhausted, and ceases to perceive the object ; but on directing 
 the axis of the eye a little to one side of the object, so that an image may 
 be formed only near ike centre, the object may be again perceived, and the 
 centre in the mean time enjoying repose, will recover its power. 
 
 But the most extraordinary fact connected with the sensibility of the retina 
 is, that if part of it be strongly exercised for a time by looking at some bright- 
 coloured object, on the eye being then turned away or altogether shut, an 
 impression of spectrum will remain of the same form as the object lately con- 
 templated, but of a perfectly different colour. Thus if an eye be directed for 
 a time to a red wafer laid on white paper, and be then shut or turned to an- 
 other part of the paper, a beautifully bright green wafer will be seen ; and 
 vice versa, a green wafer will produce a red spectrum : an orange wafer will 
 similarly produce a blue spectrum ; a yellow one a violet spectrum, &c., and 
 a. cluster of wafers will produce a similar cluster of opposite colours. Then 
 if the hand be held over the closed eye lids to prevent entirely the approach 
 of light to them, the spectrum of bright objects will appear luminous sur- 
 rounded by a dark ground, and when the hand is again removed, the contrary 
 will be true. Again, if the eye be in a degree fatigued by looking at the set- 
 ting sun, or even at a window with a bright sky beyond it, or at any very 
 bright object, on then shutting it, the lately contemplated forms will be per- 
 ceived, first of one vivid colour, and then of another, until perhaps all the 
 primary colours have passed in review. These extraordinary facts prove 
 
 23 
 
854 LIGHT. 
 
 that the sensations of 1'gtt and colour, although excitable by light are also 
 producible without it. UThis truth gave occasion to Darwin's ingenious 
 theory, that th^ sensation of any particular colour, of- red, for instance, is 
 dependent upon a certain state of contraction of the minute fibres of the 
 retina, and that the fibres, when fatigued in that condition, seek relief when 
 at liberty, by throwing themselves into an opposite state, as a man whose 
 back is fatigued by bending forward, relieves himself not by merely standing 
 erect, but by bending the spine backwards which new condition in the eye, 
 whether produced by light or by any other cause, gives the sensation of 
 green. He applied his explanation similarly to all other cases of colour. 
 It is remarkable that the colours which thus appear opposite to each other in 
 kind are those which, when the solar spectrum produced by a prism, as 
 described a few pages back, is painted round a wheel or circle, are opposite 
 to each other in place. 
 
 There are persons who, although having distinct perceptions of form, and 
 of light and shade, have not the power of distinguishing colours. It is com- 
 mon for such persons to deem pink and pea-green (naturally opposites) the 
 same colour, and therefore, not to distinguish difference of colour in a red 
 berry and the leaves around it. A man with this defect trusting to his own 
 judgment, has, without knowing it, dressed himself like a parrot. 
 
 ' The mind judges of external objects l>y the relative size, brightness, colour, 
 &c., of the minute hut perfect images or pictures of them formed at the 
 back of the eye on the expansion of nerve called the retina ; and the art of 
 painting is successful in proportion as it produces on a larger scale such 
 a picture, which, when afterwards held before the eye to reproduce itself in 
 miniature upon the retina, may excite the same impression as on the origi- 
 nal object." (Read the Analysis, page 325.) 
 
 "We now understand how an admirable miniature resemblance of the objects 
 before us is produced upon the retina of the eye, by the light from them re- 
 fracted in passing through the different parts of the eye ; but after all, this is 
 only a picture, and the inquiry remains which many persons would suppose 
 so simple as to be trifling, but which is in reality most curious and important 
 how we are thereby enabled to judge of the magnitudes, distances, and 
 other particulars respecting the things examined ? Here it will be found, to 
 the surprise of persons first entering upon the subject, that we learn the 
 meaning of a scene or pictorial signs only gradually, as we do of any other 
 system of signs, and that a person whose eyes, although perfect, had been 
 kept covered from infancy up to maturity, would no more " see," in the 
 complete sense of the word, that is, understood, any sense or prospect on 
 which he first opened his eyes, so as to have a perfect picture of it on his 
 retina, than a child understands or can read a printed page, when he first 
 looks into a book. Most interesting information has been obtained on this 
 subject, by observing the facts where blindness from birth has, by surgical 
 operation, been suddenly cured in persons arrived at maturity. 
 
 If a man were placed from infancy in an apartment fitted up as a camera 
 obscura, and had no means of becoming acquainted with external nature, but 
 by watching the images appearing on the screen, he could learn scarcely 
 any thing of the universe around him ; but if after a time he were allowed to 
 walk out, and to examine by the touch and by the measurement the different 
 objects whose images he was in the habit of viewing, and to ascertain what 
 size, shape and distance of an object corresponded with a certain magnitude 
 
THE EYE VISUAL ANGLE. 355 
 
 form, position, and brightness of image, the transient imagery might at last 
 be to him a very clear indication of the real particulars : making him in 
 imagination present to the objects, almost as if he went and examined them 
 with his hands. In the same manner, in a degree, the mind may be con- 
 sidered as stationed in or about the little camera obscura of the eye whence 
 it cannot itself escape to examine external nature; but must learn the mean- 
 ing of the images formed on the retina, by commanding the services of the 
 bodily limbs or members, and the other organs of sense. The judging of 
 things by sight, then, is merely the interpreting one set of signs, as judging 
 by sounds or language is interpreting another, and judging by hieroglyphics 
 or any written character is interpreting a third. The common visual signs 
 on the retina, however, are of all signs the most readily learned or under- 
 stood, from having certain fixed relations in form, magnitude and position 
 to the things signified ; while words, hieroglyphics, and written characters, 
 are quite arbitrary, and have no such relations. 
 
 Bodies differ and are distinguished among themselves chiefly by their 
 comparative dimensions, that is, their form and magnitude, or shape and 
 size : and to ascertain these and the relative distances and positions, are the 
 great objects which by the eye the mind seeks to accomplish. Now it effects 
 its ends by considering collectively. 
 
 1st. The tpace and place occupied by objects in the field of view, mea- 
 sured by what is called the visual angle. 
 
 2d. The intensity ofliyht, shade, and colour. 
 
 3d. The divergence of the rays of light. 
 
 4th. The convergence of the axes of the eyes. 
 
 We shall treat of these particulars separately in the order now stated. 
 
 ~Lst. The space and place occupied in the field of view, measured by iae 
 
 visual angle. 
 
 The term field of view is used to designate that open or visible space be- 
 fore the eyes, in which objects are seen : and it may mean either the small 
 field visible in one position of the eyes, or that which is perceived on direct- 
 Fig. 150. 
 
356 LIGHT. 
 
 ing them all round. If a man as at e, were surrounded by a large globe 
 or sphere of glass as a, through which his eye at the centre might view the 
 several objects around occupying certain situations and certain proportions 
 of the circumference, and if the sphere Ijad any equal divisions or degrees 
 marked upon it all round, he would be able at once to say exactly what 
 portion of his sphere or field of view was shadowed or occupied by any 
 single object, as the cross here shown at i, and thus to describe very intelli- 
 gibly either for his own recollection, or to inform others, its relative magni- 
 tude and situation as then appearing to him, just as he might say, on 
 looking at a tree in the garden through a common window which is a por- 
 tion of the field of view really divided by the cross bars, whether he saw the 
 whole tree through one pane or through several, and through which pane or 
 panes he saw it. It may be remarked farther, that whether the supposed 
 sphere of glass were large or small, viz., were as b or c, the part of its surface 
 apparently occupied by any object either beyond or within it, would bear the 
 same proportion to the whole surface ; if a d were a tenth of the small circle 
 or globe, c g would be a tenth of a larger. Now as men have found it con- 
 venient to consider a circle (and every circle) as divisible into 360 degrees, 
 (which are smaller, therefore, in a small than in a larger circle, although in 
 each having the same relation to the whole,) the ready mode of comparing 
 the apparent magnitude of objects is to say how many of these degrees of the 
 field of view each object occupies : and this is really what is meant by tne 
 apparent size of an object. And because the most convenient way of mea- 
 suring a portion of a circle, of which the whole is not seen, is to measure by 
 a fit instrument the angle or corner formed at its centre by lines drawn from 
 the extremities of the portion, as here the angle at e formed by the lines c e 
 and g e, the object is said either to occupy a certain number of degrees of the 
 circumference of the circle, or to subtend an angle of the same number of 
 degrees at its centre, and this angle is called the visual angle, the subject of 
 our present disquisition. 
 
 The visual angle, then, in regard to any object, is that included between 
 the lines or rays, as a u and d i } which, from the extreme points of the object, 
 
 Fig. 151. 
 
 as a d } meet and cross in the lens of the eye, and go afterwards to form the 
 extremes of the image on the retina, and, as formerly explained, the angle is 
 the same on each side of the lens, viz , towards the object or towards the 
 image. 
 
 Now if all bodies were at the same distance from the eye, the magnitude 
 of their images formed on the retina, or in other words, of the visual angles 
 subtended by them, would be an exact measure of their comparative real 
 magnitudes, as is seen in i u, the image of the great cross a d, and in i o the 
 image of the small cross b d : but it is evident here, that the cross c e, which 
 
THE EYE APPARENT SIZE OF OBJECTS. 357 
 
 is twice as large as b d, makes, because twice as far off, an image of only the 
 same size as b d, and an image therefore only half as large as that of a cross 
 a d equal in size with itself : and the same rule of proportion holds for all 
 other comparative distances at a hundred times the distance, an object 
 appearing only the hundreth part as tall, and so forth. To judge, therefore 
 by the eye, of the true size of an object, we must know its distance as well 
 as its apparent size or visual angle. 
 
 Many familiar facts receive their explanation from the law of the visual 
 angle or apparent size being less always in proportion as the distance of 
 an object is greater. 
 
 A man (instead of the cross here shown) ate?, standing near the outside of 
 a window, as b c (here represented edgeways) may to the eye of a spectator 
 within the window at h, subtend the same visual angle, or appear as tall as 
 the window, the light from the man's head passing through the top of the 
 window, and that from his 
 
 feet passing through the Fig. 152. 
 
 bottom ; but if the man 
 then moves away from, the 
 window, the eye of the 
 spectator will be able to see 
 his whole body through a 
 s'rnaller and a smaller ex- 
 tent of the window, as 
 through half its height or a c, when he is twice as distant, or at/, and through 
 the third or o c, when he shall be three times as distant, or at g, and so forth, 
 for any other distance ; so that soon a small figure of a man cut in paper, if 
 applied upon the glass, would exactly cover the part of it through which the 
 light from him entered the spectator's eye, and would then, by completely 
 hiding him from view, be an exact measure of his apparent size : and at last 
 a fly passing over the pane might equally hide him, and the fly then would 
 subtend a larger visual angle than he, that is to say, would be forming on the 
 retina a larger image than the man. Thus it often happens in reality, that 
 a person sitting near a window, and intent upon some object of study or of 
 conversation, mistakes a fly on the glass for a man at a distance; or, on the 
 contrary, a man for a fly. It is ascertained that the eye, with an ordinary 
 degree of light, can see an object which in the field of view occupies only the 
 sixtieth of a degree (or one minute.) This space is about the 100th of any 
 inch in a circle of twelve inches in diameter, the eye being supposed in the 
 centre of the circle. Now a body smaller than this at six inches from the 
 eye, or any thing, however large, placed so far from the eye as to occupy in 
 the field of view less space than this, is invisible to ordinary sight. At four 
 miles off, a man becomes thus invisible. A pin-head near will hide a house 
 on a distant hill nay, will hide even the planet Jupiter, although 1,000 
 times bigger than this earth. 
 
 In accordance with the principle now explained, a marine telescope has 
 been constructed, in which the field of view is divided by fine cross wires, 
 or otherwise, so that the person using it can say at once how much of its 
 field any object occupies. When ships are in chase, it is common, by this 
 instrument, or some other which will detect a change of visual angle, or 
 apparent size, to view the fleeing or pursuing ship ; and if the apparent 
 size be observed to increase, the conclusion follows that the ships are 
 
358 LIGHT. 
 
 nearing each other ; if, on the contrary, the size diminishes, the chased ship 
 is escaping. 
 
 By applying this rule, whenever the real size of a distant object is known, 
 the distance is ascertainable, and, vice versa, where the distance is exactly 
 known, the size is deterrainable : for it is evident that if a body, as a ship, 
 known to be 100 feet tall, occupy or subtend in the field of vision the 360th 
 part of a whole circle, or one degree, the whole circle must be in circum- 
 ference 360 times 100 (hundred) feet, or 36,000; and the diameter of any 
 circle being nearly one-third of its circumference, while, in the case Supposed, 
 the distance of the ship is the half-diameter, we learn that distance. Again, 
 if we know the distance of a ship or other object to be a mile, and if we then 
 find the visual angle subtended by the object to be the 1,000th part of a 
 circle, we know its true size to be the 1,000th part of a circle, of which the 
 half-diameter or radius is one mile. It is by applying this rule in a manner 
 to be afterwards explained, that we determine the size of the heavenly 
 bodies. 
 
 We now perceive that if the rays of light coming to the eye through a 
 plate of glass, from objects seen beyond it, could leave marks in the glass, 
 at the points where they passed, and marks capable of giving out the same 
 kind of light as the objects, there would be formed upon the glass such a 
 representation or picture of the objects formed or viewed through it, that 
 when held before the eye, it would form on the retina, the image or images 
 the same in almost all respects as the objects themselves; for from the dif- 
 ferent points of the glass, light could dart to the eye of the same kinds and 
 in the very same directions as that originally coming from the objects. 
 Now the art of painting seeks so to dispose lights, shades and colours on 
 any plane surface, as to produce the sort of representation of objects here 
 contemplated, while the picture-frame has to recall the window-frame, or 
 edge of the plate of glass through which the true scene is supposed to be 
 viewed. It is admirable how perfectly this art now accomplishes its ends ; 
 and although there are still trifling differences between the effect upon the 
 eye, of the picture and of the realities which peculiarities we shall consider 
 presently, and how they may be combated so as to render the illusion 
 almost perfect, it is not one of them, as might be supposed from the small 
 extent of the canvass, that the picture appears to the retina smaller than the 
 objects themselves. Few people, before studying this subject, are aware 
 that in a good picture the size of the figures is always made exactly such, 
 that at the distance from the eye at which the picture is meant to be 
 viewed, they produce on the retina the very same size of image as would 
 be produced by the realities seen under the aspect represented in the picture. 
 To become sensible of this, let a person look through a window-pane, with 
 the eye at the distance of eight inches from it, and let him trace with a 
 sharp point upon the glass, previously coated with gum, the outline of the 
 scene beyond perhaps a street or square, he will find that the outline of 
 a man seen there at the distance of twenty paces, and appearing perfectly to 
 coincide with the boundaries of the person, so that, if opaque, it would just 
 hide the person, will be scarcely half an inch tall, while the figure of the 
 man a few hundred paces off, will appear so small, that the minuter fractures 
 could not be distinguished, even if they could be drawn. 
 
 Now as a person who reads the description of an elephant, does not 
 deem the animal larger or smaller because of the size of letter used in the 
 printing, or in the size of the accompanying engraved representation; and 
 as a man in a picture gallery viewing minatures and larger portraits, does 
 
THE EYE APPARENT SIZE OF OBJECTS. 359 
 
 not conceive of the originals according to the size of the representations : 
 and as a man viewing a well-executed picture of a Grecian temple, never 
 dreams, unless his attention be particularly directed to the fact, that upon 
 the canvass the distant pillars of the rows are much smaller than the near 
 ones ; but in all such cases the mind merely uses the signs to help it to con- 
 ceive of the things according to previous knowledge, or to other principles 
 of judging : so in any common case of seeing, the mind takes little account 
 of the apparent size of objects, but passes instantly from the types to the 
 realities, which are, generally, more or less known, and it soon ceases to be 
 aware that the apparent size of the same object ever changes. Few persons, 
 for instance, are aware that when two friends shake hands, each appears 
 to the mere eye of the other ten times taller than when he has walked tea 
 paces away; or that a chair at one end of a room appears to a person sitting 
 at the other, only half as large as a chair in the middle of the room, but 
 such are the facts; and they may be immediately proved by holding a 
 common eye-giass or ring at a certain distance from the eye, and then 
 looking through it at any similar objects placed at different distances ; then, 
 while of a chair standing near, only a small part will be visible through the 
 ring of a distant chair the whole may be seen ; and so of any other case. 
 At five miles distance, the fleets which met on the great day of Trafalgar 
 might have been seen through a marriage-ring as the picture-frame. There 
 are occasions, however, where the usual collateral helps to the immediate 
 recognition of objects being wanting, the observer's attention is strongly 
 aroused to the facts of their diminutive appearance produced by distance ; 
 for instance, when a man on a long sea-voyage first approaches a land of 
 which the features are in a degree new to him ; as when an Englishman 
 arriving in India, scarcely believes that the little specks which he sees scat- 
 tered along the shore are commodious dwellings, or that what seem to him 
 only luxuriant herbs or bushes, are magnificent palm-trees. 
 
 For the same reason that a distant body to the mere eye appears diminutive, 
 namely, the smallness of the visual angle subtended by it, so does a distant 
 motion to the mere eye appear slow. A carriage dashing past a pedestrian 
 in the street, may surprise him by its speed; but if viewed at the same time 
 by a spectator at the top of a lofty tower near, it seems to be but crawling 
 along the pavement. A ship driven before a tempest, seems to a sailor on 
 board almost to fly through the white foam which surrounds-tar ; but if then 
 observed by a spectator on shore, as an object on the distant horizon, she is 
 scarcely perceived to change her place. A balloon high in the air, and borne 
 aiong on the wings of the wind at the rate of seventy or eighty miles an hour, 
 may still for a time leave a spectator on earth doubtful as to whether it be in 
 motion, or in what direction'it moves. The moon in her orbit wheels round 
 the earth at the astonishing rate of about 2,000 miles an hour, yet, owing 
 to her distance from it, her motion is not visible to the naked" eye of the 
 inhabitants of the earth, except by comparing her place at considerable 
 intervals. In respect to bodies still more distant than the moon, the truth 
 at present under consideration is still more striking. 
 
 Having now explained how the apparent transverse measures or breadth of 
 bodies and of space, in other words, the visual angle subtended by them, 
 is affected by their distance from the eye, we proceed to show how it is 
 affected also by their po&ition. 
 
 A globe at a certain distance from the eye, however turned, preserves the 
 same appearance in the field of view, and its outline traced upon a plate of 
 
360 LIGHT. 
 
 glass held across between it and the eye, is, like its direct shadow upon a 
 wall, always a circle ; but an egg which, held in one position, produces a 
 circular outline or image, when held in another, produces an image nearly 
 oval. A wheel when viewed sideways appears a perfect circle, when viewed 
 edgeways it appears a broad straight band or line, and in any intermediate 
 position it appears oval. The apparent form, then, of a body, is only a hint 
 to the mind from which, by former experience or instruction, it guesses at 
 the true form. If a man had never seen an egg but endways, he never 
 could have known that it was not a sphere. 
 
 If any long, straight object, as a beam, be placed with one of its ends 
 directly to the eye, that end only can be seen, and according to the case, 
 must appear a square or circle of the diameter of the beam ; if it then be 
 placed with its side directly to the eye, its whole length will be seen and if 
 placed in any intermediate position, it will appear more or less shortened ' } 
 in all cases, its outline on the retina being similar to that of its shadow 
 on a wall directly behind the person. A man has advanced on a spear 
 pointed directly to his eye without seeing, or on the end of a bar of iron 
 carried on the shoulder of a porter in the street. A common telescope held 
 with its end to the eye, appears a perfect circle, if then inclined a little, it 
 seems to jut out on one side, and as the inclination is increased, it juts out 
 more and more, until it displays its whole length. A great ship of war 
 whose stern is towards a spectator, appears a rounded building with its 
 rows of windows like those of a peaceful habitation j but as it turns, it 
 gradually reveals the long batteries of bristling cannon. A straight row of 
 a thousand similar objects, as of soldiers in rank, pillars, trees, &<?., may 
 appear to a person at the extremity as only one object of the kind, the 
 nearest individual completely hiding all the others ; but if viewed from the 
 side and at a certain distance, the individuals may be counted. 
 
 The appearance now treated of is called foreshortening, and is to be noted 
 wherever surfaces or lines are not placed so as directly to face the spectator. 
 Perhaps the most important case of foreshortening is when the eye looks 
 more or less obliquely along an extensive plane surface, the general surface 
 of the earth, for instance, or of the sea, by estimating aright the foreshortening 
 of which, we judge of the distance or situation of the objects placed upon it. 
 It will be readily perceived, that in all such cases the more distant portions 
 of the surface are progressively more foreshortened than the nearer; for a 
 man standing at a on the plane as a b, and with his eye at c, looking down 
 
 before him, seesapor i^n of 
 
 Fig. 153. the surface a <7 almost direct- 
 
 ly, or with a little foreshort- 
 ening, and an extent, as a d, 
 equal to the height of the 
 eye, will subtend in his eye, 
 an angle of 45, or half a 
 right angle, viz., the angle 
 a c d, and therefore rather 
 more than half of all that can 
 be subtended by a straight line or space from his feet to the horizon, how- 
 ever distant ; the next equal spaces, viz., d f, will subtend an angle of only 
 18, viz., d c.f, the next of 8, viz., f eg, and so on; #nd as he carries his 
 view more and more forward, the surface becomes to it more and more oblique, 
 until at last the light coming seems more to skim along the level than to rise. 
 This explains why a person having a side view of a row of separate objects, 
 
THE EYE FORESHORTENING. 861 
 
 as of men in line, trees, pillars, &c., may see through or between the nearest 
 of them, but towards the extremes sees them as if standing in closest possible 
 array, or as if forming a continued surface. The same remark explains why 
 masses of cloud scattered uniformly over the sky, may allow a spectator to 
 see wide intervals of the blue heaven over head, while all around there is a 
 dense cloudy mass appearing to rest on the horizon. 
 
 If a man standing on a hill look down upon a field or plain which is well 
 known to him, and if he see some objects near its side, and some near its 
 middle, and some near its distant border, he knows at once how far they are 
 from him and from one another. Similarly, when viewing the ocean from a 
 lofty cliff, and seeing ships scattered over its face, he may judge correctly of 
 their distance, for he can see only a certain extent of ocean which becomes 
 to him as a known field. The man stationed at the flag-staff on the High 
 Knowl of St. Helena, looks down upon the circular field of the Atlantic a 
 hundred miles broad, and can tell the distance of a sail in sight to within 
 a mile or two. Now, although the ground-plan of an extensive landscape 
 may not be so level as the face of the ocean, there is still an approximation, 
 which very considerably assists a spectator's judgment of distances. 
 
 Painters are not only careful to foreshorten, according to the proportion 
 explained above all, the objects which they portray, but they often avail 
 themselves of the principles to produce most striking effects. For instance, 
 Martin, in many of his beautiful designs, by judicious foreshortening, has 
 exhibited miles in extent of gorgeous architecture and of armed men, on a 
 space of canvass that would seem scarcely more than sufficient to receive a 
 very few figures ; he has made a single magnificent pillar or accoutred warrior 
 in the foreground, become the type which first fills the mind with admiration, 
 and then sends it along the retiring lines of beautiful perspective, where 
 every tip or edge renews the first impression. A man lying on a high table 
 or bed, with his 1 feet towards the spectator, is foreshortened into a roundish 
 heap, of which the soles of the feet hide the greater part. This is the de- 
 scription of ths painting which has been called the " Miraculous Entomb- 
 ment," and it is because an unreflecting spectator moving sideways with the 
 expectation of seeing more of the body, still sees only the soles of the feet, 
 and may suppose the body turned round so as to keep the feet towards him, 
 that the painting has received its appellation. For nearly the same reason, 
 the eye of a common full-faced portrait may seem to follow a spectator to 
 whatever part of the room he goes, for by moving to one side he cannot 
 see the side of the eye-balls. It is related of a murderer, that he was im- 
 pelled to commit suicide by observing that the eyes of the portrait of his 
 victim were always fixed upon him. A rifleman portrayed as if taking aim 
 directly in front of the picture, will appear to every spectator in the room 
 to be pointing at him especially. To terrify young ladies, a little arch Cupid 
 has been similarly represented with his arrow pointed directly at them, and 
 just ready to let it slip from his bended bow. 
 
 As the painter, availing himself of a knowledge of the principles now ex- 
 plained, by which the eye usually judges of size and distance, may produce 
 on his canvass the most charming illusions, so may the tasteful landlord, in 
 his ornamental gardens and pleasure-grounds, by working his levels into 
 artificial undulation of hill and dale, and clothing these with tree and edifice 
 of magnitudes to correspond make the eye of a spectator luxuriate in the 
 contemplation of the supposed extensive plains, lofty mountains, widespread 
 lakes, and distant padogas all within the narrow space of an acre or two; 
 thus, by other means, producing on the retina the same impressions as Claude 
 Poussin, or Wilson has done by the finest pictures. 
 
362 
 
 LIGHT. 
 
 When any objects or mass of objects is foreshortened, by one part being 
 farther from the eye than another, that part appears also in a proportion 
 smaller than the other. For example, in a straight row of similar houses, 
 pillars, trees, &c., (see the next cut,) those nearest to the eye will, on a glass 
 held before the eye to receive their images, from the largest images, and there 
 will be a gradual diminution from the largest to the least, so that lines drawn 
 upon the glass along the tops and bottoms of the images would tend to a 
 point, called for a reason to be explained below, the va?rishwg point. Thus 
 a person looking from a window along a straight street, must, to see the 
 chimney of the nearest house, look through the top of the window, and to see 
 the street door must look through the bottom ; but the most distant house, 
 both top and bottom, may be concealed from view by a little spot upon the 
 glass at the height of the eye. This remarkable tapering of foreshortened 
 objects may of course be strikingly observed on looking at any correctly, 
 made drawing or engraving meant to represent a retiring row of similar ob- 
 jects; such drawing being in truth an attempt to realize by art, on the 
 surface of a sheet of paper, the appearance of the objects as seen through a 
 window or aperture the size of the paper. 
 
 The art which gives rules for tracing objects on a plane surface, as they 
 would appear to an eye looking at them through that surface, if transparent, 
 with their various degrees ; first, of apparent diminution on account of dis- 
 tance ; and, secondly, of foreshortening on account of the obliquity of view, 
 is called, from the Latin word signifying to look through the art of perspec- 
 tive. It regards entirely the two particulars now mentioned ; and notwith- 
 standing the terror with which, in the imaginations of many young painters, 
 the study of it is clothed, by reason of the mathematical difficulties with which 
 it has usually been mixed up, it is in itself exceedingly simple. We hope 
 that a person capable of ordinary attention, will, after what we have already 
 said, and after the few additional remarks which we have still to make on the 
 appearances of nature, be able readily to understand the great rules of per- 
 spective. Although, without a knowledge of these rules, a quick eye soon 
 enables its possessor to sketch from nature with much truth ; and although 
 the two instruments, the camera obscura, already described, and camera lu- 
 cida, to be described in a future page, give almost mathematical accuracy to 
 drawings from nature, without requiring other skill in the draughtsman than 
 
 to trace and make permanent, 
 
 Fig. 154. w ith ink or pencil, the lines of 
 
 light which he sees on the 
 paper ; still the subject is so 
 interesting to all who attempt 
 to sketch, and indeed to all 
 who wish to look intelligently 
 either at nature or at the 
 works of art, that none who 
 have the opportunity of study- 
 ing it should neglect it. 
 
 Supposing a straight row of 
 similar objects, as of the stone 
 blocks or pillars represented 
 here from a or b to S, to be 
 viewed by a person stationed 
 
 near the side and end of the row, as over the point O, then, because, as 
 already explained, objects to the eye appear smaller in exact proportion to 
 
THE EYE PROSPECTIVE. 363 
 
 their increased distance from it, the second block, if twice as far off as the 
 first, would appear only half as large ; the third if three times as far, would 
 be only one-third as large, and so on to any extent and for any other pro- 
 portions ; and if the 1,000th of any other block, owing to its distance, sub- 
 tended to the eye an angle less than the sixtieth of a degree of the field of 
 view, it would be altogether invisible (as explained at page 333,) even if 
 nothing intervened between it and the eye. Then, where the row ceased to 
 be visible from the minuteness of the parts, or from the fact of the nearer 
 objects concealing the more remote, it might be said to have reached its 
 vanishing point. When a student of perspective has learned all that regards 
 the vanishing point in relation to a line, and the corresponding vanishing 
 line, in relation to a surface, he has learned half of his art. The above cut 
 considered as the representation of a street running directly south to S, 
 sketched from a window looking along its centre, will serve as a useful 
 illustration. 
 
 It is important, first, to remark, that in any case of a straight line, or^, 
 row of objects thus vanishing from sight, as here the line or row a $, in 
 whatever direction it points, whether east, west, north, or south, &c., in that 
 direction, too, will its remote or vanishing extremity appear to be from the 
 eye. In this sketch the row a S is supposed to run directly south ; and 
 although the eye to see the near end of it, would have to look towards the 
 left hand, or in a degree east, still every successive pillar would be more and 
 more nearly south, and the point in the heavens, or in the picture, or in a 
 transparent plane before the eye, where the line would vanish, would be so 
 nearly south from the eye, and not to the east, because the pillars happened 
 to the east of the individual, that no ordinary measure would detect the 
 little want of correspondence ; then similarly, if there were more rows of 
 objects, as of pillars, houses, trees, &"c., parallel to the first, but considerably 
 apart from each, as the lines here a S, b S, d S, &c., still all would vanish 
 or seem to terminate in the very same point of the field of view. The rea- 
 son of this is easily understood. Let us suppose a line drawn directly south 
 from the eye to the point S, between the* parallel lines of pillars, houses, 
 and trees, a S, b S, d S, &c., also pointing directly south ; and let us sup- 
 pose the two rows of pillars to be 100 feet apart, then evidently for the 
 same reason as the space between the top and bottom of the pillars, that is 
 to say, their height becomes apparently less and less as their distance from 
 the eye increases, so will the space between each pillar and its opposite, or 
 between it and the point corresponding to it in the visual ray along which 
 the eye looks, become apparently less, and therefore the lines *of pillars 
 really 100 feet apart from each other, and 50 feet from the visual ray, will, 
 at a certain distance from the eye, (viz., where a space of 50 or 100 feet is 
 apparently reduced to a point,) appear to join, and the three lines will ap- 
 pear to meet in that point, beyond which none of them can be visible, and 
 which is therefore the vanishing point of all. The conception of this truth 
 may be facilitated by supposing a planet to be visible to the exact point of 
 the heavens at the moment of observation ; then, if the three parallel lines 
 were continued on to the planet, and were visible all the way, they would 
 arrive there with the interval between them just as when they left the 
 earth'; but as a planet, although thousands of miles in diameter, owing to its 
 distance from the earth, appears on earth only as a point, much more would 
 two lines only 100 feet apart be there undistinguishable in place by human 
 sight. And what is true of space of 100 feet between parallel lines, is 
 equally true of a mile or of thousands of miles. As a general ru!e, there- 
 
364 
 
 LIGHT. 
 
 Fig. 155. 
 
 fore, it holds, that all lines in nature parallel to each other, when represented 
 in perspective, teed towards an end in the same vanishing point; and that 
 point is the situation where the line terminates, along which the eye looks 
 when directed parallel to any one of the real lines. And this is true only of 
 lines all in the same level or horizontal plane, viz., such as might lie along 
 the surface of the sea, but also of lines placed one above another, as those 
 running along the tops and bottoms of the pillars here, or along the walls, 
 roofs, and windows of the houses, or along the roots and summits of the 
 trees, and indeed of all lines in whatever situation, provided they are paral- 
 lel to the visual ray, and therefore to one another. And the truth holds 
 equally with respect to lines which do not vanish at the " point of sight," or 
 centre of the picture, as with respect to those which do. When it is ascer- 
 tained, therefore, that a line or boundary of any natural or artificial object 
 has a certain inclination to the axis of the picture, or to what we have de- 
 scribed as the principal visual ray, then also is it known that all parallels 
 t$) that line have their vanishing point in the same spot of the field of view, 
 and a line supposed to be drawn from the eye to the heavens, or really 
 drawn from the eye to the picture in that direction, marks upon the picture 
 the true vanishing point. 
 
 It will be understood why, in a long arched tunnel or a cathedral, with 
 many longitudinal lines on its floor, walls, roof, &c., all such lines seen by 
 an eye looking along from one end, appear to converge to a point at the 
 other, like the radii of a spider's web; and why similarly in the represen- 
 tation of a common room, viewed from one end, all the lines of the corners, 
 
 tops and bottoms of windows, 
 floor, stripes, on a carpet, cor- 
 ners of tables, &c., being paral- 
 lel to each other, tend to the 
 same vanishing point as V. 
 The appearance of the lines in 
 the floor of this room may re- 
 call that of the furrows in a 
 ploughed field as seen from 
 one end, when they appear 
 like the ribs of a fan spread 
 out towards the spectator. 
 The same considerations will 
 explain the phenomenon often 
 to be observed, of two little 
 clouds seen near each other, 
 and almost motionless for a 
 
 time in the distant sky, but which on approaching the spectator with the 
 wind, appear gradually to separate, and in a corresponding degree to enlarge, 
 until one of them sweeps past considerably to the right hand, and the other 
 considerably to the left; after this, they lessen and approximate as they be- 
 fore enlarged and separated, and at last beyond the spectator, appear as 
 small and as near as when first observed. Clouds being so mutable and un- 
 certain in their forms, persons have been led to deem all apparent changes 
 in them, of form, size and place, to be real changes, and not, as they gene- 
 rally are, mere optical or perspective illusion. 
 
 By far the most important vanishing point in common scenes is the middle 
 of the horizon or level line, and in a picture properly placed, it is at the exact 
 height of the eye. It is marked S in the figure before the last, and V in the 
 
THE E Y E P ERSPECTIVE. 365 
 
 last figure. Because in houses, the roofs, foundations, floors, windows, &c., 
 are all horizontal, the vanishing points of their lines must be somewhere in 
 the horizon, and if the spectator be in the middle of a street or of a building, 
 and be looking in the direction of its walls, their vanishing point will be in 
 the centre of the scene or picture ; if he be elsewhere, it will be to one side. 
 In holding up a picture-frame, through which to view a scene suitable for a 
 picture, it would be found most generally befitting to raise it until the line of 
 the horizon appeared to cross it at about one-third from the bottom : this 
 fact becomes the reason of the rule in painting, so to place the horizontal line. 
 In beginning a picture, this line is usually the first line drawn on the canvas, 
 marking the place of the vanishing points of all level line and surfaces. And 
 the eye of the spectator is supposed to be placed before the middle of it and 
 generally about as far from the picture as the picture is itself long, such being 
 the extent of view which the eye at one time most conviently commands. 
 
 Understanding now that the apparent or perspective direction of all lines 
 in a scene is towards their vanishing points as above discovered, and the rule 
 having been given for determining these points in a drawing, we proceed 
 to show how much of a line drawn to any vanishing point belongs to the 
 known magnitude of any object through which it passes ; in other words, 
 how much an object is in perspective foreshortened in consequence of its 
 distance and obliquity of position. 
 
 If we suppose A S P to represent a plate of glass seen edgeways, and that 
 towards the point S in it, an 
 
 eye is looking from the Fig. 156. 
 
 point D, evidently then, a ij\ 
 
 line from P continued in the 
 direction of R and beyond 
 until vanished from sight, 
 
 would have as its perspec- 
 tive image or representation 
 
 on the glass a line reaching =-p ^""T^"""? ^^^3^1? 
 
 from P to S, S being, 
 
 moreover, the point ofsiyht here, and the pictorial vanishing point of the 
 line. Now to divide the representative line P S so as to correspond with 
 any given portions of the original line P R, &c., it would only be necessary 
 to draw other lines from the place of the eye I) to cut or touch the original line 
 in the situations desired, and these lines would cut the perspective line S P 
 as required : for instance, the portion of true line a b would be represented 
 by that portion of the image-line S P included between the two lines a D, 
 and b D, and so of any other portions. There are figures drawn on many 
 mathematical scales by which such problems as this can be at once opproxi- 
 matively solved ; and it would be possible, by trigonometrical calculation, to 
 solve them exactly in all cases : but the most generally convenient mode in 
 practice is to sketch on the intended drawing (as that of which the bounda- 
 ries are given in the next eut)the kind of measure shown above, by setting 
 off from the point of sight S, a distance on the horizontal line, as at I), equal 
 to the distance of the eye from the picture, and then by oblique lines from D 
 drawn upon the baseline P R, to cut the perpendicular line P S in the situa- 
 tions desired as is seen in the last figure, which differs from fig. 161 
 chiefly in having the point of distance marked be/ore its point of sight, in- 
 stead of, as here, laterally. And the line P S being always cut by the oblique 
 line from D in proportion to the length of base-line between P and the ex- 
 tremity of the oblique line, a horizontal line drawn through any point in if, 
 
866 
 
 LIGHT 
 
 cuts in corresponding proportions all the other lines which have their vanishing 
 points in the horizontal line at S, for instance a S, b S, &c. Thus, to draw 
 in perspective, on the surface above represented and prepared, a chess-board 
 or board of squares, it is necessary to set off the breadth of the board on the 
 
 Fig. 157. 
 
 I) 
 
 base-line to the right and left of P, viz., at b and a, and then to draw to the 
 point of sight as a vanishing point, the lines a S and b S, part of which lines 
 will, therefore, represent the sides of the board, and then to draw the diago- 
 nal b I), which, for the reason above stated, will cut the lines P S and a S in 
 proportion to the length of base-line to the right of their extremities ; a efb, 
 therefore, is a square seen in perspective, and any number of smaller in- 
 cluded squares are made by drawing lines from the vanishing point to equal 
 divisions on the base, and making cross lines where the diagonal cuts these. 
 Much of the delight which the art of painting is calculated to afford is lost 
 to the world, because persons in general know not how to look at a picture. 
 Unless a spectator places himself where he can see the objects in true per- 
 spective, so that he may fancy himself looking at them through a window or 
 opening, every thing must appear to him false and distorted. The eye 
 should be opposite the point of sight of the picture, and, therefore, on a level 
 with the line of the horizon, and it should be at the required distance, which 
 is generally at least as great as the length of the picture. But blame not 
 unfrequently rests also with the artist, from his having neglected the study of 
 perspective. It is very common, for instance, to see miniature resemblances 
 of architectural structures so foreshortened and tapered, that the eye, to see 
 them in true perspective, would require to be within an inch of the paper ; 
 whence at the usual distance of ten or twelve inches they are seen as hideous 
 distortions. The specimens, in the few preceding pages, necessarily exem- 
 plify in a degree this error, because the point of distance had to be marked 
 where there was but a small page. These figures, therefore, by any person 
 studying the subject particularly, should be drawn on a scale so much larger 
 as to allow the eye really to view them at the distance supposed. 
 
 A means of judging of the dimensions of the bodies by the visual angle, but 
 which depends neither on the absolute size of the image, nor on the fore- 
 shortening of the ground plane on which the body stands, is to use known 
 objects in view as measures for others near them which are unknown. 
 
 If any person of our acquaintance be standing at some distance from us 
 near another person who is a stranger, we know how tall the stranger is by 
 taking the acquaintance as a measure. 
 
 In pictorial representations of objects little familiar as to many people are 
 the Egyptian pyramids, the bodies of the whale, the elephant, the camel, &c., 
 human beings may be represented around them to serve as measures for the 
 
THE EYE PERSPECTIVE. 367 
 
 less known object. The Colossus of Rhodes seen from afar, might to a 
 stranger have appeared but an ordinary statue of a man, but the exact mag- 
 nitude would have been known as soon as a ship of known dimensions were 
 seen sailing into port between his gigantic limbs. 
 
 When an unpracticed eye is first directed from a distance to a great ship of 
 war, it will on many accounts dwell upon the object with wonder and admi- 
 ration ; but it may not judge truly of the enormous magnitude until it be 
 near enough to perceive the sailors climbing on the rigging, and appearing 
 there, by comparison, as flies or little birds appear among the branches of a 
 majestic tree. 
 
 By having a measure of this kind presented to us, the magnitude and eleva- 
 tion of great edifices are rendered more obvious. The magnificient pile of 
 St. Paul's in London becomes still more striking, when we discover visitors 
 looking from the balconies near the summit-cross. They appear so minute 
 among the surrounding huge masses that a person is for a while disposed to 
 doubt whether they be men : but the fact once ascertained, the grandeur of 
 the temple is most impressive. 
 
 Manny persons cannot distinguish between the little pilot balloon (some- 
 times dispatched before the great one to show the direction of the wind) and 
 the great balloon itself, until with the last they perceive the aeronauts as 
 little black points suspended under the globular clouds. 
 
 Strangers visiting Switzerland, on first entering the valleys there, are often 
 much deceived as to their extent. Being familiar generally with more lowly 
 hills and shorter valleys at home, which, however, from being near to the 
 eyes, form bulky images, and having at first no other measure, they almost 
 universally underrate the Alpine dimensions : they will wonder, for instance, 
 in the valley of Chamouny, that they should be travelling swiftly for hours 
 without reaching the end, where on entering they did not anticipate a drive 
 of more than half an honr. 
 
 The author once sailed through the Canary Islands, and passed in view 
 of the far-famed Peak of Teneriffe. It had been in sight during the afternoon 
 of the preceding day, at a distance of more than 100 miles, disappointing 
 general expectation by appearing then only as an ordinary distant hill rising 
 out of the ocean, but next morning, when the ship had arrived within about 
 twenty miles of it, and while another ship of the fleet, holding her course six 
 miles nearer to the land, served as a measure, it stood displayed as one of 
 the most stupendous single objects which on earth, and at one view, human 
 vision can command. The ship in question, whose side, showing its tiers 
 of cannon, equalled in extent the front of ten large houses in a street, and 
 whose masts shot up like lofty steeples, still appeared but as a speck rising 
 from the sea, when compared with the huge prominence beyond it, towering 
 sublimely to heaven, and around which the masses of cloud, although as lofty 
 as those which sail over the fields of Britain, seemed still to be hanging low 
 on its sides. Teneriffe alone of very high mountains, rises directly and 
 steeply out of the bosom of the ocean, to an elevation of 13,000 feet, and as 
 an object of contemplation, therefore is more impressive than even the still 
 loftier summits of Chiraborazo or the Himalayas, which rise from elevated 
 plains, and in the midst of surrounding hills. 
 
308 LIGHT. 
 
 It is because objects which are nearly on a level with us, as contrasted with 
 such as are either much above or much below, are in general more nume- 
 rously surrounded by other known objects which serve as measures of 
 comparison, that we judge so much more correctly of the size and distance 
 of those near our level than of others. 
 
 A man walking like ourselves on the sea-shore or other level, is at once 
 fully recognized ; and probably it may not occur to us, that he appears smaller 
 on account of the distance ; but if the same man be seen afterwards at an 
 equal distance above us, collecting the sea-fowl's eggs on the face of a cliff, 
 or below us, gathering shells on the beach when we ourselves have reached 
 the height, he appears no bigger than a crow ; yet in all the cases he is where 
 the same bulk forms the same magnitude of image on the retina. 
 
 Even on a horizontal plane, if the general surface be bare and uniform, 
 single distant objects appear very diminutive. This is true, for instance, of 
 a man seen apart from his caravan, while journeying across a s-iudy desert j 
 bul a man viewed at an equal distance, in the midst of a cultivated land- 
 scape, appears of his natural size. The same is true of a boat or ship seen 
 out on the high sea, as constrasted with the like viewed in a harbor where 
 other known objects are near them. 
 
 We may now understand why the sun and moon, at rising or setting, 
 appear to us much larger than when they have attained meridian height 
 although, if we examine them by any measure of the visual angle, as simply 
 by looking at them through the same ring or tube, we find that there is 
 scarcely a difference ; and singularly we find the difference to be, that the 
 orbs appear horizontally even less than when seen on the meridian, owing 
 to our being then about 4,000 miles more distant from them. The sun 
 and moon as they appear from this earth are nearly of the same size, 
 each occupying in the field of view about the half of a degree, or as much as 
 is occupied by a circle of a foot in diameter when held 125 feet from the 
 eye which circle, therefore, at that distance, and at any time, would just 
 hide either of them. Now when a man sees the rising moon apparently 
 filling up the end of a street, which he knows to be 100 feet wide, he very 
 naturally believes that the moon then subtends a greater angle than usual, 
 until the reflection occurs to him, that he is using as a measure, a street known 
 indeed to be 100 feet wide, but of which the part concerned, owing to its 
 distance, occupies in his eye a very small space. The width of the street 
 near him may occupy 60 of his field of view, and he might see from between 
 the houses many broad constellation instead of the moon only, but the width 
 of the street far off may not occupy, in the same field of view, the twentieth 
 part of a degree, and the moon, which always occupies half a degree will 
 there appear comparatively large. The kind of illusion now spoken of is 
 yet more remarkable when the moon is seen rising near still larger known 
 objects, for instance, beyond a town, or a hill which then appears within the 
 luminous circle. Any person who from the river-side terraces of Greenwich 
 has observed the sun setting beyond London, with St. Paul's Cathedral 
 included in the glorious picture, will recollect a most interesting example of 
 our present subject. That our ocular judgment of the size of the sun or 
 moon is thus influenced 'by the presence or absence of objects of comparison, 
 and not by the place of the bodies in the sky, is proved by the fact, that a 
 person viewing these bodies at any eleva-tion from the. bottom of some of the 
 Swiss valleys, where he might almost suppose himself placed at the centre 
 of the earth, and looking abroad along an endless extent of precipices if he 
 
THE EYE INTENSITY OF LIGHT. 369 
 
 can closely compare them with certain known magnitudes of ridge or forest 
 bounding his view, sees them as large as they appear from other situations 
 when rising beyond a low horizon. Another proof is aiforded by the case 
 of a balloon at a great elevation seen crossing the disc of the sun or moon, 
 and then appearing, however large in reality, as an absolute speck within 
 the vast luminous area. In a future paragraph it will be explained, that 
 another circumstance contributes to cause the sun and moon, when low, to 
 appear larger than when high, namely, their apparent dimness, owing to the 
 obstruction of their light in traversing the low dense atmosphere. 
 
 It may be remarked here, that the visual estimate formed of the great 
 size of the sun and moon when they are seen on the horizon, is not an 
 illusion, as is popularly supposed, but an approximation to truth, still pro- 
 digiously short of the reality. When we see a distant tree, or a house, or 
 a hill, apparently within the circumference of one of these orbs, it is really 
 true that the orb is larger than the tree, or house, or hill, just as another 
 more distant hill would be larger than nearer objects similarly surrounded 
 by its outline; but the celestial body is so much larger, that even if the 
 whole British Isles could be lifted away from the earth, and suspended near 
 the moon, as a map in the sky, they would hide from a spectator on earth 
 but a small part of the disc of the moon. 
 
 Having now shown that the visual angle or apparent size can be a measure 
 of the distance of any object only when the true size also is known, or 
 of the trjue size when the distance is known, we proceed to examine other 
 means which the eye commands for guessing at distances. 
 
 2d. Intensify of liylit, shade and colour. (See the Analysis, pages 325 
 
 and 355.) 
 
 It has already been explained that light, like every other influence, radi- 
 ating from a centre, becomes rapidly weaker as the distance from the centre 
 increases, being, for instance, only one-fourth part as intense, at double 
 distance, and in a corresponding proportion for other distances ; while it is 
 still farther weakened by the obstacle of any transparent medium through 
 which it passes. Now the eye soon becomes sufficiently familiar with these 
 truths, to judge from them, with considerable accuracy, of - the comparative 
 distances of objects. 
 
 The fine Gothic pile of "Westminster Abbey may break upon the view in 
 some situations where nearer edifices, and perhaps some minor imitations of 
 its beauties, already fill the eye with their strong lights, but the misty or 
 less distinct outline of the venerable pile may warn the approaching stranger 
 of its true magnitude, and prepare him for the enjoyment which a nearer 
 inspection of its grandeur and perfection is to afford. 
 
 A small yacht or pleasure-boat may be built according to the same model 
 or with the same comparative dimensions as a first-rate vessel of war, and 
 may be in view from the shore at the same time, only so much nearer than 
 the ship, that both shall form images of the same magnitude on the retina of 
 a spectator. In such a case, an unpractised eye might have difficulty to 
 discriminate, but to an old seamen, the bright lights of the little vessel, 
 contrasted with the softer or more misty appearance of the larger, would 
 declare the truth at once. A haziness occurring in the atmosphere between 
 the little vessel and the eye might considerably favour the illusion. 
 
 In a fleet of ships, if the sun's direct rays fall upon one here and there 
 
 24 
 
370 LIGHT. 
 
 through openings among the clouds, while the others remain in shade, the 
 former, in appearance, starts towards the spectator. In like manner, the 
 mountains of an unknown coast, if the sunshine fall upon them, appear 
 comparatively near, but if the clouds again intervene, they recede and mock 
 the awakened hope of the approaching mariner. 
 
 A conflagration at night, however distant, appears to spectators, generally, 
 as if very near, and inexperienced persons often run towards it with the 
 hope of soon arriving, but find, after miles travelled, that they have made 
 but a little part of the way. 
 
 A person ignorant of astronomy deems the heavenly bodies so much 
 nearer to the earth than they are, merely because of their being so bright 
 or luminous. The evening star, for instance, seen in a clear sky, over some 
 distant hill-top, appears as if a dweller on the hill might almost reach it 
 for the most intense artificial light which could be placed on the height 
 would be dim in comparison with the beauteous star; yet to a dweller on 
 the hill it appears just as distant as to one on a plain; nay, at a thousand 
 miles farther west, and, therefore, nearer, the appearance would still be 
 nearly the same. 
 
 The concave of the starry heavens appears flattened above, or as if its 
 zenith were nearer to the earth than its sides or horizon, because the light 
 from above having to pass through only the depth or thickness of the atmo- 
 sphere, is little obstructed, while of that which darts towards any place 
 horizontally through hundreds of miles of dense vapour-loaded air, only a 
 small part arrives. 
 
 The sun and moon appear larger at rising and setting than when midway 
 in heaven, partly, as already explained, because they can then be easily com- 
 pared with other objects, of which the size is known, but partly, also, because 
 "of the much less light arriving from them in the former situation, while 
 their diameters are nearly the same. 
 
 A fog or mist is said to magnify objects seen through it. The truth is, 
 that because it diminishes the intensity of the light from, them, it makes 
 them appear farther distant without lessening the visual angles subtended 
 by them ; and because an object at two miles, subtending the same angle 
 as an object at one mile is twice as large, the conclusion is drawn that the dim 
 object is large. Thus, a person in a fog may believe that he is approaching 
 a great tree, fifty yards distant, when the next step throws him into the bush 
 which had deceived him Two friends meeting in a fog have often mutu- 
 ally mistaken each other for' persons of much greater stature. A row of 
 fox-glove flowers on a neighbouring bank has been mistaken for a company 
 of scarlet-clad soldiers on the more distant face of the hill. There arc, for 
 similar reasons, frequent misjudgings in late twilight and early dawn. The 
 purpose of a thin gauze screen interposed between the spectators in a theatre 
 and some person or object meant to appear distant, is intelligible on the same 
 principle : a boy near, so screened, appears to be a man at a distance. The 
 art of the painter uses sombre colours when his object is to produce in his 
 picture the effect of distance. On the alarming occasion of a very dense 
 fog coming on at sea, where the ships of a fleet are near to each other, 
 without wind, and where there is considerable swell or rolling of the sea, 
 much damage is often done : but it is to be remarked in such a case, that the 
 size of each ship approaching to the shock, is always, in the apprehension 
 of the crew of the other ship, exaggerated. 
 
 The celebrated Spectre of the Bracken, among the Hartz Mountains, is a 
 good illustration of our present subject. On a certain ridge, just at sunrise 
 
THE EYE INTENSITY OF LIGHT. 371 
 
 a gigantic figure of a man had often been observed walking, and extraordi- 
 nary stories were related of him. About the year 1800 a French philosopher 
 and a friend went to watch the apparition ; but for many mornings they 
 paraded on the opposite ridge in vain. At last, however, the monster was 
 seen, but he was not alone ; he had a companion, and singularly he and his 
 companion aped all the motions and atitudes of the observer and his com- 
 panion ; in fact the spectres were merely shadows of the observers, formed 
 by the horizontal rays of the rising sun falling on the morning fog which 
 hovered over the valley between the ridges ; and because the shadows were 
 very faint, the figures were deemed distant, seeming men walking on the 
 opposite ridge, and because a comparatively small figure seen near, but sup- 
 posed distant, appears of gigantic dimensions, these shadows were accounted 
 giants. 
 
 While the comparative intensities of light coming from bodies considered 
 as wholes, or from their sides similarly exposed to the source of light fur- 
 nish an indication of their different distances from the observer, the compara- 
 tive intensities from their sides dissimilarly or unequally exposed to the 
 source of light, and which, therefore, reflect light to the eye, or are illumi- 
 nated in different degrees, furnish an indication of the forms and attitudes 
 of the bodies. In observing, for instance, a white house exposed to the sun, 
 it is seen that the side directly receiving the rays is highly illuminated or 
 bright, while the other sides are less so, and are said to be in the shade a 
 shade which is more or less deep in proportion as there are few or many 
 sources of reflected light near it. The different faces or walls of such a house 
 are to the sense of the observer, as strongly distinguished from each other, 
 by the mere difference of shade, as if they were of different colours, or as if 
 they were examined by the touch, or by walking round them. If the object 
 were a ball instead of a square house, there would still be as great differences 
 of shade in the half not receiving direct rays, but the parts, instead of forming 
 abrupt contrasts like the walls of a house, would appear to melt into each 
 other, marking the beautiful round contour of the object. The consideration 
 of all such cases forms the subject of chiaro-oscuro, so interesting to the 
 pajnter. 
 
 Had there not been in nature the provision of light and shade now des- 
 cribed, the sense of sight would have been of comparatively little use, and a 
 mass of.things in the light, if of the same colour, would have been as little 
 distinguishable from one another by a person looking directly at them, as 
 things forming a mass, or shadow are distinguishable by a person looking at 
 the shadow. It is this provision, therefore, which enables us, independently 
 of colour, to distinguish the profile or outlines.of different bodies placed near 
 to. each other, and to distinguish in the same body the protuberent or other 
 form of the surfaces which is towards the observer. But for this, it would 
 have been impossible to distinguish, for instance, between a white wall when 
 naked and when having various white objects placed before it j and it would 
 have been impossible to distinguish between the rounded figures, if similarly 
 coloured, of a flat circle, a sphere and a cone, all directly opposed to the eye ; 
 but in reality, by some difference of shade, the white objects are distin- 
 guished from the wall, and in the three geometrical figures, the uniformly 
 bright surface of the circle, the soft rounded shadowing of the sphere and 
 the shade coming to a point on the cone, at once declare the true forms. But 
 for the shadowed parts, the facade of a white palace of varied architecture 
 would have been an unmeaning sheet of lights : the lights, however, and 
 shadows produced by the juttings and recesses, mark the variety of surface 
 
372 LIGHT. 
 
 most completely; and the round pillar is distinguished from the square, and 
 every pediment, and capital, and architectural ornament, stands out plea- 
 singly conspicuous. But for light and shade again, the " human face divine." 
 would have been an unmeaning patch of flesh, for there are few lines in it 
 but those made by different exposures to the lights, and yet its every 
 prominence and depression, and every momentary change, are so truly indi- 
 cated to the eye that it becomes full of meaning or expression. How well 
 mere light and shade serve to convey what the eye has to learn of a scene 
 or object, may be perceived by examining any of the admirable engravings 
 which now abound, and which, although made up entirely of degrees of 
 shade, or of black and white, are scarcely inferior in expression to finished 
 paintings. 
 
 The student of painting soon learns that the lines called outlines, by which 
 he first sketches subjects, do not exist at all in nature, and have to be again 
 effaced in his finished work : for they only mark the place where lights and 
 shades happen to meet. Much may be conveyed to the mind, however, by 
 a mere outline, and particularly of lines if different breadth and thickness 
 are used to mark the situation of the fainter and deeper shadows. 
 
 The subject of chiaro-oscuro is not so simpl as, from the fact of the sun 
 being the great source of light, might at first be supposed; for although this 
 be true, still every body which reflects the sun's light becomes a new source 
 to those about it, and the shading of a picture must have reference to all such 
 sources, and to the colours of the body itself, and of the neighbouring bodies. 
 
 In looking -at an extended landscape, it is seen that the near objects con- 
 sidered as wholes, are comparatively bright, that their shadows are strongly 
 marked, and that their peculiar colours are everywhere easily distinguishable 
 as of flowers, fruit, foliage, &c., but of objects farther off, the colours, with 
 increasing distance become dim, the lights and shadows melt into each other 
 or are confused, and the illumination altogether becomes so faint that the 
 eye at last sees only an extent of distant blue mountain or plain appearing 
 bluish, partly because the transparent air through which the light must pass 
 has a blue tinge, and partly because the quantity of light which can arrive 
 through the great extent of air is insufficient to exhibit the detail. TJie 
 ridge called Blue Mountains in Australia, another of the same name in 
 America, and many others elsewhere, are not really blue, for they possess 
 all the diversity of scenery which their climates can give, but to the eye 
 which first discovered them, bent on them generally from a distance : they 
 all at first appeared blue, and they have retained the name. 
 
 In a good picture, where, upon canvass stretched on a frame, the artist has 
 disposed the lights, shades and colours in the very situations and with the 
 intensities which they would have had on coming from the real scene to the 
 eyes, through a plate of glass filling up the frame, all that we have now 
 been saying is strictly exemplified. In the foreground, the objects are large 
 and bright, but as the scene is supposed to be gradually more remote, the 
 size and brightness correspondingly diminish, until, at last, there is only a 
 dim mixture of bluish or grayish masses forming the horizon and sky. 
 
 A child during what may be called the education of the sense of sight, 
 has a strong perception of the vast differences of appearance which things 
 assume according to their accidental distance from the eye, their position, 
 their exposure to light, &o. ; for many of these differences, being at first 
 calculated to deceive the young judgment, have, from time to time, been 
 noted by him with surprise. Thus, a boy when he first discovers that a ship 
 which at the quay, with her white sails spread out ; concealed from him 
 
THE EYE PERSPECTIVE. 373 
 
 half the heavens, is in an hour or two afterwards, seen by him on the dis- 
 tant horizon as a dark speck hardly big enough to hide one star, has his 
 attention strongly awakened, and he feels surprised; or, again, when he 
 discovers that the faint blue unchanging mass which he had always observed 
 bounding in one direction, the view from the house of his infancy, is a dis- 
 tant mountain side, thickly inhabited and corvered with fields and gardens, 
 where in succession, all the bright colours of the different seasons predo- 
 minate he is equally struck. But as soon as experience has enabled him 
 to interpret readily and correctly, the visual signs under every variety of 
 circumstance, his attention passes so instantly from them to the realities 
 which alone are interesting to him just as it might pass from the paper and 
 printing of a newspaper to the important intelligence communicated by them 
 that he very soon ceases Jo be aware that the sign, which, in every case, 
 similarly suggests the object, is not, also, in every case similar to itself, and 
 the very same true and complete representation of the reality. The prejudice 
 that the sign is of this nature becomes quickly so strong, that even a diffi- 
 cult effort has been made by a grown person again to attend to the mere 
 appearances, in any scene of which the realities are known. 
 
 This attempt to analyze mere appearances, and which, in one sense, is an 
 attempt to unlearn something, or to retrograde, is called, as already : "stated, 
 the study of perspective. When it regards the apparent reduction of size and, 
 the foreshortening of bodies under various circumstances, it is called linear 
 perspective when it regards the fading of light and the modifying of colour 
 it is called aerial perspective. As the art of painting depends entirely upon 
 the understanding of these two departments, the gradual progress which it 
 has made in different countries is a measure of the degree in which the com- 
 mon prejudice that things appear as they are has, in them, been overcome. 
 Where this prejudice exists, any untaught person conceives a good painting 
 to be merely a miniature representation drawn according to a certain re- 
 duced scale, as of arf inch to a yard, and in which all the demensions of 
 things are to be measured as simply as in the reality while the colour, as 
 to vividness, &c., should perfectly agree with the originals. This statement 
 is remarkably illustrated by the facts, that children in their rude atempts to 
 paint, always aim at realizing the notion of the art above detailed, and that 
 such has been the first stage of painting in every country. In Europe now, 
 owing to the labours of men of genius, art in painting may be said almost 
 to rival nature, producing scenes as lovely as the finest of nature's scenes, 
 and scarcely distinguishable from them : but in other countries, as in China 
 and India, among the native artists, the first stage of the art is still in exist- 
 ence. In a Chinese picture, owing to the absence of perspective propor- 
 tions, an extensive subject is only a collection of portaits of men and things 
 drawn all on the same scale, and placed near one another, and where all the 
 colours are as vividly shown as if the objects were only a few feet from the 
 eye ; there, the figures at the bottom or fore-ground are supposed to repre- 
 sent the objects nearest to the spectator, while the figures higher up are sup- 
 posed to be of more remote objects, all appearing as they might be seen in 
 succession by a person who had the power of flying over the country. This 
 kind of picture or representation, although not natural, if all viewed at once, 
 may communicate more information than a single common painting, for it is 
 equivalent to many such. In Europe, lately, the principle has been again 
 usefully acted upon for certain purposes, as for representing on one long 
 sheet or on a succession of sheets, connected in a suitable manner, the banks 
 of a river or a line of a road. The banks of the Rhine particularly have thus 
 
374 LIGHT. 
 
 been admirably portrayed, so that, the spectator directing his eye along the 
 paper feels almost as if carried in a balloon to view in detail the whole of 
 the real and enchanting scenery. The principle might, perhaps, with ad- 
 vantage, be acted upon still more extensively for instance, to produce, 
 instead of common maps or charts of countries, true bird's-eye views, over 
 which the eye, moving from place to place, and at every new point of sight, 
 would see a certain portion of the country, as a bird or aeronaut would, the 
 sketch being supposed to be taken from that certain elevation deemed most 
 suitable for the ends in view. 
 
 3d Divergence of the rays ofliglit. (See Analysis,"page 355.) 
 
 This is the next circumstance to be mentioned by which the eye judges 
 of distance. Supposing the line E F to mark t(je place and breadth of the 
 pupil of the eye, the light entering from an object at a which is near (it is 
 
 here placed nearer than an object 
 
 Fig. 1-58. could be seen in reality,) is very 
 
 divergent, or is spreading with a 
 large angle j from b the pencil of 
 rays is less divergent, or opens 
 with a smaller angle ; from c it is 
 less divergent still, and so on. 
 Now the eye, to form an image 
 
 on its retina requires to exert a bending power exactly proportioned to the 
 divergence of the received rays : and it appears to have a sense of the effort 
 made, which becomes to the person a kind of measure of the distance of the 
 object. This divergence of the rays entering the eye, is the chief circum- 
 stance in which the most perfect painting must still differ in its effect upon 
 the eye from a natural scene for, first, in the natural scene, the objects are 
 generally more distant than their representation can be ; and secondly, while, 
 in nature, every object, according to its distance, is sending rays which reach 
 the eye with different divergence, and which rays, therefore, can produce 
 distinct images on the retina at any one time, only of the objects which are 
 at the same distance from the eye, the rays from a picture, which is a 
 single plane surface, come from every part with the same divergence, and 
 the eye must feel a disappointment in not having to accommodate its power 
 of bending, to the different distances attempted to be portrayed on the can- 
 vass. It might be expected that this kind of disappointment would be more 
 felt on looking at a common picture placed a few feet from the eye, than at 
 the sort of picture called panorama, which is on a larger scale and propor- 
 tionately more distant, but such is not the case : and the reason seems to be 
 that in the former the illusion is not intended to be complete, the fact of its 
 being but a picture not being at all concealed, and the eye is therefore at 
 once told to expect a difference of feeling ; but in the panorama, the whole 
 circumstances are arranged to deceive the eye entirely, if possible, and to 
 make it believe that the images on the retina are formed by light from the 
 objects themselves, then to the eye, really deceived in all other particulars, 
 the non-accordance with nature in this one is strongly, and, by some persons, 
 even painfully felt, so as on their first entering the place to cause headache 
 or giddiness. The illusion and consequently the pleasure from viewing any 
 picture may be made more complete by the spectator using lenses or spec- 
 tacles, such that the focal distance shall be equal to the distance of the paint- 
 ing from the eye; because such lenses, as was formerly explained, would 
 render all the rays entering the eye nearly parallel, and therefore very nearly 
 such as would arrive from objects at a considerable distance. 
 
THE EYE PERSPECTIVE. 375 
 
 4:th. Convergence of the axes of the eyes. (See the Analysis, page 355.) 
 
 This is the last circumstance to be mentioned, by which a person through 
 the eye, judges of the distance of objects. In consequence of there being 
 in the two eyes corresponding parts which must be similarly affected by any 
 object, that the person may have a single vision of it, as was explained in a 
 former page, the axes of both eyes must point to the object, and if it happen 
 to be very near, they will meet and cross each other so near the face as to 
 produce the appearance of squinting, seen when a person trying to look r.t 
 the point of his nose, but if the object be more distant, the obliquity will be 
 less, until at last the eyes directed to a thing at a very great distance, will 
 have their axes almost parallel. The last figure may serve also to explain 
 this subject. Supposing E and F to mark the place of the two eyes if the 
 object looked at be near them, as at a, they must be very much turned 
 inwards, that their axes may meet at a ; if it be at b, they will be less 
 turned, if at c less still, and so forth. 
 
 When the eyes are not directed to any thing in particular, the axes gene- 
 rally become parallel, or as if they were pointed to a very distant object ; and 
 because this happens generally when persons are reflecting on things which 
 are absent and seen only by the mind's eye, it is an expression of counten- 
 ance held to mark contemplation or thoughtfulness. 
 
 The direction of the visual axis is another particular, like the divergence 
 of light as to which a mere picture, can never produce upon the eye pre- 
 cisely the effect of the^ objects themselves. To see a picture, the axes must 
 meet at it, and generally, therefore, at a few feet from the eye ; while to see 
 the objects of nature, they often do not meet nearer than at miles. By a 
 glass, however, as will be explained a little farther on, it is possible to correct 
 also this defect, and to render the optical illusion, as regards still objects, 
 absolutely complete. 
 
 When a picture has to represent objects supposed far from the eye, the 
 farther the picture itself is placed from the eye supposing the figures to be 
 made proportionately large, the more nearly perfect will the illusion become, 
 because the divergence of rays and convergence of the axes (the two circum- 
 stances in which the effect of a mere picture on the eye must always differ 
 from the effect of a real scene) will be in proportion more nearly natural. 
 This explains in part why the picture called panorama (from Greek words 
 signifying a view of the whole} is an exhibition so charming ; for usually the 
 pain ting is far removed from the eye, and is drawn on a proportionately large 
 scale, and the eyes feel that the light comes from a considerable distance, and 
 that their axes do not need to converge very much ; and when in such a case, 
 the first impression of the want of absolute conformity to nature has passed 
 away, the illusion becomes nearly complete. But a not less important pecu- 
 liarity in the panorama is, that instead of being a painting on a plane surface 
 like common pictures, and embracing only a small part of the field of view, 
 it is on a curved surface which entirely surrounds the spectator,. and on which 
 all the objects visible in the various directions from the supposed place are 
 seen in the very situations which in nature they hold ; and the spectator is 
 enabled to conceive much more distinctly of each particular by seeing it in 
 relation to others around. Few persons can forget the impression made on 
 them by the first panorama which they may have seen ; and after increased 
 maturity of judgment, they will discover still more and stronger reasons for 
 admiring this almost miraculous mode of instantly transporting them to any 
 distance, beyond seas and other dangers, to contemplate at their ease the 
 
876 LIGHT. 
 
 most interesting scenes on earth, represented under the most favourable cir- 
 cumstances of position, light and weather. Hence few persons of good taste 
 neglect the opportunity now, in most great towns so frequently offered, of 
 obtaining at little cost so high a gratification. 
 
 To correct the slight remaining optical defects of a common panorama, a 
 large lens may be used, of which the focal distance is equal to the distance 
 of the picture from the eye. This has the effect of diminishing the diver- 
 gence of the rays until it becomes exactly that which belongs to the supposed 
 remoteness of the objects, and it also bends the light so that the axes of the 
 eyes may be nearly parallel. The author has found a convenient mode of 
 using the lens for such a purpose to be to cut out two round pieces from 
 opposite sides of it, and to form thenuinto a pair of spectacles : from one 
 lens three pairs may be formed. Panorama exhibitors should keep such 
 lenses or spectacles for the use of visitors. 
 
 The effect of the magnitude and distance of the ordinary large panoramic 
 views might, with the assistance of proper glasses, be had from even the 
 smallest picture on engraved representation embracing the same field ; and it 
 is remarkable that some enterprising person has not undertaken to publish 
 sets of interesting views fitted to be used in that way. A common panorama, 
 occupying a circular wall of 150 feet in circumference and twenty feet high, 
 may be reduced and still retaining the same truth of proportions, to appear 
 on a piece of paper five feet long and eight inches high or broad ; and if 
 this were set up in a suitable frame, like a wall, round the head of a spectator, 
 while its edges were concealed by drapery or otherwise, and the eye could 
 only view it through fit glasses placed in its centre and made to turn round 
 so as to command the whole, it could not by any ordinary spectator be dis- 
 tinguished from the large panorama. With the art of lithography, now so 
 well adapted for producing soft representations of scenery, the expense of 
 such views might be very moderate, allowing them to form a common part of 
 library furniture. When we reflect upon the expansion of mind obtained by 
 travelling, and that not a few of the advantages would follow a familiarity 
 with a good selection of panoramic views, it is not perhaps too much to sup- 
 pose that courses of instruction in geography, history, &c., may before long 
 be illustrated by this most interesting mode of aiding the conception and 
 memory. 
 
 Common paintings and prints may be conside%d as detached parts of a 
 panoramic representation, showing as much of that general field of view 
 which always surrounds a spectator, as can be seen by the eye kept in one 
 place, and looking through a window or other opening of moderate size. 
 The pleasure from contemplating these is much increased by using a lens or 
 such spectacles as above described. There is in the 
 Fig. 159. shops such lens, with the title of optical pillar ma- 
 
 chine, or diagonal mirror, fitted up so that the print 
 to be viewed is laid upon a table beyond the stand of 
 the lens, and its reflection in a mirror supported diago- 
 nally over it, is viewed through the lens. The illu- 
 sion is rendered more complete in such a case by 
 having a box, as a b, on the bottom of which the paint- 
 ing is laid, and at the top of which the lens and mirror, 
 fixed in a smaller box at a, are made to slide up and 
 down to allow of a ready adjustment of the focal dis- 
 tance. This box used in a reverse way becomes a 
 perfect camera obscura. The common show-stalls seen in the streets are 
 
THE EYE PA IN TIN a REPRESENTING MOTION. 377 
 
 boxes made somewhat on this principle, but without the mirror ; and 
 although the drawings or prints in them are generally very course, they are 
 not uninteresting. To children whose eyes are not yet very critical, some 
 of the show-boxes afford an exceeding great treat. 
 
 A still more perfect contrivance of the same kind has been exhibited for 
 some time in London and Paris under the title of Casmoramo, (from Greek 
 words signifiying views of the world, because of the great variety of views.) 
 Pictures of moderate size are placed beyond what have the appearance of 
 common windows, but of which the panes are really large convex lenses 
 fitted to correct the errors of appearance which the nearness of the pictures 
 would else produce. Then, by adding various subordinate contrivances, 
 calculated to aid and heighten the effects, even shrewd judges have been led 
 to suppose the small pictures behind the glasses to be very large pictures, 
 while all others have let their eyes dwell upon them with admiration, as 
 magical realizations of the natural scenes and objects. " Because this contri- 
 vance is cheap and simple, many persons affect to despise it ; but they do not 
 thereby show their wisdom : for to have made so perfect a representation 
 of objects, is one of the noblest triumphs of art, whether we regard the 
 pictures as drawn in true perspective and colouring, or the lenses which 
 assist the eye in examining them. 
 
 It has already been stated that the effect of looking through such glasses 
 at near pictures, is obtainable, in a considerable degree, without a glass, by 
 having the pictures very large, and placing them at a corresponding distance. 
 The rule of proportion in such a case is, that a picture of one foot square at 
 one foot distance from the eye, appears as large as a picture of 60 feet square 
 at 60 feet distance. The exhibition called the Diorama is merely a large 
 painting prepared in accordance with the principle now explained. In prin- 
 ciple it has no advantage over the cosmoramo or the show-box, to compensate 
 for the greater expense incurred, but that many persons may stand before it 
 at the same time, all very near the true point of sight, and deriving the 
 pleasure of sympathy in their admiration of it, while a slight motion of the 
 spectator does not make his eye lose the right point of the view. 
 
 A round building of prodigious magnitude has lately been erected in the 
 Regent's Park in London, on the walls of which is painted a representation 
 of London and the country around, as seen from the cross on the top of St. 
 Paul's Cathedral. The real scene is unquestionably one of4he most extra- 
 ordinary which the world affords, and this representation of it combines the 
 several advantages of the circular view of the panorama the size and dis- 
 tance of the great diorama and that from the details being so minutely 
 painted, distant objects may be examined by a telescope or opera-glass. 
 
 From what has now been said, it may be understood, that for the purpose 
 of representing still-nature, or mere momentary states of moving objects, a 
 picture truly drawn, truly coloured, and which is either very large to correct 
 the divergence of light and convergence of visual axes, or if small, is viewed 
 through a glass, would affect the retina exactly as the realities. But the 
 desideratum still remained of being able to paint motion. Now this, too, 
 has been recently attempted, and in many cases with singular success, chiefly 
 by making the picture transparent, and throwing lights and shadows upon it 
 from behind. In the exhibition of the diorama and cosmorama there have 
 been thus represented with admirable truth and beauty such phenomena as 
 the sunbeams occasionally interrupted by passing clouds, and occasionally 
 gilding the varied scene : perhaps darting through the windows of a venerable 
 cathedral and illuminating the interesting objects in its interior ^the rising 
 
378 LIGHT. 
 
 and disappearing of mist over a landscape running water, as, for instance, 
 the cascades among the sublime precipices of Mount St. Grothod, in Switzer- 
 land ; and one of the most striking scenes of all, a great fire or conflagra- 
 tion. In the cosmoramo of Regent Street, the great fire of Edinburgh was 
 admirably represented ; first, that noble city was seen sleeping in darkness 
 as the fire began, then the conflagration grew and lighted up the sky, and at 
 short intervals, as the wind increased, or as roofs fell in, there were bursts 
 of flame towering to heaven, and vividly illuminating every wall or spire which 
 caught the direct light then the clouds of smoke were seen rising in rapid 
 succession and sailing northward upon the wind, until they disappeared in 
 the womb of distant darkness. So naturally was all this represented, that 
 no stranger can have viewed the appalling scene with indifference, while on 
 those who knew the city, the effect can scarcely have been weaker, than if 
 they had witnessed the reality. The mechanism for producing such effects 
 is very simple; but spectators, that they may fully enjoy them, need not 
 particularly inquire about it. 
 
 It is remarkable, when the imagination is once excited by some beautiful 
 or striking view, how readily any visual hint produces clear and strong 
 impressions. One day in the cosmorama, a school-boy visitor exclaimed 
 with fearful delight that he saw a monstrous tiger coming from its den among 
 the rocks; it was a kitten belonging to the attendant, which by accident 
 had strayed among the paintings. And another young spectator was heard 
 calling that he saw a horse galloping up the mountain side; it was a 
 minute fly crawling slowly along the canvas. There is, in this department, 
 a very fine field yet open to the exercise of ingenuity, for the contemplation 
 of pictures representing motion or progressive events, may be made the 
 occasion of mental excitement the most varied and intense. For instance, 
 there are few scenes on earth calculated to awaken more interesting reflec- 
 tions on the condition of human nature than that beheld by a person who 
 sails along the river Thames from London to the sea, a distance of about 
 sixty miles, through the wonders which on every side there crowd on the 
 sight the fores'ts of masts from all parts of the world the glorious monu- 
 ments of industry, of philanthropy, of science the endless indications of 
 the riches, the high civilization, and progressive happiness of the people. 
 Now this scene was lately in one of our theatres, strikingly portrayed by 
 what was called a moving panorama of the southern bank of the Thames. 
 It was a very long painting, of which a part only was seen at a time gliding 
 slowly across the stage, and the impression made on the spectators was that 
 they themselves were sailing down the river in a steamboat, and viewing the 
 fixed realities. In the same manner might be most interestingly represented 
 the whole coast of Britain, or any other coast, or any line or road, or even a 
 line of balloon flight. There was another moving panorama exhibited about 
 the same time at Spring Garden, aiming at an effect of still greater difficulty, 
 viz., to depict a course of human life ; and the history chosen was that of the 
 latter part of Bonaparte's career. Scenes representing the principal events, 
 were, in succession, made to glide across the field of view, and were so 
 designed that the real motion of 'the picture gave to the spectator the feeling 
 that the events were then in progress ; and with the accompaniments of clear 
 narration and suitable music, they produced on those who viewed them the 
 most complete illusion. The story began by recalling the blow struck at 
 Bonaparte's ambition in the battle of Trafalgar; and to mark how completely, 
 by representations of various movements and situations of the battle, the spec- 
 tators were in imagination made present to it, the author may mention that 
 
T H E E Y E P A I N T I N G REPRESENTING MOTION. 379 
 
 on the occasion of his visiting the exhibition, a young man seeing a party of 
 British represented as preparing to board an enemy's ship, started from his 
 seat with a hurrah, and seemed quite confounded when he discovered that 
 he was not really in the battle. To the views of Trafalgar succeeded many 
 others, similarly introduced and explained, in each of which the hero himself 
 appeared : there were his defeat, at Waterloo his subsequent flight his 
 delivery of himself to the British admiral his appearing at the gangway of 
 the Bellerophon to thousands of spectators in boats around, while in Plymouh 
 harbour, previous to his departure for ever from the shores of Europe his 
 house and habits during his exile, with various .picturesque views of St. 
 Helena ; and last of all, that solemn procession, in which the bier with his 
 lifeless corpse was moving slowly on its way to the grave under the willow- 
 tree. The exhibition now spoken of might have been made better in all 
 respects, yet in its mediocrity it served to prove how admirably adapted such 
 unions of painting, music, and narration are to affect the mind, and there- 
 fore to become the means of conveying most impressive lessons of historical 
 fact and moral principle. 
 
 Painting, whether employed to portray scenes of entirely still-nature, or 
 scenes involving some kind of motion as above described, has still, as its 
 great aim or end, merely to represent interesting subjects, and to give to, the 
 spectator as much as possible that clear conception of them which is obtained 
 by ocular examination of realities; and thus, as a system of visual signs of 
 thought, it becomes, like language, which is a system of audible signs, a 
 means of expanding the boundaries of individual human existence into wider 
 space and time, and thus of elevating human nature. While it portrays only 
 strict matters of fact, whether of past or present time, as particular human, 
 individuals, objects of natural history, the beautiful and magnificent scenes 
 of nature, interesting events which the artist has the means of faithfully 
 representing, &c., it may be called truly historical painting, embodying the 
 materials of true history, both natural and civil, and then it is of singular 
 value. But even when applied to other purposes, it may be fraught with 
 delight ; and just as language, of which the grand object or use is to express 
 strict truths, has still been admirably employed in giving a permanent exist- 
 ence to a variety of fictions, from the wildest fables and rhapsodies to the 
 historical plays and novels of modern times, as those of Shakspeare and of 
 Scott which plays and novels, although not furnishing true portraits of 
 individual human nature, are yet most correct portraits of general human 
 nature so may painting be employed in embodying fictions adapted to its 
 peculiar powers, and it may do so in a manner to prove the artist endowed 
 with the highest degree of human genius. It should always be recollected, 
 however, that what is usually dignified with the name of historical painting, 
 really bears to historical truths only the kind of relation which novels and 
 plays bear to it, and often approaches even less nearly to the truth ; for it 
 pretends to relate a thousand minute circumstances which no history has 
 preserved, and which, therefore, only the imagination of the artist can supply. 
 Thus when a painter, knowing that Lucretia stabbed herself in the presence 
 of her father and others, after the crime of Tarquin, exhibits a woman dying, 
 and a certain number of persons around her in horror and astonishment, he 
 no more represents the real Lucretia and her friends than he represents any 
 other particular young woman and her friends; for he is quite assured that 
 not one of the figures in such a picture is a portrait of the individual whose 
 name it bears; his picture, therefore, in so far, is as untruth or fiction, while 
 it very probably has some of the additional errors and even absurdities so 
 
380 LIGHT. 
 
 common among historical painters, in respect of national usage in costume, 
 religion, manners, &c, and in respect to the general personal appearance, 
 as when a Rubens, wishing to represent Sabine or other ladie,s, gave them 
 the Dutch corpulency deemed comely in his own country, although it 
 strikingly contrasted with the true forms of Italian or Grecian nymphs. 
 From all this it appears that historical pictures may often be regarded as 
 portraitures, not of the realities, but of comedians acting scenes in historical 
 plays intended to represent the realities. 
 
 In dealing with the events of ordinary history, there is no strong reason 
 why artists may not please themselves and their spectators as we have now 
 been describing ; but it may admit of doubt whether similar liberties should 
 be allowed with respect to religion Yet any painting of the last supper, for 
 instance, or of the ascension, is not more true than a theatrical representation. 
 To judge of the nature of such a picture we have only to suppose any of the 
 events recorded in the New Testament to be represented by a painter in 
 China with the countenances seen on Chinese tea-boxes ; such a representa- 
 tion would appear in Europe revoltingly absurd; but the common practice 
 here is only a degree better, Italian countenances being usually substituted 
 for the Jewish; and twenty painters, undertaking the same subject gene- 
 rally put different persons into all the situations. Then it can produce no 
 pleasing impression on a Christian's mind to be told, that an admired paint- 
 ing of the crucifixion was made chiefly from the body of an executed mur 
 derer; or that fora praised representation of the triumphal entry into Jeru- 
 salem, the painter had deemed his own physiognomy the most befitting for 
 the principal figure, while he copied the portrait of Voltaire as a specimen 
 9 ' the bad Jews, of Newton as a specimen of the good, and of wife, cousins, 
 acquaintances, and old clothes-men, to make up the remaining groups. With 
 the knowledge that such things have often been, it need not surprise that 
 many persons of correct feeling turn with horror from all these mimicries 
 and falsehoods, to seek their idea of God and his providence in the sublime 
 descriptions of his attributes, which written language conveys and which all 
 creation, in a mute language not less impressive, so strongly confirms. When 
 men generally could not read, and as a mass were extremely ignorant, various 
 means of fixing their attention upon religious subjects might be useful, and 
 therefore proper, as sacred plays, certain processions, pictures, &c., which 
 have now in many countries ceased to be either; but a person of good sense 
 will continue to regard with a certain respect whatever at any time may have 
 contributed to reclaim portions of mankind from barbarism and wickedness 
 to the just appreciation as the divine charities of a pure religion. 
 
 There are in painting other classes of fictions, which pretend to nothing 
 beyond fiction, and which yet are truly admirable; such are personifications 
 of the virtues and vices, serving to recommend the practice of the former, 
 and to deter from that of the latter almost all Hogarth's works are of this 
 character, and evince the highest mental acumen and genius : then may be 
 mentioned the personifications of what have been called the elements and 
 powers of nature, including many of the personages of the heathen mytho- 
 logy then other generalizations of the characteristics of human or other 
 nature, as scenes of domestic affection, of the play of the passions, &c., &c.; 
 and because many subjects when so sketched, are intelligible to the eye with 
 the suddenness of lightning, where longest verbal description would convey 
 the idea but imperfectly, the art of painting, in regard to them, possesses a 
 truly magical and inestimable power. 
 
 As painting, whether employed to represent matters of fact or of fiction, 
 
THE TELESCOPE. 381 
 
 can accomplish its ends only through the means of drawing or linear per- 
 spective, and of shading and colouring, or aerial perspective, these subjects 
 require to be studied by every artist with great attention ; but it is important 
 for all to be aware that the greatest mastery over these, which are merely the 
 mechanical parts of the art, will go a very short way towards producing good 
 performances, unless there be present also the genius to select or to compose 
 subjects worthy of being represented, indeed, will go little farther to make 
 a painter than the learning of mere penmanship goes to make a historian or 
 a poet. This remark seems the more necessary, because there is in human 
 nature a disposition to value so -much the means by which important ends are 
 attained, that often the end itself is forgotten in' the contemplation of the 
 means as when a person, perceiving that money will procure all desirable 
 things, at last becomes the insane miser, and dies from want of the common 
 necessaries of life rather than touch his hoarded treasures : while among 
 painters, as among persons of other occupations, the talent for the inferior 
 or more mechanical departments of the art, is more common than for the 
 higher. Do we not see the subordinate accomplishments of the painter, by 
 not a few, both artists and pretended connoisseurs, supposed to be the prin- 
 pal? But this is evidently to value the dress or clothing instead of the 
 person ; or like the bibliomaniac, to regard the type and binding of books 
 more than the subject. To prove how unessential what is called high- 
 finishing in painting, is to the complete attainment of the purposes of the 
 art, we may instance the cartoons of the immortal Raphael, which to the 
 mere mechanic in art appear almost daubs, although exciting such enthu- 
 siasm in the superior mind : and many of the mere sketches of genius are 
 to a true taste more precious than some of the most labored pieces in our 
 galleries. As it is of no importance to a man who sees approaching the 
 friend of his heart, whether it be by day-light or candle-light, or with the 
 source of light above or below, &c., provided there is light enough for him 
 to distinguish clearly the friend of his heart, so is it of no importance how 
 any interesting subject is represented, provided the picture vividly excite a 
 true conception of the subject. A painter will discover the difficulties which 
 a brother artist had to surmount in representing an object in some particular 
 predicament, as regards the light, &c., and may estimate the talent accord- 
 ingly : but the great proportion, even of the most accomplished ordinary 
 spectators, will generally be looking beyond the sign to the thing signified, 
 heedless of the artist's difficulties. In consequence, however, of the preju- 
 dice in favor of l( a sweet or adorable bit of colouring," as it will sometimes 
 be called and which in truth may have the merit of most natural colouring, 
 there are preserved in many galleries pictures disgusting in almost all other 
 respects, as of drunken Dutch boors, with fiery noses and physiognomies 
 degrading to human nature, &c., &c., on seeing which the man of taste 
 deplores that the art of painting should so often have been prostituted by 
 clever men to the vile purpose of representing things of worse than no interest. 
 
 " When the image formed, as above described, beyond a lens, is viewed in 
 the air by an eye placed still farther beyond in the same direction, the 
 arrangement, according to minor circumstances, constitutes either the 
 common TELESCOPE or MICROSCOPE." Read the whole second paragraph 
 of the Analysis, page 325. 
 
 The name of telescope (a compound Greek term, signifying to see far, as 
 microscope signifies to see what is small,) applies to that wondrous instru- 
 ment of modern invention b^ the use of which the intelligent soul may be 
 
382 LIGHT. 
 
 said, on the beams of light as its path, to dart widely into space for the pur- 
 pose of contemplating the distant glories of creation ; or again, by which it 
 can command distant objects instantly to approach, for the purpose of conve- 
 nient inspection. In ancient times, a man', while looking with admiration 
 on the bright face of the moon, might have exclaimed " How pleased would 
 I be, had I the power to fly upwards to that celestial orb, the better to under- 
 stand its nature and beauties ;" but he could little have anticipated that the 
 day was coming when human ingenuity would find means in a great measure 
 to satisfy the wish : now the telescope is this means, for one which merely 
 doubles apparent magnitudes, shows the moon exactly as she would appear 
 to a person who had ascended towards her from the earth a distance of 120,000 
 miles, while one of greater power produces effects correspondingly great. 
 But to examine the heavenly bodies is by no means the only use of the 
 telescope, man being often extremely interested to discover what is passing 
 at a distance on the surface of the earth around him. Thus, by a telescope, 
 the military chief obtains a close view of approaching friends or foes, while 
 they are yet concealed from the naked eye. in the blue mist of distant moun- 
 tain or plain and similarly 'the sea-captain, while persons around him per- 
 ceive only a little speck on the far horizon, discovers there a ship of class and 
 nation at once evident to him, and with the crew of which, by the additional 
 use of signal flags, he is enabled readily to converse At midnight, a tele- 
 scope directed to a distant cathedral, may so effectually call it into the presence 
 of the observer, that on the clock-turret may be watched the progress of the 
 slow hands which tell of the unceasing lapse of time. A man, in the midst 
 of a wide plain, or on a lofty hill-top, or far on the face of a lake, who might 
 suppose himself quite alone and unseen, would yet, by a telescope, be 
 instantly placed under the observation of whoever chose to watch him. And 
 the same might happen to a man within the high walls of his own garden, 
 or even within his house near an open window, if a straight line could reach 
 him from some station where an observer was. Some remarkable cases of 
 actions, imagined by the parties to have been done in perfect secrecy, have 
 thus been brought to light. 
 
 Now the telescope, with its extraordinary powers, exhibits but another 
 modification of the simple case (described at page 341, and exemplified in 
 the camera obscura, &c.,) of an image formed for visual inspection beyond 
 a lens And we shall here explain that its powers depend altogether on the 
 two circumstances, first of its large lens collecting, for the formation of the 
 image (subsequently transferred to the observer's retina) a thousand times 
 or more the quantity of light which the naked pupil could receive ; and 
 second, of its forming by this light an image, to which the eye may be brought 
 very near, so as to examine it with magnifying glasses of any power. 
 
 To understand this well, we must recall, that the nature of the bending of 
 light in passing through a lens is such, that all the rays reaching the lens 
 
 Fig. 160. 
 
 from any point of a visible object in front, and forming what is called a pencil 
 of light as that spreading from the point A of the cross here represented, to 
 
THE TELESCOPE; 383 
 
 the lens L are collected in a corresponding point, as a, at the focal dis- 
 tance beyond the lens, so as always to meet the central ray of the pencil 
 here, (the direct line A a,)and, therefore, when the light comes from above 
 the centre of the lens, the focal meeting is below, as shown here; and when 
 it comes from below, the meeting is above; then the same happening as re- 
 gards every visible point of the object (the rays from only the two extreme 
 points A and B are here represented) at corresponding points beyond the 
 lens in the space between a and 6, the collected light, if received on a white 
 screen placed there, as in the camera obscura, will make apparent to an eye 
 in any direction a beautiful inverted image of the object. Now in the place 
 where the rays meet to form this image, if no screen be interposed, the rays, 
 although not lost or destroyed, but merely cross each other in the air, with- 
 out interference, nearly as they previously crossed in the lens, and spread 
 again beyond the focal points, or towards c as here shown, as they origi- 
 nally spread from the several points of the object itself; an eye, therefore, 
 placed anywhere beyond c, must receive portions of the pencil from every 
 point of the image, and may see the image in the air as it would see an 
 object situated where the image is, in the focus of the lens. This may be 
 observed at once by holding a spectacle-glass or any lens at a proper distance 
 between an object and the eye. 
 
 Now a telescope is merely a long tube, blackened within to exclude and 
 destroy useless light, and having a large lens, called the object-glass, filling 
 its distant end, to gather the light from the objects in front, and with that 
 light to form images towards the other or near end of the tube, where the 
 eye may conveniently inspect them. These images, for a purpose to be im- 
 mediately explained, are examined through another lens called the eye-glass, 
 which is fixed in a small tube made to slide backwards and forwards in the 
 large, so as to admit of the focal distances being adjusted to the power of 
 different eyes, &c. The accompanying sketch shows the progress of the 
 
 light from the object A, through the object-glass L, to form an image 5 a, 
 and afterwards to be bent by the eye-glass, 1), so as to enter the pupil of 
 the eye at E, where the rays cross, to form the last image on the retina. 
 
 In the simple telescope with only two lenses, as above represented, called 
 the astronomical telescope } or the night- telescope , because chiefly used at night, 
 the image is inverted ; but this is a circumstance of no importance in view- 
 ing the heavenly bodies; to fit the instrument, however, for viewing terres- 
 trial objects, it is necessary to place in the tube another simple or compound 
 lens, which shall form a second image from the first, and by inverting a 
 second time, shall produce an image equally upright. 
 
 To determine how much larger an object will appear when viewed through 
 a certain telescope, for instance, through one with an object-glass of three 
 feet focus, than when viewed by the naked eye, we must recollect that the 
 image is formed in the focus of the object-glass, or at I a, in the last figure, 
 and subtends from the centre of that glass or lens, the same visual angle as 
 the object itself (a fact explained at page 357,) and to an eye placed there 
 
384 LIGHT. 
 
 would appear of the same size as the object, but if the eye be brought nearer 
 to the image than the centre of the object-glass, the image will appear by so 
 much taller and broader, nd thus, as compared with the object, may be 
 called so much magnified. Now as the naked eye cannot see distinctly an 
 object nearer to it than at about six inches, because of the great divergence 
 of light from a nearer radiant point, the telescope in question, without an 
 eye-glass, would allow the eye to come only six times nearer to the image 
 than when at the centre of the object-glass and would only magnify the 
 diameter six times; but if then an eye-glass, as D, of half an inch focus 
 were placed half an inch from the image, so as to render the rays of every 
 pencil parallel, and therefore fitted to the powers of the eye, while the dif- 
 ferent parcels would cross each other a little way beyond the glass, as shown 
 above, an eye placed to receive in its pupil the crossing parcels, would see 
 the image as large as if at half an inch from it, and therefore 72 times nearer 
 than if viewed from the object-glass, and therefore again as of 72 times greater 
 diameter. Now, as in all cases, the image in a telescope is in the focus both 
 of the object-glass and eye-glass, and is therefore nearer to the latter than to 
 the former in proportion as their focal distances differ, the magnifying power 
 is measured by that difference in the case at present supposed, the differ- 
 ence is as 72 to 1, and 72 is the magnifying power of the telescope. The rule 
 is generally thus expressed, " divide the focal distance of the object-glass by 
 that of the eye-glass, and the quotient is the magnifying power." It is always 
 to be remembered, that if the diameter of an object be magnified 10 times, 
 the surface is magnified 100 times, and so in proportion for other jiumbers. 
 
 With such means of aiding the sight, then, it is that we discover the 
 mountains of our moon, and can even measure their altitudes; that we can 
 see the four beautiful moons of the planet Jupiter; that we can perceive 
 marks and irregularities on the surfaces of the other planets, enabling us to 
 say at what rate they severally whirl round their axes, experiencing the 
 phenomena of day and night; and that we can determine many other 
 interesting particulars. 
 
 The discovery of the telescope is said to have been first made accidentally 
 by the children of a Dutch spectacle-maker, while playing with their father's 
 work ; but it was turned to no use until Galileo, led by science, fell upon it 
 again, and with the knowledge of its worth, obtained from it the most sub- 
 lime results. The human heart can rarely have throbbed with such delight 
 as Galileo's when he first directed his optic tube to the heavens, and through 
 it contemplated so many glorious objects before unseen by human eye; as 
 the planet Venus, our beautiful morning and evening star, appearing not a 
 circle, but a crescent like our moon in her quarters as the satellites of Ju- 
 piter the rings of Saturn myriads of stars until then invisible to man; 
 and in a word, when he beheld the undoubted proofs of the true system of 
 the universe, as his genius had before conceived it, uniting the greatest 
 simplicity with unspeakable grandeur. 
 
 The Galilean telescope was simply a large object-glass to collect much 
 light, with a small concave eye-glass placed so as to intercept the converg- 
 ing rays before they reach their focus, and to change their convergency into 
 the parallelism which the eye could command. This telescope, although 
 magnifying less than that made of two convex glasses, as above described, 
 still, from occasioning no loss of light by the crossing of rays in forming an 
 image, was of considerable power. The common opera-glass is a telescope 
 made on this principle. 
 
 It was explained at page 338, that a ray of light, in being bent or refracted 
 
THE TELESCOPE. 385 
 
 by transparent media, as by a lens, is also divided into rays of the different 
 colours seen in the rainbow. Hence an image formed behind a simple lens 
 has coloured edges or fringes. This fact rendered the images of small ob- 
 jects much magnified, in the first made telescopes, very indistinct : and, but 
 for the important discovery made by Dollond, the optician, that different 
 kinds of glass have dispersive and refractive powers of different relative 
 force, so that a concave lens of a certain curve applied to a convex lens 
 might completely counteract the dispersion of colour by the latter, while it 
 left enough of the convergence of the rays for the formation of an image 
 refracting telescopes would have always been very imperfect. Dollond 
 called his telescopes achromatic, or not-colouring. It is very remarkable, 
 that he had the fortune to obtain some glass for his purposes, more suitable 
 than any which has been procured since, or which could be made by known 
 rules, until the late improvements in the manufacture suggested by the inge- 
 nuity of Mr. Farraday. The author of this work carried abroad with him 
 a small telescope of old Dollond's, which often gave more correct information 
 respecting minute colored objects at a distance, as signal flags at sea, than 
 much larger glasses of modern make. 
 
 The MICROSCOPE of greatest power and with the form called compound, 
 in its structure approaches very closely to the telescope, the chief difference 
 being, that while in the telescope a large distant object forms in the focus of 
 the object-glass an image exactly as much smaller than itself as the distance 
 of the image from the glass is less in the microscope conversely, a small 
 object placed near the focus of the object-glass produces a more distant 
 image, as much larger than itself as the image is more distant than it and 
 in the one case as in the other, the image is viewed through an appropriate 
 eye-glass. The object-glass in the telescope is large, in the microscope it is 
 generally very small. If, in the latter, an object-glass be used of one-eighth 
 of an inch focal distance, and the object be so placed that its image is formed 
 at six inches, the image will be of diameter 48 times as great as the object, or 
 will have nearly 2 ; 500 times as much surface : and if that image be viewed 
 through an eye-glass of half an inch focus, the image will appear still twelve 
 times larger, or 30,000 times larger than the object. 
 
 A simple convex lens is called a single microscope, and it magnifies, as 
 already explained, chiefly by allowing the eye to be brought much nearer to 
 the object than the distance at which the object could be ssen without the 
 glass ; but even where the distance of the eye and object is not changed, a 
 lens interposed will still magnify by bending the light, as at d and /', making 
 that which comes to the eye at e from 
 
 the top of such an object as the little Fig- 162. 
 
 cross a, to appear to come from b, 
 and that from the bottom to come 
 from Cj thus magnifying the cross 
 here represented by the black lines 
 to appear of the size represented by 
 the dotted lines. A concave lens 
 minifies for the contrary reason. 
 
 Perhaps there is not a greater treat for a person who has feeling for the 
 beauties of nature, than to explore with the microscope. While the telescope 
 lifts the mind to the contemplation of boundless space occupied by myriads 
 of suns, and exhibits this globe of ours as less, compared with the universe 
 around it, than a leaf is compared with a forest, or one grain of sand com- 
 pared with all which lies on the ocean's shore, the microscope again excites 
 
 25 
 
380 LIGHT. 
 
 new astonishment by showing on a leaf, or in a single drop of some water in 
 which the leaf has been infused, thousands of living creatures, and of crea- 
 tures not imperfect because thus small, but endowed with organs and parts 
 as complex and curious as those of an elephant. And he who admires the 
 curious structure of a honey-comb, may bend his eye through the microscope 
 upon the cut surface of a willow-branch, or of other wood, there to see a 
 similar structure more -wonderful still ; or he may compare the lace of a 
 fly's wing with the most perfect which human art can weave; or the beauti- 
 ful proportions and perfection of the limbs and weapons of an insect, invisi- 
 ble, perhaps, to the naked eye, with any larger objects of the kind already 
 known to him. 
 
 Telescopes and microscopes might with propriety be both called micro- 
 scopes, for often the telescopic object subtends to the naked eye even a smaller 
 angle than the objects which the microscope examines. The minutest visible 
 insect at hand may hide from the eye a planet at a distance. The image in 
 the telescope, however, is always much smaller than in the microscope be- 
 cause the rays from a distance being nearly parallel, must form the image 
 nearly in the principal focus of the object glass ; while for the microscope, 
 the rays from the near object being very divergent may be made to form the 
 image far beyond that focus, and therefore proportionally larger. 
 
 " Light falling on very smooth or polished surfaces, is reflected so nearly 
 in the order in which it falls, as in many cases to appear to the eye as if 
 coming directly from the objects originally emitting it, and such sur- 
 faces are called MIRRORS ; the surface of which is flat as well as polished, 
 is called a plane mirror." (Read the Analysis, page 325.) 
 
 If, on a marble slab, or other flat surface, (represented here at M R, with 
 the edge supposed towards the spectator,) a ball 
 Fig. 163. were projected from A perpendicularly towards 
 
 n., D, the ball would rebound directly back to A, 
 
 but if projected obliquely, as from B to D, it 
 would not return to the first situation B, but to 
 b, a corresponding situation on the 'opposite side 
 of the perpendicular, thus making the angle of 
 the return or reflection equal to the angle of 
 approach or incidence; the same would be true 
 of a ball approaching obliquely from any other 
 point, as C, and rebounding to c. Now light 
 'is reflected from polished surfaces according to 
 the same law, so that an eye at A would see 
 itself as if placed at d, an eye at b would see an 
 object really at B, as if it were at e, and so forth. 
 Where the existence of a mirror is not suspected, the objects reflected from 
 it are held to be realities placed beyond where it is. A wild animal will 
 attack its image in a glass : and the fable says that a dog crossing a brook, 
 quitted the piece of meat in its mouth to catch the tempting image which he 
 saw in the water below. The reason that an object seen in a plane mirror 
 appears to be just as far beyond the mirror as its true distance on the side 
 of the spectator, is, that the diverging rays of a pencil of light have the 
 same divergence after as before reflection. , 
 
 Any plane very smooth surface reflects light as now described, and is a 
 mirror; but different substances send back very different proportions of the 
 
 ID 
 
MIRRORS. 387 
 
 light which falls on them. A highly polished metallic surface is the best 
 mirror, often returning three-fourths of the whole light. Hence, in reflect- 
 ing telescopes, the mirrors are made of polished metals. 
 
 Our common looking-glasses are really metallit mirrors, for it is the 
 smooth, clear surface of the quicksilvered tin foil behind the glass which re- 
 flects the light, the glass itself merely serving the purpose of preserving the 
 metallic surface perfectly clean and flat. There is always an imperfection 
 in such glass mirrors, when used for viewing objects obliquely, because the 
 external surface of the glass acts also as a mirror, although so much more 
 feebly than the metal behind, and forms a separate image not quite coincid- 
 ing with the other, and therefore mixing with and confusing it. 
 
 The mirror-power of glass unaided is seen from the panes of a plate-glass 
 window, which make objects in front very visible, although by no means 
 with clearness comparable to that from a metallic surface. All common 
 panes of glass in windows, or in print frames, &c., reflect as much light as 
 plate-glass, but the reflection being irregular because the surface is irregular, 
 scarcely attracts notice. 
 
 The smooth surface of a fluid ^ a mirror, which is, moreover, horizontal ; 
 and when that surface is metallic, as of mercury, the mirror is most perfect. 
 In water, spirits, oil^or any other liquid, it is also perfect, but feebler. 
 
 The mirror of liquid quicksilver is sometimes used by astronomers in 
 observing the apparent altitudes of the heavenly bodies, for the image in the 
 mirror appearing exactly as much below the horizon as the object is really 
 above it, half the distance between them is the true height. 
 
 A varnished picture or any japanned surface, is a mirror: nay, even a 
 polished table of mahogany or other wood as it is well known among play- 
 ful children. The author, while writing this, has before him a table covered 
 with black leather, and in that covering, as 'a mirror, he clearly sees the 
 bright objects beyond the table. Polished stones, as marble slabs, &c., reflect 
 as much as glass. Even a surface of air may be a mirror, as where a cold 
 and dense stratum happens to lie in contact with a warmer and rarer stratum. 
 In such cases, where particular causes have unequally heated different levels 
 of the atmosphere, the trees, islands, c., happening to be below, are reflected 
 from above, and appear as if in the sky. This phenomena is called mirage. 
 It is often to be observed over the burning sands of Africa, where the air is 
 much heated ; and elsewhere certain kinds of mists and thin clouds produce 
 a similar effect, causing, for instance, a ship to appear as if suspended aloft, 
 with keel uppermost. 
 
 In certain cases, an object seen by the light reflected from a mirror appears 
 reversed, as when the right hand of a person standing before a glass becomes 
 the type for the left hand of the image ; or when a tree, or rock, or moun- 
 tain, seen in the mirror of a lake, has its top downwards. 
 
 It is on this account, that a man painting his own portrait from a mirror, 
 is apt to reverse all the accidental characteristics of the countenance or per- 
 son, not the same on both sides ; and if, as is generally true, one eye be 
 higher than the other, or the nose be a little to one side, a very incorrect 
 resemblance will be produced. Hence also a person whose countenance is 
 at all thus peculiar never sees himself in a mirror as he appears to others; 
 and a belle or beau, who has decided that a curl is more graceful on the left 
 temple, may unconsciously leave it on the right. 
 
 By an image, however reflected from a first mirror to a second, and from 
 that to the eye, persons may see the object, or themselves, if they choose, 
 as others see them. What a pity that there are not some moral mirrors to 
 answer an analogous purpose ! 
 
388 LIGHT. 
 
 A candle placed between two parallel mirrors fixed on opposite sides of a 
 room, makes visible in either glass to a spectator near the middle of the 
 room an endless straight line of lights. If the glasses be inclined to each 
 other, the lights will appear as if placed in the circumference of a circle, of 
 which the centre is where the prolonged mirrors would meet : this fact is 
 well illustrated in the beautiful toy called the kaleidoscope. It is possible 
 to place a few mirrors in such situations around an apartment, that a man 
 entering it, may see himself multiplied into a crowd, and a few ornamental 
 pillars may produce the effect of thousands formed into long colonades of 
 retiring lines. 
 
 The sun or moon reflecting in a still lake, appear as they do in the sky ; 
 but if the surface of the water become at all ruffled by the breeze, instead 
 of one distinct image, there will be a long line of bright tremulous reflection. 
 The reason of this appearance is, that every little wave, in an extent perhaps 
 of miles, has some part of its rounded surface with the direction or obliquity 
 which according to the required relation of the angles of incidence and reflec- 
 tion, fits it to reflect the light to the eye, and hence every wave in that extent 
 sends its momentary gleam, which is succeeded by others. 
 
 Although the external surface of glass reflects but a small part of the light 
 which falls upon it, being, therefore, a feeble mirror, still curiously, if light 
 which has entered a piece of glass, fall very oblique- 
 Fig. 164. ]y U p 0n the back or internal surface, instead of 
 passing out there, it is more perfectly reflected than 
 it would be by the best metallic mirror. This light 
 from A entering a piece of glass at B, is entirely 
 reflected at C, the back of the piece, and escapes 
 at D towards E. The back of a wedge of glass, 
 or common prism, thus becomes a perfect mirror. 
 It is this fact which enabled Dr. Wallaston to construct that beautiful little 
 instrument called by him the Camera Lucida. The two surfaces at the 
 back of the small prism of glass A become mir- 
 Fig. 165. rors, the first reflecting to the second, and the 
 second. to the eye at E, the objects in the land- 
 scape before it, while the eye also sees through 
 the glass to the paper below at B, and may sup- 
 pose the imagery to be feebly portrayed on the 
 
 r 
 
 A\ 
 
 ~~ paper: with a pencil that appearance is made 
 permanent, and a correctly-drawn outline of the 
 scene is at once obtained The instrument for 
 assisting draftsmen is still simpler than the ca- 
 mera obscura. Other modifications of the instru- 
 ment have since been contrived. 
 
 The same fact of the internal surface of a transparent mass becoming a 
 mirror, gives us the explanation of that apparition or phenomenon so 
 admired before it was understood, and not less admired since the rainbow, 
 or arc in the sky, as in France and elsewhere it is named an object which 
 the poets of nature have almost worshipped for its beauty, and which few of 
 us can cease to remember as one of the delights of our boyish days, when 
 we saw it stretching over the haunts of our young pleasures, and may have 
 pursued it in the hope of catching some of the falling rubies and emeralds, 
 or bright coloured dew of which it might be composed. 
 
 When a partial shower of rain falls on the side of the landscape opposite 
 to where the sun is shining, there immediately appears in the shower a 
 
MIRRORS. 389 
 
 variegated arch, red at its external border or confine, and then successively 
 orange, yellow, green, &c. (in the order of the colours of the prismatic spectrum 
 described at page 337,) towards its inner border. Its centre is directly oppo- 
 site to the sun, or at the end of the straight line supposed to be drawn from, 
 the sun through the eye of the spectator towards the opposite horizon ; and 
 being, therefore, always under the horizon, the bow is less than a semicircle. 
 The diameter of the circle of which the bow is a part, occupies nearly 82 
 of the field of view, that is to say, the bow always coincides with a hoop of 
 one foot diameter held eight inches from the eye. There is a second bow of 
 much fainter light than the first, and with the colours in reverse order : it is 
 of 108 diameter, and therefore external to the other. 
 
 Now the explanation of this miracle of beauty is simply as follows. 
 While -the sun shines upon the spherical drops of falling rain, its light falls 
 upon the whole central part of any drop, passes completely through, but that 
 portion which enters near the e%ge of the drop, as at a, is refracted or bent, 
 and reaches the back surface of the drop a't y so 
 slantingly, or an angle so great, that it suffers there Fig. 166. 
 
 an entire reflection instead of being transmitted ; the 
 ray, therefore is turned to b, where it escapes from 
 the drop, and as here shown, descends to the earth 
 or eye in the direction b e. Thus every drop of rain 
 on which the sun shines is a little mirror suspended 
 in the sky, and is returning at a certain angle all 
 round it, viz., at an angle of 41, a portion of the 
 light which falls on it ; and the eye placed in the re- 
 quired direction receives that reflected light. If in this case, however, there 
 were reflection only, and not also refraction with separation of colours, the 
 rainbow would be only a very narrow resplendent arc of white light formed 
 of millions of little images of the sun; but 
 in truth, because the light, which enters Fig. 167. 
 
 near the edge of the drop, traverses the 
 surface very obliquely, it is much bent or 
 refracted before its reflection, as seen at a, 
 and is divided into rays of its seven col- 
 ours, as it would be on passing through a 
 prism (as explained at page 337 ;) and 
 this division or separation continuing after 
 the light again escapes from the drop at b, 
 instead of one white ray descending from 
 each drop to a certain point of the earth, 
 seven rays descend (here marked by dotted lines from the figure 1 on the 
 left hand to 7, 6, 5, &c., on the right, and with separation greater than oc- 
 curs in reality to make it very evident,) and of these rays, an eye can only 
 receive one at a time from the same drop, which drop will then appear of the 
 colour of the ray : but for the same reason that seven eyes placed in a line 
 from above downwards, as at 7, 6, 5, &c., on the right would be required to 
 see the seven colours from one drop in the centre of the bow, so one eye look- 
 ing in the direction of seven drops situated in a corresponding row, as from 
 1 -to 7 on the left, will catch the lower or red ray of the upper, the orange 
 or second ray of the next, the yellow or third ray of that which follows, and 
 so on, while it will lose all the others, and thus will see the several drops 
 as if they were each of one colour only. Of such elements, then found in 
 the same relative directions all around the eye ; the glorious arch is formed. 
 
390 LIGHT. 
 
 No two eyes can see the same rainbow, that is, can receive light from the 
 same drops at the same time ; and the same eye does not for two instants 
 receive light from the same drops. This rainbow can never appear to a per- 
 son on a plain, unless when the sun is within 41 of the horizon, for other- 
 wise the centre of the rainbow would be more than 41 under the horizon, 
 and therefore the whole circumference would be below it too. 
 
 We have described above what is called the 
 
 Fig. 168. principal bow, formed in the drops by two 
 
 refractions, and one reflection of light. To 
 produce the fainter second or external bow, 
 mentioned above, and of which the colours are 
 in reverse order, the light which enters on the 
 under side of the drop as at a, is reflected first 
 at y, then again at b, and escapes at c towards 
 the eye : after* two reflections as well as two 
 refractions. As the semi-diameter of the bow 
 is 54, it may be visible when the internal bow is not. 
 
 An artificial rainbow may be produced in sunshine at any time by scatter- 
 ing water-drops from a bush or otherwise ; and a rainbow is often seen among 
 the spray of a lofty waterfall, or of a stormy sea. The cut-glass ornaments 
 of chandeliers, &c., produce colours on the same principle as rain-drops; as 
 do also mist and particles of frozen water between a luminous body, and the eye 
 exhibiting the circular coloured halos often observed around the sun and moon. 
 A white halo is light reflected from the external surfaces of drops or particles. 
 
 "Mirrors may l>e plane, convex, or concave; and certain curvatures v;ill 
 produce images by reflection, just as lenses produce images by refraction ; 
 in consequence, there are reflecting telescopes, microscopes, &c., as there 
 are refracting instruments of the same names." (See the Analysis, 
 page 325.) 
 
 While a plane surface reflects light, so that what is called the image in it 
 of a known object may readily be mistaken for the reality, convex or con- 
 cave mirror's reflect as if every distinct point of them were a separate small 
 plane mirror, and their effects on light correspond with a relative inclination 
 of the different parts. The only forms of much importance are the regularly 
 spherical or parobolic concave and convex mirrors. We shall now find that 
 these produce on light similar effects with lenses, only the concave mirror 
 answers to the convex lens, and the convex mirror to the concave lens. It 
 is the concave mirror which gathers the light to form images in the most 
 perfect telescopes that exist, as those of Herschel and others. Admirable as 
 is the refracting telescope, it still falls short in certain respects of the teles- 
 cope acting by reflection. 
 
 In a hollow sphere, or part of a sphere with 
 polished internal surface, if rays radiate from the 
 centre in all directions, they reach every part 
 perpendicularly, and therefore are thrown back 
 to the centre. Thus if A B were a concave 
 spherical mirror, of which C. were the centre, 
 rays issuing from C. would, in obedience to the 
 law that the angles of incidence and reflection 
 are equal, again meet at C. 
 
 It can be proved also, that any ray parallel to 
 
CURVED MIRRORS. 
 
 391 
 
 the axis, falling upon such a mirror, will be Fig. 170. 
 
 reflected inwards so as to cut the axis half- 
 way between the mirror and its centre, viz., 
 at D, the centre being C. Then as all pa- 
 rallel rays must meet in the same point, 
 that point becomes a focus, as already ex- 
 plained for lenses, and there an image of the 
 sun will be formed when the mirror is held 
 directly towards the sun. This point is called the principal focus of the 
 mirror. 
 
 For the same reason that parallel rays meet in the focus, so will rays, 
 issuing from the focus towards the mirror, become parallel, after reflection, 
 as seen above or in the figure at page 271 ; and if they be then caught in a 
 second and opposite mirror, as also represented at page 271, corresponding 
 effects will follow. 
 
 Now, for a concave mirror, as already explained for a lens, when rays fall 
 on it obliquely from one side of the axis, their focus will be on the opposite 
 side, and therefore the mirror will form an inverted image of any body 
 placed before it, just as the lens does ; and the image will be near or distant 
 and large or small, according to the divergence of the approaching rays, 
 exactly as happens with lenses j and thus the camera obscura, magic lantern, 
 telescopes and microscopes, may all be formed by mirrors, as they may be 
 by lenses. Moreover, concave mirrors magnify, as concave lenses of the 
 opposite names do. The two subjects of images by refraction and by reflec- 
 tion run so nearly parallel, that it would be useless repetition here to enter 
 upon the detailed consideration of the latter subject, and we shall therefore 
 content ourselves with showing why a concave mirror magnifies and why a 
 convex mirror minifies. 
 
 A concave mirror magnifies "be- Fig. 171. 
 
 cause the light from the top of the 
 cross at A, reaching the mirror 
 where it can be reflected to an eye 
 placed at F, viz., at E, seems to 
 the eye to come from C, and the 
 light of B similarly appears to 
 come from D, so that the cross A 
 B, by the reflection, seems to the 
 eye to be of the greater dimensions C D. 
 
 In the convex mirror, again, for corresponding reasons, the cross A B 
 appears only as C D, and therefore much smaller than the reality. 
 
 Concave or magnifying mirrors are often 
 used by persons in shaving. 
 
 A convex mirror is a common ornament of 
 our apartments, exhibiting a pleasing minia- 
 ture of the room and its contents. 
 
 Any polished convex body is a mirror, and 
 therefore the ball of the human eye is one, in 
 which we may contemplate most perfect 
 miniatures of surrounding things. It is the 
 image of the window or of the sun in the 
 
 convex mirror of the eye, which painters usually represent by a spot of 
 white paint there ; and a similar luminous spot or line must be made when 
 
 Fig. 172. 
 
392 . 'LIGHT. 
 
 they have to represent almost any of the pieces of furniture which have 
 rounded polished surfaces as bottles, glasses, smooth pillars, &c., 
 
 Convex lenses thus are also mirrors to all the objects around them, and 
 very strikingly so, owing to the perfection of the form of a lens. The 
 polished back of a watch, often, in the same way, attracts the attention of a 
 child, who wonders to see there so clearly " the little baby." 
 
 It has been mathematical amusement to calculate what kind of distor- 
 tion mirrors of unusual forms will produce, and then to make distorted draw- 
 ings, which, when reflected from such mirrors, might produce in the eye the 
 natural image of the objects. 
 
 When a concave mirror is used for a telescope, the image formed in front 
 of it, and to be examined through the magnifying eye-glass, may be viewed, 
 first, as in Herschel's telescope, by the spectator turning his back to the 
 real object, and looking in at the mouth of the telescopic tube, near to the 
 edge of which the image is. thrown by a slight inclination of the mirror at its 
 bottom : or, secondly, as in the Neictonian telescope, through an opening 
 in the side of a tube, after being reflected by a small plane mirror, placed 
 diagonally in the centre of the tube : or, thirdly, as in the Gregorian 
 telescope, through an opening cut in the principal mirror or speculum, after 
 being reflected towards that opening by a small mirror placed in the centre 
 of the tube ; this last arrangement .is that preferred for smaller telescopes, 
 because the spectator, while seeing the image, is also looking in the direc- 
 tion of the object. 
 
 Reflecting telescopes have the advantage of being perfectly achromatic, 
 that is, of producing no coloured or rainbow edges to the images ; for com- 
 pound light is reflected, although not refracted entire, all the colours follow- 
 ing the same law of equal angles of incidence and reflection. 
 
 Herschel's largest telescope had a niirrow^ of 48 inches in diameter, and 
 therefore received about 150,000 times more light than an unassisted eye, 
 forming with that light, at a focal distance of 40 feet, a large image admira- 
 bly distinct. It was with such a telescope that, in the obscurity of remote 
 space, Herschel discovered the immense planet rolling along, which in honour 
 of his royal patron, he called th% Georgium Sidus, but which now, by 
 the decision of the scientific world, bears his own name ; arid with such 
 he discovered moons before unseen of other planets, and he unravelled the 
 celestial nebulas and clustered -stars of the milky way and, in a word, un- 
 veiled vastly more than had before been done, the system of the boundless 
 universe. If this world were to last for millions of years, the discoveries 
 made by Herschel's telescope would mark a memorable epoch of its early 
 history. 
 
 "Light returned from, or passing through bodies of rougher or irregular 
 
 surface, or which have other peculiarities, is so modified as to produce all 
 
 i those phenomena of colour and varied brightness seen among natural 
 
 bodies, and giving them their distinctive characters and beauty." (See 
 
 the Analysis, page 325.) 
 
 General remarks on this part of our subject were made in the beginning 
 of the section, in the explanations of how objects not self-luminious become 
 visible by reflecting the light of other bodies, and of how the prism sepa- 
 rates a ray of white light into rays of the several colours which are seen 
 also in the rainbow which rays, on being again mixed, become white light 
 
COMPARISON OF LIGHT AND SOUND. 893 
 
 as before : and much beyond these remarks we have not the intention of 
 now proceeding To give a full account of the matters that might come 
 within the scope of this department, would occupy the pages of a large 
 volume, for there would be to pass in review the various opinions which 
 have existed on the intimate nature Q$ light, the facts connected with what 
 has been called the polarization of light, the relation of light in its double 
 refraction, to the ultimate structure of material masses, &c., all which sub- 
 jects are in certain respects highly interesting, but as some of them are not 
 yet completely investigated as respecting others various opinions prevail, 
 as they involve few matters yet applied to common use, as the reasonings 
 about them are far removed from ordinary trains of thinking, and refer to 
 facts altogether unknown to common observation, we hold them not to be 
 fit parts of a popular treatise on light. We may state, however, that persons 
 who have the leisure and mathematical preparation necessary for pursuing 
 the study, will find their labour in it richly rewarded. 
 
 What we deem necessary here to add, is, that white light, in falling upon 
 any transparent substance, as air, water, glass, &c., reduced to thin plates of 
 films, is so affected, that for certain degrees of thinness, different for each 
 substance, it is decomposed, and is reflected or is transmitted, not as white 
 light, but as some of the colours of the rainbow, and the colour reflected in 
 any case, is always the opposite or complement of that which is transmitted, 
 that is to say, is such that the two brought together make white light as 
 before. The facts may be studied as Newton originally studied them, in the 
 thin plate of air which occupies the space between a convex lens and a plane 
 surface of glass upon which the lens is laid, in which plate, as the distance 
 from the point of the apparent contact of the glasses increases, there are 
 all degrees of thinness, and with these appear successive rings of vivid co- 
 lours. The same trpth is exemplified in the colours of a soap-bubble, which 
 brighten as the bubble swells and becomes of thinner substance, and are 
 different as the thickness is different and greater from above downwards ; 
 and it is exemplified also in the colours seen in the fissures of cracked 
 ice or crystalline spars, and in numerous other common facts. Now, what- 
 ever be the reasons of such decomposition of light and the explanation is 
 not yet complete we cannot doubt that in natural bodies generally, the 
 colours, opacity, transparency, &c., depend entirely upon the volume and 
 arrangement of the minute fibres or plates, with included interstices, which 
 constitute the volume or structure of each mass Accordingly, whatever 
 changes that arrangement may change also the colour of the mass Thus, 
 by drawing a certain number of minute lines on a certain extent of any me- 
 tallic surface, we may make it of what colour we please ; and mother-of-pearl 
 owes its vivid colours and beauty entirely to its furrowed or striated surface, 
 as is proved by our making an impression of that surface on sealing wax and 
 perceiving that the wax exhibits similar colours. 
 
 The investigations in progress respecting the phenomena of light, are fur- 
 nishing new proofs of the extreme simplicity of nature, amidst the boundless 
 extent and most curious variety. When men thought of the sense of touch 
 only as it exists at the tips of the fingers, or on the general surface of the 
 body they were far from suspecting that the sense of hearing had the near 
 relation to it which subsequent discoveries have proved, and still less, that 
 the sense of sight was only yet a finer touch than hearing. But step by 
 step they have ascertained, 1st. in relation to sound, that the air through 
 which it usually reaches the organ of hearing, is a material fluid as much as 
 
394 LIGHT. 
 
 water, consisting of the same or smaller particles, only more distant among 
 themselves, and that a motion or trembling in the air,, by affecting nerves 
 exposed in the ear, produces the sensation of sound by slight repeated pressures 
 on these nerves, as the trembling in a log of wood caused by the action of a 
 saw produces a peculiar sensation of touch in the nerves of a hand laid on the 
 log ; and, moreover, that sound in all its varieties, is merely such trembling 
 affecting a structure of nerve in the ear, which nerve is made as much more 
 readily excitable than the nerves in the fingers or general cutaneous surface, 
 as the action or impulse of trembling air is more delicate than the stronger 
 pressures of common occurrence. And, 2dly, in the investigation respecting 
 light, this kind of comparison is carried a step farther, for it is become 
 matter almost of certainty that the sensation of light is produced in the 
 suitable nervous tissue of the eye, called the retina by a trembling motion in 
 another fluid than air, which fluid pervades all space, and in rarity or sub- 
 tilety of nature surpasses air yet more than air does water or solids ; and 
 that, while in sound, different tones or notes depend on the number of 
 vibrations in a given time, so in light do different colours depend on the 
 number and extent of the vibrations. Can human imagination picture to 
 itself a simplicity more magnificent and fruitful of marvellous beauty and 
 utility than all this ? But yet farther, as air answers in the universe so many 
 important purposes besides that of conveying sounds although this, alone, 
 comprehends language, which almost means reason and civilization so also 
 does the material of light minister in numerous ways, in the phenomena of 
 heat, electricity and magnetism. 
 
 The truths now positively ascertained with respect to the nature of light 
 and vision, are among those in the wide field of human iaquiry, which, . 
 acting on ordinary apprehension, most forciby place the student, as it were, 
 in the very presence of Creative Intelligence, awakening in him the most 
 elevated thoughts of which the human mind is capable. Had there been no 
 light in thje universe, all its other perfections in regard to man had existed in 
 vain. This earth would have been to its human inhabitants what any 
 unknown shore would be to exiles abandoned upon it after their eyes were 
 put out: every movement might be to their destruction, for their perceptions, 
 being limited by the length of their arms, and of their fearful groping steps, 
 the wretched beings separating when impelled by hunger to search for food, 
 would probably scatter to meet no more. But the material of light exists, 
 pervading all space, and certain impressions made upon it in one place, ex- 
 tend rapidly over the universe, the progressive impression being called a ray, 
 or beam of light. The beams of light, then from all parts coming to every 
 individual, may be regarded as millions of supplementary arms or feelers 
 belonging to the individual, and which reach to the end of the universe, so 
 that each person, instead of being as a blind point in space, becomes nearly 
 omnipresent \ then these limbs or feelers have no weight, they are never in 
 the way, they impede nothing, and they are only known to exist when their 
 use is required ! 
 
 But this miracle of LIGHT would have been totally useless, and the para- 
 dise of earth would have been to man still a dark and dreary desert, had there 
 not been farther the twin miracle of the EYE, an organ of commensurate deli- 
 cacy to perceive the light. In the Eye we have to admire the round cornea, 
 of such perfect transparency, placed exactly in the anterior of the ball, (and 
 elsewhere it had been useless,) then exactly behind this, the beautiful curtain, 
 the iris, with its opening, called the pupil, dilating and contracting to suit the 
 
PERFECTION OF THE EYE. 395 
 
 intensity of light and exactly behind the iris, again, the crystalline lens pos- 
 sessing important and remarkable properties, and which, by acting on the 
 entering light, forms of it on the retina beautiful pictures or images of the 
 objects in front ; the most sensible part of the retina being where the images 
 fall. Of these parts and conditions, had any one been otherwise than as it is, 
 the whole eye had been useless, and light useless, and the great universe use- 
 less to man, for he could not have existed in it. Then, farther, we find that 
 this precious organ, the eye, is placed in the person, not as if by accident, 
 any where, but aloft on a befitting eminence, where it becomes the glorious 
 watch-tower of the soul ; and, again, not so that to alter its direction the 
 whole person must turn, but in he head, which, on a pivot of admirable 
 structure, moves while the body is at rest ; besides that, the ball of the eye 
 itself can roll in its place, and is furnished with muscles which, as the will 
 directs, turn it with the rapidity of lightning to sweep along the horizon, or 
 take in the whole heavenly concave ; then is the delicate orb secured in a 
 strong socket of bone, and there is over this the arched and padded eyebrow 
 as a cushion, to mitigate the shock of blows, and with its inclined hairs to 
 turn aside any descending perspiration or other moisture which might incom- 
 mode ; then is there the soft and pliant eyelid with its beauteous fringes, 
 incessantly wiping the polished surface, and spreading over it the pure mois- 
 ture poured out from the lachrymal glands above, of which moisture the 
 superfluity, by a fine mechanism, is sent into the nose, there to be evapor- 
 ated by the current of the breath ; still farther, it is to be noted, that instead 
 of there being only one such precious organ, there are two, lest one, by acci- 
 dent, should be destroyed, but which two have so entire sympathy, that they 
 act together as only one more perfect; then the sense of sight continues 
 perfect during the period of growth from birth to maturity, although because 
 the eye then increases in size, the distance between the lens and the retina is 
 constantly increasing ; and the pure liquid which fills the eye, ifrrendered tur- 
 bid by accident or disease, is by the actions of life, afthough its source be the 
 thick red blood, gradually restored to transparency. The mind which can 
 suppose or admit that, within any limits of time, one single such apparatus 
 of vision could have been produced by accident, or without design, must 
 surely be of extraordinary character, or must have received unhappy bias in 
 its education ; but the mind which can still farther admi.t that the millions 
 of human eyes which now exist on earth, all equally perfect, can have sprung 
 from accident and that the millions of millions of other eyes throughout 
 the almost innumerable species of the living creation, where each is adapted 
 to the peculiar nature and circumstances of the animal which bears it, can 
 be accident ; and, lastly, that the countless millions of all these well adapted 
 kinds, which have existed in past ages, were all but accidents the mind 
 which can admit this, must have some of its highest faculties either 
 benumbed or destroyed. 
 
 As a concluding reflection with respect to vision, we may remark, that all 
 the provisions above considered have mere utility in view, for any one of 
 them wanting would leave a necessary link in the chain of creation wanting : 
 but we have shown, in a preceding part of the work, that if there had been 
 white light only, susceptible as now of different degrees of intensity and 
 shade, the merely useful purposes of vision should have been answered about 
 as perfectly as with all the colours of the rainbow a truth instanced in the 
 facts, that many persons do not distinguish colours, and that it imports not 
 whether a person view objects in the morning, or at mid-day, or at even-tide, 
 
396 LIGHT. 
 
 or through plain glass or coloured glass, provided there be light and shade 
 enough to show them clearly. While, therefore, the existence of light gene- 
 rally, and of the eye, speaks- of Creative Power and Intelligence, the exist- 
 ence of colours, or of that lovely variety of hues exhibited in flowers, in the 
 plumage of birds, in the endless aspects of the earth and heavens because 
 appearing expressly planned to give delight to animated beings, speaks of 
 Creative Benevolence, and may well excite in us towards the Being in whom 
 these attributes reside, the feelings associated in our minds during this 
 earthly scene, with the endearing appellation of " Father/' 
 
Fig. 173. 
 
MECHANISM OF THE HUMAN SKELETON. 399 
 
 I 
 
 PART V. 
 
 ANIMAL AND MEDICAL PHYSICS. 
 
 l 
 
 SECTION I. 
 Mechanism of the Human Skeleton. 
 
 HAVING now completed our study of general mechanics, we shall proceed, 
 with the light thence derived, to examine that most interesting illustration 
 of many of the truths the solid frame-work of the human body a perfect 
 work of an unerring Engineer ! 
 
 There is scarcely a part of the animal body, or an action which it per- 
 forms, or an accident that can befall it, or a piece of professional assistance 
 which can be given to it, that does not furnish illustration of some truth of 
 natural philosophy; but w?re we here to enter into much detail, we should 
 be giving minute lessons in medical science, instead of explaining general 
 laws. We shall therefore only touch upon as many particulars as will make 
 the understanding of all the others easy; trying to conclude, among our 
 illustrations, such matters of importance as would be likely to escape the 
 notice of a hasty student. 
 
 The cranium or skull has been already mentioned as an instance of the 
 arched form answering the purpose of giving strength. The brain, in its 
 nature, is so tender or susceptible of injury, that slight local pressure dis- 
 turbs its action. Hence a solid covering like the skull was required with 
 those parts made stronger and thicker which are most exposed to injury. 
 An architectural dome is constructed to resist one kind of force only, always 
 acting in one direction, viz., gravity; and therefore its strength increases 
 regularly towards the bottom, where the weight and horizontal thrust of the 
 whole are to be resisted ; but in a skull, as in a barrel or egg-shell, the mere 
 tenacity of the substance is many times greater than sufficient to resist 
 gravity, and therefore the form and securities are calculated to resist forces 
 of other kinds operating in all directions. When we reflect on the strength 
 displayed by the arched film of an* egg-shell, we need not wonder at the 
 severity of blows w^ich the cranium can withstand. 
 
 In the early fetal state, that which afterwards becomes the strong bony 
 case of the brain exists only as a tough flexible membrane. Ossification 
 commences in this membrane long before birth, at a certain number of points 
 from which it spreads, and the portions of the skull formed around these 
 points soon acquire the appearance of so many scales or shells applied on 
 the surface of the brain, and held together by the remaining membrane not 
 yet ossified. They afterwards become firmly fixed together, by projections 
 of bone from each, shutting in among similar projections of the adjoining 
 ones, until all mutually cohere by perfect dove-tailed joints, like the work of 
 a carpenter. These joints are called sutures of the cranium, and are visible 
 
400 ANIMAL AND 
 
 
 
 to extreme old age. Through early childhood, the cranium remains to a 
 certain degree yielding and elastic, causing the falls and blows, so frequent 
 . during the lessons of walking, c., to be borne with comparative impunity. 
 The mature skull consists of two layers or tables, with a soft diploe between 
 them ; the outer table being very tough, with its parts dove-tailed into each 
 other as tough wood is joined by human artificers; while the inner table is 
 harder and more brittle, (hence called vitreous) with its edges merely lying 
 in contact. 
 
 A very severe partial blow on the skull generally fractures and depresses 
 the part, as a pistol-bullet would : while one less severe, but with more ex- 
 tended contact, being slowly resisted by the arched form, often injures the 
 skull by what is correspondent to the horizontal thrust in a bridge, and 
 causes a crack at a distance from the place struck generally half way 
 round to the opposite side. The French in speaking of this effect, use the 
 term contre-coup. Sometimes in a fall with the head foremost, the skull 
 would escape injury, but for the trunk which falls upon it, and drives the 
 end of the spine against or even through its base. 
 
 In the lower jaw we have to remark the greater mechanical advantage, or 
 lever-power, with which the muscles act, than in other parts of animals. 
 The temporal and masseter muscles pull almost directly, or at right angles 
 to the line of the jaw, while in most other cases, as in that of the deltoid 
 muscle lifting the arm, the muscles act very obliquely, and with power 
 diminished in proportion to the obliquity. An object placed between the 
 back teeth is compressed with the whole direct power of the strong muscles 
 of the jaw. Hence the human jaw can crush a body which offers great 
 resistance, and the jaws of the lion, tiger, shark, and crocodile, &c., are 
 stronger still. 
 
 The teeth rank high among those parts of the animal body which appear 
 almost as if they were severally the results of distinct miraculous agencies 
 so difficult is it to suppose a few simple laws of life capable of producing 
 the variety of form and fitness which they exhibit. They constitute a beau- 
 tiful set of chisels and wedges so arranged as to be most efficient for cutting 
 and tearing, and grinding the food, with their exterior enamel so hard, that 
 few substances in nature can make an impression upon it. In early states, 
 of society, teeth were used for many purposes for which steel is used now. 
 It seems, however, as if the laws of life, astonishing to human intellect as 
 they are, had still been inadequate to cause teeth cased in their hard and 
 polished enamel, to grow as the softer bones grow ; and hence has arisen a 
 provision more extraordinary still. A set of small teeth appear soon after 
 birth, and serve the child until six or seven years of age : these then fall 
 out, and are replaced by larger ones, which endure for life ; the number of 
 the latter, however, being completed cfaly when the man or woman is full 
 grown, by the four teeth, called wisdom teeth, from coming with the person's 
 maturity, to fill up the then spacious jaw. 
 
 The spine or back bone, in its structure, has as much of beautiful and 
 varied mechanism as any part of our wonderful frame. It is the central 
 pillar of support and great connecting chain of all the other parts; and has 
 at the same time, the office of containing within itself, and of protecting 
 from external injury, a prolongation of the brain, called the spiral marrow, 
 more important to animal life than the greater part of the brain itself. It 
 has united in it the apparent incompatibilities of great elasticity, great 
 flexibility in all directions, and great strength, both to support a load and to 
 defend its important contents, as we shall now perceive. 
 
MEDICAL MECHANICS. , 401 
 
 Elasticity. The head rests on the elastic column of the spine, as softly 
 as the body of a carriage rests upon its springs. Between each two of the 
 twenty-four vertebrae or distinct bones of which the spine consists, there is 
 a soft elastic inter vertebral substance, about half as bulky as a vertebrae, and 
 which yields readily to any sudden jar : then the spine is waved or bent like 
 an italic/^ as if perceived on viewing it sideways, or in profile, and by this 
 reason, also, it yields to any sudden pressure operating against either end. 
 The bending might seem a defect in a column intended to support weight, 
 but the disposition of the muscles around is such as to leave all the elasticity 
 of that form, and a roomy thorax, without any diminution of strength. 
 
 Flexibility. The spine has been compared to a chain, because it consists 
 of many distinct pieces (twenty-four.) They are in contact by smooth rub- 
 bing surfaces, which allow of a degree of motion in all directions ; and a little 
 motion comparatively between each two adjoining pieces, becomes a great 
 extent of motion in the whole line. 
 
 The strength of the spine as a whole, is shown in the fact of a man's 
 easily carrying upon his head or back a weight heavier than himself; and the 
 strength of each separate vertebrae surrounding the spinal marrow, is evident 
 in its being a double arch, or strong irregular ring. The spine increases in 
 size towards the bottom, in the justest proportion, as it has more weight to 
 bear. The articulating surfaces of the spine are so many, arid so exactly fitted 
 to each other, and are connected by such number and strength of ligaments, 
 that the combination of pieces, becomes, in reference to motion, a much 
 stronger column than a single bone of the same size would be. 
 
 Considering the great number of parts forming the spine, and their nice 
 mutual adaption, it might be expected that injuries and diseases of the struc- 
 ture would be very frequent. The reverse, however, under natural circum- 
 stances, is true ; so that while hundreds and thousands of works have been 
 published on the diseases of almost every other part of the body, hardly any 
 have been written on spine-affections, and what have appeared are of very 
 recent date. One reason of this is, that whatever regards health and disease 
 is now much more completely analyzed than formerly; but another and the 
 chief reason is, that from a change in modern times introduced into the system 
 of education for young ldies, a considerable proportion of them having grown 
 to womanhood with weakened and crooked spines. The subject merits 
 further consideration here. 
 
 To the well-being of the higher classes of animals, a certain degree of exer- 
 cise of their various parts is not less necessary than their nourishment, and 
 if, during the period of growth, such exercise be withheld by any cause, the 
 body never acquires its due proportions and strength. To prompt young 
 creatures to the required exertion, nature has given them an overflow of life 
 and energy, as evinced in the ever-changing occupation of a child in the quick 
 succession of its ideas, in its jumping and skipping, and using all the modes 
 of roundabout action to expand muscular energy, instead of seeking, as in after 
 life, to accomplish its ends in the shortest ways : and as seen among the 
 inferior animals, in the play of kittens, puppies, lambs, &c. But, strongly as 
 nature has thus expressed herself, tyrant fashion, with^a usual perversion of 
 common sense, had of late times, in England, for young women of the higher 
 classes formed a school discipline, directly at war with nature's dictate ; so 
 that a stranger arriving from China, might almost suppose it our design to 
 make crooked and weak spines by that discipline, as it is the design in China 
 to make little feet by the iron shoe. The result is the more striking, when the 
 brothers of the female victims, and who of course have similar constitutions. 
 
 26 
 
402 ANIMAL AND 
 
 are seen to be robust, healthy, and well-formed. A peasant-girl, when her 
 spirits are buoyant, is allowed to obey her natural feeling, and at proper times 
 to, dance, and skip, and run, until healthy exhaustion asks that repose which 
 is equally allowed; and thus she grows up strong and straight, but the young 
 lady is receiving constant admonition to curb all propensity to such vulgar 
 activity, and often, just in proportion as she subdues nature, she receives the 
 praise of being well-bred. The multifarious studies, also, of the latter come 
 powerfully in aid to the admonition, by fixing her for many hours everyday 
 to sedentary employment; and the consequences soon follow, of weakness 
 in the body generally from the want of the natural quantity and variety of 
 muscular exertion, but weakness of the back particularly, from the manner 
 in which the sitting is usually performed. It would be accounted great cruelty 
 to make a delicate girl stand all day, because her legs would tire, but this 
 very cruelty is in almost constant operation against her back, as if backs could 
 not tire as well as legs. When she is allowed to sit down because she has 
 been long standing, great care is taken that the muscles of the back, which 
 still remain in action as she sits, shall not be at all relieved; for, from the 
 idea that it is ungrace%il to loll, she is either upon a stool which has no back 
 at all, or upon a very narrow chair with a perpendicular back. Now neither 
 of these seats relieve her spine, the stool, however, being less hurtful than 
 the chair, because it allows the spine to bend in different ways so as to rest 
 the different sets of muscles alternately, while the chair keeps the spine 
 constantly upright and nearly unmoved. The excessive fatigue soon causes 
 the spine, somewhere, to give way and to bend, and the curvature often 
 becomes permanent. Ajid, as when a bend takes places in one situation, 
 there immediately follows an opposite bend above or below, to keep the centre 
 of gravity ef the body always directly over the base, the curve thus becomes 
 double, like an italic/, and the distortion is rendered complete. In bending 
 the spine is sometimes also partially rotated or twisted, so as to show from 
 behind that waving profile which should be seen only from the side. 
 
 When owing to such discipline the inclination of the back has once been 
 begun, it is often rapidly increased by the means used to correct it. Strong 
 stiff stays are put on to support the back, as is said, but which in reality, by 
 superseding the action of the muscles placed there l|y nature as the supports, 
 cause these to lose their strength, and to be unable, when the stays are with- 
 drawn, to support the body. Longer sittings in the narrow upright chair are 
 then recommended, and sometimes the back is forcibly stretched by pullies, 
 so the patient is kept all day and night lying on an inclined board, losing her 
 health, &c. ; the only things guarded against being, the patient should take 
 due exercise and air, and should rest properly when she is not taking exer- 
 cise. With many persons the prejudice had at last grown up, that strong 
 stays should be put on at a very early age, to prevent the first approach of 
 the mischief, and that children^should always be made to sit on straight- 
 backed chairs, or to lie on hard planes; and it is probable, that if these cures 
 and preventives had been adopted as universally and strictly as many deemed 
 them necessary, we should now scarcely have in England a young lady of 
 healthful form. What would be said of the person who should try to 
 improve the strength and shape of a young race-horse or greyhound, by 
 binding tight splints or stays round its beautiful young body, and then 
 tying it up in a stall! But this is the kind of absurdity and cruelty which 
 has been so commonly practised in this country towards beings than whom, 
 as nature offers them, the universe surely contains none more faultless. 
 
 A pernicious prejudice, with respect to such curvature or distortion of the 
 
MEDICAL MECHANICS. 403 
 
 spine, long existed, namely that it was a scrofulous affection ; and many 
 mothers concealed it as much as possible, and sought remedy from quacks 
 far from home. In consequence, until within a few years, the management, 
 of spine diseases was chiefly in the hands of some irregular members of the 
 profession, and a rich source of wealth it became to them, from many of 
 their remedies being calculated rather to prolong than to cure the evil. The 
 practice in such cases, however, has now fallen into the hands of the pro- 
 fession generally ; the science having detected the true cause of* the evil, its 
 frequency is already diminished. It has been shown that to prevent the 
 disease is easy, and that the best cures are those conducted on the general 
 principles of improving the health of the patient by fit regimen, of prescrib- 
 ing such exercises as may directly strengthen the affected part, and of caus- 
 ing the patient, "when reposing to assume positions which directly counteract 
 the morbid tendency. 
 
 Some might expect here a long description of machines employed in the 
 treatment of spine affection : but the list of those which are useful or safe 
 is very short : a sofa to rest upon during the day and a fit bed for the 
 night; (the " hydrostatic bed/') proposed by the author of this work, and 
 described ia the next chapter, has certain advantages ;) choice of pleasant 
 means of taking exercise, such as the skipping-rope, shuttle-cock, dum-bells, 
 a rope-ladder to climb, a winch to turn, &c. : and where it is much desired 
 that the young lady should employ herself in the sitting attitude, as in 
 practising music, a chair may be used with crutches rising from its side, or 
 with straps descending from pullies in an overhanging canopy or crane, and 
 kept tight by proper weights at their distant ends, to support the head and 
 shoulders. The author has had a small crane of wood made, which well 
 answers the last mentioned purpose, and may be attached to a common 
 chair. It would be out of place here to detail those particulars of constitu- 
 tional treatment which, in peculiar habits, maybe required to aid the effects 
 of the means above described 
 
 The ribs Attached to twelve vertebrae in the middle of the back, there 
 are the ribs or bony stretchers of the cavity of the chest, constituting a struc- 
 ture which solves, in the most perfect manner, the difficult mechanical pro- 
 blem of making a cavity with solid exterior, which shall yet be capable of 
 dilating and contracting itself. Each pair of corresponding ribs may be 
 considered as constituting a hoop : which hangs obliquely down from the 
 place of attachment behind, so that when the forepart of all the hoops is 
 lifted by the muscles the cavity of the chest is enlarged. 
 
 We have to remark the double connection of the rib behind, first .to the 
 bodies of two adjoining vertebrae, and then to process or projection from 
 the lower, thus affecting a very steady joint, and yet leaving the necessary 
 freedom of motion : and we observe the forepart of the rib to be joined in the 
 breast-bone by flexible cartilage, which allows the degree of motion required 
 there without the complexity of a joint, and admirably guards, by its elasti- 
 city against the effects of sudden blows or shocks. 
 
 The muscles, which have their origin on the ribs and their insertion into 
 the bones of the arm, afford us an example worth remembering of action and 
 reaction being equal and contrary. When the ribs are fixed, 'these muscles 
 move the arm ; and when the arm is fixed, as by resting on a chair or other 
 object, they with equal force move the ribs The latter occurrence is seen 
 in fits of asthma and dyspnoea. 
 
 The human skeleton, with its naked ribs, is so associated in the common 
 mind, with ideas of death and loss of friends, and all the terrors of doubtful 
 
404 ANIMAL AND 
 
 futurity, that to most persons it is an object of abhorrence; but to the philo- 
 sophic mind, which rises superior to place and time, the so admirable adap- 
 tation of all the parts to their purposes, and of parts which, being purely 
 mechanical, are perfectly understood, makes it independently of all profes- 
 sional considerations, an object of the most intense interest. Such mechanism 
 reveals, by intelligible signs, the hand of the Creator; and a man may be 
 said sublimely to commune with his Maker, who contemplates and under- 
 stands the structure aright. 
 
 The shoulder-joint is remarkable for combining great extent of motion 
 with great strength. The round head of the shoulder-bone, that it may turn 
 freely in all ways, rests upon a shallow cavity or socket in the shoulder- 
 blade; and the danger of dislocation from this shallowness is guarded against 
 by two strong bony projections above and behind. To increase the range of 
 motion to the greatest possible degree, the bone called the shoulder-blade, 
 which contains the socket of the arm, slides above itself upon the convex 
 exterior of the chest, having its motions limited in certain directions by its 
 connection, through the collar-bone or clavicle, with the sternum. 
 
 The scapula or blade-bone is extraordinary as an illustration of the me- 
 chanical rules for combining lightness with strength. It has the strength of 
 the arch from being a little concave, like the dished wheel already described, 
 and its substance is chiefly collected in its borders and spines, with thin 
 plates between, as the strength of a wheel is collected in its rim, and spokes, 
 and nave. 
 
 The bones of the arms, considered as levers, have the muscles which 
 move them attached very near to the fulcra, and very obliquely, so that the 
 muscles, from working through a short distance, comparatively with the 
 displacement of the resistances at the extremities require to be of great 
 strength. It has been calculated that the muscles of the shoulder-joint, in 
 the exertion of lifting a man upon the hand, pull with the force of two 
 thousand pounds. 
 
 Notwithstanding all the securities of the shoulder-joint now described in 
 the infinite variety of twists, and falls and accidents to which men, in the 
 busy scene of society, are liable, the joint is frequently dislocated, that is, 
 the rounded head of the humerus or arm-bone slips from his socket, with 
 instant lameness as a consequence. 
 
 In the treatment of dislocations and fractures of the frame-work of the 
 human body, the surgeon cannot avoid displaying strikingly either his pro- 
 fessional skill or ignorance. With what ease does the displaced arm or thigh- 
 bone return to its socket, under the guidance of the skilful hand ; and to 
 what horrible, and often unavailing torture, is the patient subjected, when, 
 in such a case, ignorance dares to act ! It is very painful to allow the imagi- 
 nation to dwell upen the records of ancient surgery, and to be made present, 
 as it were, to the stretching of patients on the rack with pullics and power- 
 ful engines, to do, what better information could have accomplished with 
 such gentleness. And would that the records of modern times contained 
 no instances of individuals crippled for life by bad practice. To a 
 practioner in this branch, impunity and a quiet conscience can now be se- 
 cured only -by his having a perfect knowledge of anatomy, and familiarity 
 with the laws of mechanical philosophy. 
 
 With our present information on these subjects, we are suprised at the 
 detail of the practices and errors promulgated in former times, owing to im- 
 perfect knowledge of mechanics, even by authors of the highest credit. It 
 would hardly be believed that so distinguished an ornament of English sur- 
 
MEDICAL MECHANICS. 405 
 
 gery as Mr. Pott, should assign as one reason for not pulling by the hand 
 or foot, in reducing a dislocation of the shoulder or hip, that the intervening 
 joints prevented the strain from reaching the part desired. 
 
 Some surgeons, possessing a certain degree of knowledge in mechanics, 
 but only that degree which is dangerous, having heard that the lever was 
 a powerful engine, have tried to replace bones 'solely by leverage, as it was 
 called. Thus, a man's dislocated arm has been placed over the back of a 
 chair as a fulcrum, or over the top of a door, and while the weight of the 
 suffering body was hanging to it on one side as the resistance, force has been 
 applied to the other side, enough sometimes to break the bone, or to tear 
 away the ligaments and soft parts about the joint. 
 
 Other surgeons, after learning in the same way the effects of the pulley, 
 have wished to do all by irresistible extension, and instead of borrowing the 
 moderate assistance which might be useful, have torn muscles and ligaments 
 from their attachments. 
 
 It is not the object of this work to enter into an extended examination of 
 the accidents which befall the body requiring mechanical skill for their proper 
 management, for this would be to deliver a course of instruction on practical 
 surgery; but it is wished to awaken the attention of the medical student to 
 those valuable general principles which may furnish direction in most diffi- 
 culties. Knowing these principles, and possessing good sense, he will often, 
 be a more effective minister of his art than a man full of learned precedents, 
 who knows them not. To make this lesson more impressive to his young 
 readers, the author may take the liberty of adducing his own experience. 
 When he was himself very young, and had not yet had extensive practical 
 experience, he was thrown into a situation where a heavy medical charge 
 developed upon him, and where, through accidents among a numerous crew, 
 during a very eventful voyage, which led to intercourse with, the savage 
 inhabitants of unfrequented coasts, he had, within twenty-six months, more 
 practice in singular wounds, dislocations, and fractures, than falls to the lot 
 of many practitioners during a life : in that time he became strongly im- 
 pressed with the importance to the medical man of such knowledge as he now 
 recommends : and he had reason to rejoice that although Natural Philosphy 
 was not then much insisted upon in the course of professional education, cir- 
 cumstances had led him to look carefully at the body through that medium. 
 
 The os humerij or bone of the upper arm, is not perfectly cylindrical, but 
 like most of the other bones called cylindrical, it has ridges to give strength, 
 on the principle explained in the chapter " on strength of materials." 
 
 The elbow joint is a correct hinge, and so strongly secured that it is rarely 
 dislocated without fracture. 
 
 The fore-arm consists of two bones with a strong membrane between 
 them. Its great breadth, from this structure, affords abundant space for the 
 origin of the many muscles which go to move the hand and fingers : and the 
 very peculiar mode of connection of the two bones, gives man that most 
 useful faculty of turning the hand round, into what are called the positions 
 of pronation and supination, exemplified in the action of twisting or of 
 turning a gimblet. 
 
 The old surgeons, who acted frequently by rules of routine rather than 
 by reasons, in the accident of fracture to one or both bones of the fore-arm, 
 often applied a tight bandage, which pulled the bones at the fractured part 
 close to each other, and thus injured the future shape and strength of the arm. 
 
 The wrist. The many small bones forming the wrist have a signal effect 
 
406 ANIMAL AND 
 
 of deadening, in regard to the parts above, the shocks or blows which the 
 hand receives. 
 
 The annular ligament is a strong band passing round the joint, and 
 keeping all the tendons which pass from the muscles above to the fingers, 
 close to the joint. It answers the purpose of so many fixed pullies for 
 directing the tendons : without it, they would all, on action, start out like 
 bow-strings, producing deformity and weakness. 
 
 The human hand is so admirable, from its numerous mechanical and 
 sensitive capabilities, that one opinion at one time prevailed, that man's 
 superior reason depended on his possessing such an instructor and such a 
 servant. Now, although reason, with hoofs instead of fingers, could never 
 have raised man much above the brutes, and probably could not have secured 
 the continued existence of the species, still, the hand is no more than a 
 fit instrument of the Godlike mind which directs it. 
 
 The pelvis, or strong, irregular ring of bone on the upper edge of which 
 the spine rests, and from the sides of which the legs spring, forms the centre 
 of the skeleton. A broad bone was wanted here to connect the central 
 column of the spine with the lateral columns of the legs, and a circle was 
 the lightest and strongest. If we attempt still farther to conceive how the 
 circle could be modified so as to fit it for the spine to rest on, for the thighs 
 to roll in, for muscles to spring from, both above and below, for the person 
 to be able to sit, &c., we shall find, on inspection, that all our anticipations 
 are realized in the most perfect manner. In the pelvis, too, there are the 
 thyroid hole and ischiatic notches, furnishing subordinate instances of con- 
 trivance to save material and weight : they are merely deficiencies of bone 
 where solidity could have given no additional strength. The broad ring of 
 the pelvis protects most securely the important organs placed within it. 
 
 The hip joint exhibits the perfection of the ball and socket articulation. 
 It allows the feet to move round in a circle, as well as to have the great 
 range of backward and forward motion, exhibited in the action of walking. 
 When we see the elastic, tough, smooth cartilage which lines the deep socket 
 of this joint, and the similar glistening covering of the ball or head of the 
 thigh-bone, and the lubricating synovia poured into the cavity by appro- 
 priate secretaries, and the strong ligaments giving strength all around, we 
 feel how far the most perfect of man's work falls short of the mechanism 
 exhibited in nature. 
 
 The thiyh-bone is remarkable for its two projections near the top, called 
 trochanters, to which the moving muscles are fixed ; and which lengthen 
 considerably the lever by which the muscles work. The shaft of the bone 
 is not straight, but has a considerable forward curvature. Short-sightedness 
 might suppose this a weakness, the bone being a pillar to support a weight ; 
 but the bend gives it in reality the strength of the arch, to bear the action 
 of the mass of muscles called vastus, which lies and swells upon its fore part, 
 
 The knee is a hinge joint of complicated structure, claiming the most 
 attentive study of the surgeon. The rubbing parts are flat and shallow, and, 
 therefore, the joint has little strength from form ; but it derives security from 
 the numerous and singularly strong ligaments which surround it. The liga- 
 ments on the inside of the knees resemble, in two circumstances, the annular 
 ligaments of joints, viz., in having a constant and great strain to bear, and 
 yet in becoming stronger always as the strain increases. The line of the leg, 
 even in the most perfect shapes, bends inward a little at the knee, requiring 
 the support of the ligaments ; and in many persons it bends very much ; but 
 
MEDICAL MECHANICS. 407 
 
 the inclination does not increase with age. The legs of many weakly in- 
 kneed children become straight by exercise alone. This inclination at the 
 middle joint of the leg, by throwing a certain strain on the ligaments, gives, 
 in such actions as jumping, running, &c., an increase of elasticity to the limb. 
 
 In the knee there is a singular provision of loose cartilages between the 
 ends of the bones. They have been called friction-cartilages, from a 
 supposed relation in use to friction-wheels, but their real effect seems to be, 
 to accommodate, in the different positions of the joint, the surfaces of the 
 rubbing bones to each other. 
 
 Under the head Pneumatics, we shall find that the bones forming the 
 joints, are held together, independently of their ligaments, by a constant 
 pressure of the atmosphere, amounting in the knee, for instance, to upwards 
 of sixty pounds. 
 
 The great muscles on the fore-part of the thigh are contracted into a ten- 
 don a little above the knee, over and in front of which the tendon has to 
 pass to reach the top of the leg, where its attachment is. The part of the 
 tendon over the joint becomes bony, and forms the patella or knee-pan, 
 often called the pulley of the knee. This peculiarity enables the muscles 
 to act more advantageously, by increasing the distance of the rope from the 
 centre of the motion. The petella is, moreover, a sort of shield or protection 
 to the fore-part of this important joint. 
 
 The leg below the knee, like the fore-arm already described, has two 
 bones. They offer spacious surface of origin for the numerous muscles 
 required for the feet, and they form a compound pillar of greater strength 
 than the same quantity of bone as one shaft would have had. The indi- 
 vidual bones also are angular instead of round, hence deriving greater power 
 to resist blows, &c. 
 
 The ankle-joint is a perfect hinge of great strength. There is in front of 
 it an angular ligament, by which the greater part of the tendons passing 
 downwards to the foot and toes are kept in their places. One of these ten- 
 dons passes behind and under the bony projection of the inner ankle, in a 
 smooth, appropriate groove, exactly as if a little fixed pulley were there. 
 
 The heel, by projecting so far backwards, is a lever for those strong mus- 
 cles to act by, which form the calf of the leg and terminate in the tendo- 
 achillis. The muscles by drawing at it, lift the body, in the actions of 
 standing on the toes, walking, dancing, &c. In the foot of the negro, the 
 heel is so long as, in European estimation, to appear ugly ] and its great 
 length rendering the effort of smaller muscles sufficient for the various pur- 
 poses, the calf of the negro's leg is smaller than of other races of men. 
 
 In a graceful human step the heel is always raised before the foot is lifted 
 from the ground, as if the foot were part of a wheel rolling forward ; and 
 the weight of the body, supported by the muscles of the calf of the leg, as 
 just described, rests for the time on the fore-part of the foot and toes. There 
 is at that time a bending of the foot in a certain degree. But where strong 
 wooden shoes are used, or any shoe so stiff that it will not yield and allow 
 this bending of the foot, the heel is not raised at all until the whole foot rises 
 with it, so that the muscles of the calf are scarcely used, and in consequence 
 soon dwindle in size, and almost disappear. Many of the English farm- 
 servants wear heavy stiff shoes, and in London may constantly be seen is 
 the drivers of country wagons, with fine robust body and arms, but with legs 
 which are fleshless spindles, producing a gait most awkward and unmanly. 
 The brothers of these men, otherwise employed, are not so mis-shapen ; 
 and even they themselves, when they choose to become soldier, and are 
 
408 ANIMAL AND 
 
 trained in military exercises, lose their peculiarity. What a pity that, for 
 the sake of a trifling saving, graceful nature should be thus deformed. An 
 example of an opposite kind is seen in Paris, where, as the streets have no 
 side pavements, and the ladies are obliged consequently to walk almost con- 
 stantly on tiptoe, the great action of the muscles of the calf has given a 
 conformation of the leg and foot, to match which the Parisian belles proudly 
 challenge all the world, not aware, probably, that it is a defect of their 
 city to which the boasted peculiarity is mainly due. 
 
 A person confined to his bed for a weqk or two by sickness, has generally 
 to remark a much greater wasting of the legs than of the arms : the reason 
 of which is, that the muscles of the leg, in ordinary cases, being more in 
 use than those of the arms, their ordinary bulk is more dependent on use, 
 and they suffer a corresponding change from inaction. 
 
 Such facts as now mentioned, bear directly on the subject so near the 
 hearts of many English mothers, viz., the weak and crooked backs of their 
 daughters. From such they may understand that strong stays, which in 
 part supercede the action of the muscles placed by nature around the spine 
 to support it, cause these muscles to dwindle, and afterwards, when the sup- 
 port of the stays fails or becomes unequal, leave the back to bend or twist. 
 Stays, therefore, can neither help to make strong and well-formed backs, 
 originally, nor can they be a remedy after the weakness has commenced. 
 A healthy young woman from the country, with spine lying deep between 
 the firm cushions of muscles which support it, if, according to town fashion, 
 braced up in tight stays, will frequently, at the end of a short time, exhibit 
 such a wasting of the flesh, that the' points of bone in the spine may be 
 counted by the eye, all the way down. 
 
 The arch of the foot is to be noticed as another of the many provisions 
 for saving the body from shocks by the elasticity of the supports. The heel 
 and the ball of the toes are the two extremes of the elastic arch, and the 
 leg rests between them. 
 
 Connected with elasticity, it is interesting to remark how imperfectly a 
 wooden leg answers the purpose of a natural one. The centre of the body, 
 when supported by the. wooden leg, which always remains of the game 
 length, must describe, at each step a portion of a circle of which the bottom 
 nob of the leg is the centre; and the body is, therefore, constantly rising and 
 falling; but with the natural legs, which, by gentle flexture at the knee, 
 are made shorter or longer in different parts of the step as required, the body 
 is carried along in a manner perfectly or nearly level. In like manner, a 
 man riding on horseback, if he kept his back upright and stiff, has his head 
 jolted by every step of the trotting animal; but the experienced horseman, 
 even without rising in the stirrups, by letting the back yield a little at each 
 movement, as a bent spring yields during the motion of a carriage, can 
 carry his head quite smoothly along. 
 
 In a general review of the skeleton, we have to remark, 1st, the nice 
 adaptation of all the parts to one another, and to the strains which they have 
 respectively to bear; as in the size of the spinal vertebrae increasing from 
 above downwards the bones of the leg being larger than thos of the arm, 
 and so on. 2dly, the objects of strength and lightness combined ; as by the 
 hollowness of the long bones their angular form their thickening and 
 flextures in particular places where great strain has to be borne the enlarge- 
 ment of the extremities to which the muscles are attached, lengthening the 
 lever by which these acts, &c. 3dly, we have to remark the nature and 
 
MEDICAL MECHANICS. 409 
 
 strength of material in different parts, so admirably adapted to the purposes 
 which the parts serve : there is bone for instance, in one place nearly as 
 hard as iron, where, covered with enamel, it has the form of teeth, with the 
 office of chewing and tearing all kinds of matter used as food; in the cranium 
 again, bone is softer but tough and resisting ; in the middle of the long bones 
 it is compact and little bulky, to leave room for the swelling of the muscles 
 lying there; while at either end, with the same quantity of matter, it is 
 large and spongy, to give a broad surface for articulation ; and in the spine, 
 the bodies of the vertebrae, which rest on an elastic bed of intervertebral sub- 
 stance are light and spongy, while their articulating surfaces and processes 
 are very hard. Tn the joints we see the tough elastic smooth substance called 
 cartilage, covering the ends of the bones, defending and padding them, and 
 destroying friction. In infants we find all the bones soft or gristly, and there- 
 fore calculated to bear with impunity the falls and blows incidental to their 
 age ; and we see certain parts, where elasticity is necessary or useful, re- 
 maining cartilage or gristle for life, as at the anterior extremities of the ribs. 
 About the joints we have to remark the ligaments which bind the bones to- 
 gether, possessing a tenacity scarcely equalled in any other known substance; 
 and we see that the muscular fibres whose contractions move the bones and 
 thereby the body, because they would have rendered the limbs clumsy 
 even to deformity had they all passed over the joints to the parts which 
 they have to pull, attach themselves, at convenient distances, to a strong 
 cord called a tendon, by means of which, like a hundred sailors at a rope 
 they make their effort effective at any distance. The tendons are remarka- 
 ble for the great strength which resides in their slender forms, and for the 
 lubricated smoothness of their surfaces. Many other striking particulars 
 might be enumerated, but these may suffice. Such, then, is the skeleton 
 or general frame-work of the human body ; less curious and complicated, 
 perhaps, than some other parts of the system which we have yet to examine, 
 but so perfect and so wonderful, that the mind which can attentively con- 
 sider it without emotion is in a state not to be envied.* 
 
 This living force of man has been used as a working power in various 
 ways, as in turning a winch pulling a rope walking in the inside of a 
 large wheel to move it, as a squirrel or turnspit dog moves his little wheel, 
 &c. Each of these has some particular advantage : but the mode in which 
 for many purposes, the greatest effect may be produced, is for a man to carry 
 up to a height his body only, and then to let-it work by its weight in de- 
 scending. A bricklayer's labourer would be less fatigued to lift twice as 
 many bricks to the top of a house in the course of a day, by ascending the 
 ladder without a load, and raising bricks of nearly his own weight over a 
 pully each time in descending, than by carrying fewer bricks and himself 
 up together, and descending again without a load, as is still usually done. 
 
 Reflection, independently of experiment, would naturally anticipate the 
 above stated result, for the load which a man should be best able to carry, 
 is surely that from which he can never free Jiimself, the load of his own 
 body. Accordingly the strength of muscles and disposition of parts are all 
 such as to make his body appear very light to him. 
 
 The question which was agitated with such warmth some time ago as to 
 
 * In the second and third editions of this work a criticism was introduced at this 
 place of a treatise on "Animal Mechanics," published as a part of the Library of 
 Useful. Knowledge; in which treatise the author from imperfect acquaintance with 
 Natural Philosophy had fallen into many grave errors. That note, having answered its 
 purpose is not repeated here. 
 
410 ANIMAL AND 
 
 the propriety of condemning men and women to work on the tread-mill re- 
 ceives an easy decision here. They work by climbing on the outside of a 
 large wheel or cylinder, which is turned by their weight, and on which, to 
 avoid falling from their proper situation, they must advance just as fast as it 
 turns. There are, on the outside of the cylinder, projections or steps for the 
 feet, and the action to the workers is exactly that of ascending an acclivity. 
 Now, as nature has fitted the human body for climbing hills as well as for 
 walking on plains, the work of the tread-mill, under proper restrictions as 
 to duration, must be nearly as natural and healthful as any other. Its 
 effects have ultimately proved it to be so. 
 
 Animal power being exhausted in proportion, as well to the time during 
 which it is acting, as to the intensity of force exerted, there may often be a 
 great saving of power by doing work quickly, although with a little more 
 exertion during the time. Suppose two men of equal weight to ascend the 
 same stair, one of whom takes only a minute to reach the top, and the other 
 takes four minutes, it will cost the first but a little more than a fourth part 
 of the fatigue which it costs the second, because the exhaustion has relation 
 to the time during which the muscles are acting. The quick mover may 
 have exerted, perhaps, one-twentieth more force in the first instant, to give 
 his body the greater velocity which was afterwards continued, but the sloth 
 supported his load four times as long. 
 
 A healthy man will run rapidly up a long stair, and his breathing will 
 scarcely be quickened at the top ; but if he walk up slowly, his legs will 
 feel fatigued, and he will have to wait some time before he can speak calmly. 
 
 For the same reason coach-horses are much spared by being made to, 
 gallop up a short hill, and then being allowed to go more slowly for a little 
 time, so as to rest at the top. 
 
 The rapid waste of muscular strength which arises from continued action, 
 is proved by keeping the arm extended horizontally for some time. Few 
 persons can continue the exertion beyond a minute or two. In animals 
 which have long horizontal necks, there is a wonderful provision of nature 
 in a strong elastic substance on the back or upper part of the neck which 
 nearly supports the head independently of muscular exertion. 
 
 In farther illustration of the truth that strength is saved in many cases by 
 doing work quickly, we may recall the fact explained at page 60, that a body 
 thrown or shot upwards with double velocity, rises four times as far when 
 that with a single velocity, or half the other. 
 
 "Instruments. 1 ' 
 
 The following remarks regard some instruments used by medical men, and 
 which range under the present division of " Mechanics." 
 
 The obstetric forceps As the blades beyond the joint or fulcrum are 
 longer than the handles, the pressure on the head included in them is less 
 than that exerted by the hand that uses them, but its degree should always 
 be kept present to the mind of the operator. 
 
 The vectiSj or lever used instead of the forceps just mentioned, is a dan- 
 gerous instrument in unskilful hands. In fact whenever it is used as a 
 lever, in the common acceptation of the term, with some part of the pelvis 
 as the fulcrum, the use of it is a piece of unskilful cruelty; for the soft parts 
 between the bone and the instrument are bruised not only with the whole 
 force of the hand, but with twice or thrice as much, according as the resist- 
 
MEDICAL MECHANICS. 411 
 
 ance is nearer to the fulcrum than to the hand. The instrument is safely 
 used, only when the operator makes one of his hands the fulcrum, and uses 
 the other as the power, or makes different parts of the same hand answer 
 both purposes ; and then there is a resemblance between the action of the 
 vectis and of a hook. 
 
 The levator, or lever for rising the broken and depressed portion of the 
 skull in trepanning, has a fulcrum attached to it by a joint. Care should 
 be taken to place this fulcrum where its pressure cannot be injurious. 
 
 The circular-saw or crown-saw of the trephine, should be worked with a 
 quick motion and gentle pressure, for the reason given at page 107, when 
 treating of cutting instruments. The purpose is thereby attained, both 
 sooner and better, and the head of the patient is less shaken. 
 
 For the same reason, in amputation, a light and quick motion of the 
 straight saw causes less jarring, and effects an easier division of the bone. 
 
 In using the amputation knife, the speed, neatness and success of the 
 operation are all favoured by blending the drawing or saw motion of the knife, 
 with the pressure towards the bone 
 
 These last observations are of a hundred similar, which might be made to 
 prove the extreme importance to a surgeon of having familiarity with the 
 use of tools and instruments. Perhaps a person cannot better acquire this 
 than by practising, while young, some amusing work of carpentry. Manual 
 dexterity, and a little readiness at mechanical contrivance, so frequently 
 prove of importance to persons.in all stations, that it is a great defect in the 
 prevailing system of general education, not to cultivate them with greater 
 attention. ^ 
 
 The tooth-key is an instrument found in many hands, and persons who do 
 not pretend to more than the lowest degree of skill in the healing art, are 
 not afraid to use it. The consequence is, that perhaps scarcely a day passes 
 in which teeth are not broken and jaws splintered, and gums bruised even 
 to sloughing, by the unskilful or awkward use of it. The common tooth-key 
 may be compared to wheel and axle, the hand of the operator acting on two 
 spokes of the wheel to work it, while the tooth is fixed to the axle by the 
 claw, and is drawn out as the axle turns. The gum and alveolar process of 
 the jaw form the support on which the axle rolls. The common errors in 
 tooth-drawing by the key, are these : 
 
 1st. Turning the key towards that side where the adjoining teeth are so 
 close that the tooth to be drawn cannot pass, without either breaking one of 
 them, or being itself broken. Sometimes two or even three teeth are thus 
 unfixed instead of one. 
 
 2d. Neglecting the natural inclination of the tooth. By winding it round 
 in the direction in which it already inclines, and in accordance with a bend 
 which is generally found in it, the operation is easy and safe ; but by draw- 
 ing it in the opposite way, it not unfrequently is broken, or it splinters the 
 part of the jaw-bone in which it was set. 
 
 3d. If the tooth-claw be blunt, its point may slip upon the tooth, so as 
 to produce a jar which is very apt to break the tooth 
 
 4th. Unless the axle or fulcrum of the key be made to rest as evenly as 
 possible on the gum, it will tear or otherwise injure the gum. It should 
 rest, if possible, over the part of the bone in which the tooth is set, for if 
 not as when a back tooth is drawn by an instrument resting on a part con- 
 siderably anterior to it the twist produced is painful, and there is danger 
 of splintering. 
 
412 ANIMAL AND 
 
 A man who, after making these reflections, operates leisurely a few times 
 on the dead subject, will be able to give instant and safe relief to very com- 
 mon and most intense suffering. And it is hardly excusable in any medi- 
 cal man who may be placed where a professed dentist cannot be procured, 
 to neglect acquiring a talent so easy. 
 
 Some dentists, by a strong forceps made for the purpose, pull teeth 
 directly out; others by a simple sharp-pointed lever push them out; and 
 others use a forceps in the manner of the tooth-key, by resting one side of 
 it on the gum as a fulcrum, and then giving it a twisting motion : in the 
 latter case, the resting side of the forceps is formed like the bolster of a 
 tooth-key. But much more in all cases depends on the dexterity of the 
 operator than on the form of the instrument. 
 
 Steel trusses for ruptures are among the blessings to suffering humanity 
 which modern ingenuity has supplied. From the unhealthy employments 
 of some men, and the early dissipations or unnatural modes of life of others, 
 debilitated constitutions are frequent, and are often transmitted to offspring; 
 and one of the lamentable effects is that weakness of the flesh forming the 
 sides of cavities which, and on occasions of strong effort, allows, at particular 
 points the living parts to protrude from within, so as to form tumours under 
 the skin. A person perfectly healthy may suffer the same injury from a 
 very violent strain or pressure on the abdoman. The occurrence is called 
 hernia or rupture ; the most common hernia is that of the small intestine 
 through the groins. 
 
 Formerly this occurrence disabled for life. A man who had hernia was 
 discharged from the army or navy ; he could not ride on horseback, or take 
 usual exercise ; he could not lift a weight ; and in a word he often became 
 a miserable burden to himself and others. Now, by fitting the pad of a 
 good steel truss to the part, the rupture is as perfectly restrained, as if the 
 hand of a skilful surgeon were constantly there. The truss may be put on 
 and off, with as little reflection or trouble, as a part of the ordinary dress, 
 and the man becomes again almost as fit for all the duties of life as if he 
 were without his ailment. 
 
 The old and still common form of the steel truss is that of a half or three- 
 quarter hoop, so bent and tempered, that when put upon the patient, one 
 end which has a pad upon it, presses with a given force, over the opening by 
 which the rupture protrudes, and the remainder tightly embraces the body. 
 The objections to this kind of truss are, the difficulty of making it to fit 
 exactly; its being rather troublesome to put on and off; and its pressing 
 disagreeably round the body. 
 
 Another kind of truss, free from these objections, consists of a little more 
 than half a hoop, with a pad at each end : one of the pads supports the 
 weakness, and the other rests upon the centre of the back, to bear all the 
 strain there, while the hoop itself passing round the hip farthest from the 
 hernia, reposes loosely on the hip. This truss may be called self-adjusting, 
 for it almost of itself falls into its place, and needs no fastenings ; the same 
 truss fits all persons of one size, whatever their shape; and the strength may 
 be adjusted by changing the number of plates in the spring-hoop. 
 
 Tourniquets, creches, splints, &c., &c., are so simple in all respects as not 
 to merit special notice here. 
 
 This section contains some of the reflections which, in contemplating the 
 human skeleton, occur to a person familiar with mechanical philosophy : and 
 the more complete such a person's knowledge is of anatomy, physiology, 
 
MEDICAL MECHANICS. 413 
 
 surgery and medicine, the more numerous will be the professional objects on 
 which this philosophy will shed a light, dissipating doubt and error. The 
 author has not entered into more minute detail, because it would have been 
 encroaching upon the office of the teachers of particular departments, and 
 because he thinks that any one who is not enabled, by the examples here 
 given, to make the applications of the general laws to all possible cases, 
 may account the study f the healing art unsuited to the faculties with 
 which he is endowed. 
 
414 FLUIDITY IN RELATION TO ANIMALS. 
 
 
 
 PART V. 
 
 (CONTINUED.) 
 
 SECTION II. DOCTRINES OF FLUIDITY IN RELATION TO 
 
 ANIMALS. 
 
 In the preceding sections on the laws of fluidity, occasional illustrations 
 have been taken from the animal economy- but there are many otherpar- 
 ticulars of the same class and of great interest, which it is convenient to 
 consider apart under the four heads of, 1st. The circulation of the Hood ; 
 2d. The respiration and voice; 3d. The \digestion^ and 4th. The pelvic 
 phenomena. It is important to remark here, that this section cannot be 
 understood by a person ignorant of those which precede. 
 
 THE CIRCULATION OF THE BLOOD. 
 
 PERHAPS there are few points more remarkable in the history of the pro- 
 gress by which man has arrived at his present knowledge of the universe, 
 than that he was so long ignorant of the fact that the blood in his own and 
 in other animal bodies is constantly circulating. England claims as one of 
 her sons, the man whose powerful intellect at last established this truth, in 
 opposition to strong appearances, and to the most fixed prejudices. Dr. 
 Harvey published his proofs in the year 1619. A person who tries to ima- 
 gine what the science of medicine could have been while it took no account 
 of this fact, on which, as a basis, much of the certain reasoning about the 
 phenomenon of life must rest, is prepared for what old medical books exhibit 
 of the writhings of human reason, in attempts to explain and to form theories, 
 while a fatal error was mixed with every supposition. The chief circum- 
 stances which prevented the earlier discovery of the circulation was, that on 
 examining dead bodies, the arteries were always found empty of blood ; 
 which was the reason, also, of those vessels being called arteries or air- 
 tubes. 
 
 We now know, that as the Thames water spreads over London in pipes. 
 to supply the inhabitants generally, and to answer the particular purposes of 
 brewers, bakers, tanners, and others, and is then in great part returned to 
 where the current sweeps away the impurities, so, nearly in the human 
 body, does the blood spread from the centre, through the arteries, to nourish 
 all the parts, and to supply material of secretion to the liver, the kidneys, 
 the stomach, and other viscera; and returns from these by the veins towards 
 the heart and lungs, to be purified and to have its waste replenished, that it 
 may again renew its course. 
 
 The circulation may be more particularly described thus : From the left 
 
THE CIRCULATION. 415 
 
 chamber or ventricle of the strong muscular mass, the heart, a large tube 
 arises, called the aorta ; and b*y a continued division or ramification, opens 
 a way for the bright scarlet blood to the very minutest part of the living 
 frame, the extreme divisions of twigs being so small that they are called 
 capillary or hair-like tubes. At the termination of these vessels, the blood, 
 after answering the purposes of general nutrition, &c., by which it loses its 
 bright colour, enters the commencement of the venous tree, or returning 
 channel ; and gliding successively from smaller to larger branches, returns 
 towards the right chamber or ventricle of the heart requiring purification and 
 partial renewal." Considering the great arterial and venous systems of the 
 body as twin trees the scarlet and the purple, with corresponding and 
 meeting branches, and with trunks which touch each other at the heart it 
 will appear that they again similarly meet or inosculate by their extreme 
 roots, and thus form a continued or circular channel. The root of the venous 
 tree, by which the blood spreads from the right chamber of the heart through 
 the lungs is called the pulmonary artery, and that of the arterial tree, by 
 which the blood returns to the left chamber, is called the pulmonary vein. 
 Both of these ramify in the spongy masses of the lungs forming a great part 
 of the pulmonary substance. Fresh material for the blood is brought from 
 the digestive organs by the I icteal absorbents and thoracic duct, and is con- 
 stantly pouring into a large vein near the heart, to be completely mixed with 
 the dark or returning blood by a violent agitation or churning during its 
 passage through the heart. The mixture, on leaving the right ventricle, is 
 strained through the minute ramifications, the vessels in the lungs, and at 
 the same time is exposed to the action of the air entering the cells of the 
 lungs in respiration, by which exposure the dark purple blood becomes again 
 pure scarlet, and when it reaches the left chamber or ventricle, is ready to set 
 out on its journey as before, charged with new life and nutriment. The 
 two chambers or ventricles of the heart have each an anti-chamber or auricle, 
 (so called from an external resemblance to a dog's ear,) into which the blood 
 is first received from the veins ', and there are valvular doors between the 
 auricle and ventricle, which allow the blood to pass readily into the ventricle, 
 but oppose its recoil during the ventricular contraction. Similarly acting 
 valves are placed between the ventricles and great arteries. There are valves, 
 also, in many of the veins, over the body, to secure the natural course of 
 the circulation. Besides the important change of purification which the 
 blood undergoes in passing through the lungs, its composition is much 
 influenced by the action of the kidneys, of the exhalent of the skin, and of 
 the liver, the two former relieving it of superfluous moisture and salts, the 
 last from a large quantity of matter in the form of bile. 
 
 The description given above of the circulation of the bloo'd is only an out- 
 line; and yet by showing the manner in which fresh material enters, it con- 
 tains more than Harveyknew of the subject. In this department of knowledge, 
 as in most others, we have advanced from the very general and vague, to the 
 more particular and precise , and just as the general nature of steam was 
 known long before it served in steam-engines, and as the period of the moon's 
 revolutions had been accurately observed for thousand of years before, the 
 fluctuations in her velocity could be calculated so as to make her the mari- 
 ner's best guide in his courses across the ocean, so, when Harvey had proved 
 the general fact of the circulation of the blood, he had left much yet to be 
 done, by observing and raising from subordinate facts, to render the know- 
 ledge available for the many useful purposes which it is calculated to serve. 
 
 Within a few years only, has the importance of the subordinate circum- 
 
416 FLUIDITY IN RELATION TO ANIMALS. 
 
 stances been fully appreciated, as is evinced by the numerous works 
 composed to elucidate them ; but many of which works have served only to 
 prove, that if the difficulties were to be solved by natural philosophy, medical 
 men in general had not yet studied it sufficiently to be able to use it suc- 
 cessfully. In this section it will be attempted to place certain important 
 points of the subject in a clear light : and by referring directly to the general 
 laws of nature, as explained in the body of the work, to settle existing 
 disputes on some of these points, to remove remaining doubts on others, 
 and to suggest some important new applications. 
 
 The fact of the circulation of the blood being once admitted, an inquirer 
 who contemplates the apparatus by which it is effected, is led by the general 
 analogies of nature to conclude 1st. That the ventricle of the heart, at each 
 contraction, empties itself into the great artery, as a forcing-pump at each 
 stroke empties itself into its pipe ; 2d. That the consequent jet causes a 
 wave to pass along the extremities of the arterial tree, (accounted simply 
 elastic,) so as to produce every where, what is called the pulse ; 3d. That 
 the force of the heart acting along the arteries, forces the blood through 
 their open capillary extremities into the commencing veins, and along the 
 veins back to the heart again. Now these assumptions, which Harvey 
 believed completely to describe the circulation, are all nearly true : but are 
 still so far from being either the exact or the whole truth, that they leave 
 important facts unexplained. Thus : 1st. The pulse, instead of being the 
 wave, as slowly progressive as the view above given anticipates, is almost as 
 instantaneous over the whole body as a shock of electricity; 2d. The arteries 
 are all found empty after death, although they have no power of emptying 
 themselves ; and if an artery be tied in the living body, the part beyond the 
 ligature, and cut off, therefore, from the influence of the heart, is equally 
 emptied ; 3d. The rapidity of the blood's passage through the capillaries 
 varies very much, but it does not vary in exact accordance with the changes 
 in the rapidity or force of the heart's action. These and other facts ascer- 
 tained since Harvey's day, not exactly squaring with his views, have rendered 
 further investigation necessary; and it is now additionally known 1st. 
 That the coats of the arteries are not simpl} 7 elastic but actively contractile ; 
 and 2dly. That the capillary vessels can move the blood independent of the 
 heart. In analyzing this subject, it is convenient to follow the blood round 
 from the heart to the heart again, through the three stages of, 1st. The 
 arteries ; 2d. The capillaries ; 3d. The veins. 
 
 Motion of the Wood in the arteries. 
 
 The contractions of the heart inject the blood into the arteries with a force 
 maintaining such a tension in them, that according to the interesting experi- 
 ments of Dr. Hales, recorded in his Statistical Essays, if any artery of a 
 large animal like a horse be made to communicate with an upright tube, the 
 blood will ascend in the tube to a height of about ten feet above the level of 
 the heart and will there continue, rising and falling a few inches with each 
 pulsation of the hear^ Now a column of ten feet, as explained at page 120, 
 indicates a pressure of about four and a half pounds on the square inch of 
 surface : this, therefore, is the force of the heart urging the blood along the 
 arteries into the veins. The opposing tension of the veins is much less, be- 
 cause, as will be explained under the proper head, the blood readily escapes 
 from them into the heart ; Hales found that in a tube communicating with a 
 vein, the blood stood only a few inches higher than the level of the heart. 
 
THE CIRCULATION. 417 
 
 In small animals he ascertained the tension of artery and vein to be less than 
 in large ones } and the ratios deduced for the human body, under ordi- 
 nary circumstances, were an eight feet column, or nearly four pounds per 
 inch for the arteries ; and half a foot column, or a quarter of a pound per 
 inch for the veins. 
 
 Arteries examined after death are found to consist of 1st, an outer coat 
 of strong elastic substance; 2d, a middle coat of circular fibres; and 3d, an 
 invex inner coat of smooth Kniny membrane. Their elasticity or power of 
 resisting change of dimension, and of returning to a middle state from either 
 dilatation or compression, because remaining in the dead artery, was the most 
 obvious property, and was that first attended to. Minute observation of the 
 phenomena of life has since determined the following facts, proving and illus- 
 trating a contractility resident in the fibrous coat. 
 
 1. A small living artery, cut across, soon contracts so as to close its canal 
 and arrest haemorrhage. 
 
 2. While an animal is bleeding to death, the arteries, in accommodating 
 themselves to the decreasing quantity of blood, contract far beyond the de- 
 gree to which their simple elasticity would carry them j and they relax again 
 after death. Dr. Hales took seventeen quarts of blood from a horse before 
 it died, in whose body only three quarts more were found altogether, and yet 
 the moment before death the tension of the arteries sustained a column of 
 two feet of blood in his experimental tube. 
 
 3. The artery of a living animal, if exposed by dissection to the air, some- 
 times will contract in a few minutes to a great degree : and in such a case, 
 only a single fibre of the artery may be affected, narrowing the channel like 
 a thread tied round it. (See Parry on the Pulse.) 
 
 4. When a living artery is tied, the part between the ligature and the 
 nearest branch on the side of the heart gradually contracts, and becomes at 
 last a solid or impervious cord. 
 
 5. Fluctuation in the vital action of parts, is often attended with sudden 
 increase or diminution of caliber in the arteries concerned. 
 
 Although these facts prove indubitably a contractility in the coats of arte- 
 ries distinct from their elasticity, still, because the circular fibres do not 
 resemble common muscles in colour or in chemical composition, or in being 
 immediately obedient to the stimuli of electricity, pricking, great heat, &c., 
 their contractility was by many persons for a long time denied. The dis- 
 pute, however, was often more about the words contractility and muscularity, 
 than about facts. 
 
 The pulse in the arteries, chiefly as regards its almost instantaneous occur- 
 rence over the whole system, in all states of arterial dilatation, and its great 
 strength and sharpness in very small and remote branches, points also to the 
 active contractility of the arterial coats : for, 
 
 1. Were the arterial tree in the living body a system of simply elastic tubes 
 as readily admitting of further dilatation as in the dead body, the first part 
 or trunk would affect the motion of the blood beyond it, nearly as the air- 
 vessel (see page 126) placed at the commencement of artificial arrangements 
 of water-pipes affects the motion of the water in them ; that is to say, as the 
 air-vessel converts the sudden and interrupted jets of water from pumps of 
 fire-engines, town supplying pipes, &c., into a uniform stream with scarcely 
 a remnant of shock, so, in the arterial branches, simple elasticity would 
 cause a more tranquil flow than indicated by the remarkable gushes from a 
 wounded artery, and a quieter beat than that bounding pulse of life felt in the 
 remote artery of the wrist, as sensibly, in proportion, as near the heart itself. 
 
 27 
 
418 FLUIDITY IN RELATION TO ANIMALS. 
 
 2. Were the pulse a wave advancing in tubes that yielded as readily as the 
 dead arteries in their middle states of dilatation, it would be more slowly 
 progressive from the heart to the extremities ; but it is felt so instantly over 
 the whole body, as to be commonly compared to a shock of electricity. 
 
 3. A pulse may be produced artificially in the arteries of a body recently 
 dead by filling them with water to the tension of life, and then injecting at 
 intervals, by a syringe, as much water as the heart throws off blood at a beat; 
 but although the artery is then distended nearly to the limit of its dilatability, 
 and is, therefore, rendered rigid, the beats are weaker than those of the liv- 
 ing pulse. A similar experiment, tried by connecting the artery of a dead 
 animal with the corresponding artery of a living one, has a similar result. 
 
 4. A tube extensively elastic, that it might convey a wave of liquid with 
 a velocity approaching to that of the pulse, would require to be so tense, 
 from fulness, as to be discernible always by the touch, through any imbed- 
 ing medium, such as flesh, like a hard cylinder or cord ; and it would be act- 
 ing constantly as a spring tending to straighten itself, and, therefore, would 
 be stiffening the parts through which it passed. Now the living arteries, 
 between their pulsations, are almost as soft and compressible as the surround- 
 ing flesh, and they offer no perceiveable opposition to bending, in any move- 
 ment of the parts. This may be verified by examination of the lips, for in- 
 stance, or of the fingers; but when a person sits cross-legged, the well-known 
 shaking of the suspended foot, in unison with the pulse, shows the recurring 
 efforts of the artery to straighten itself, during the moments of greater tension. 
 
 5. A bulky wave in elastic vessels would have to recoil from the extremi- 
 ties, or to pass through them as a gush ; and the recoil would be particularly 
 observeaSle near the ligature of the tied artery ; but examination has not 
 detected such effects in the living body. The operation for aneurism, in 
 which the artery is tied beyond the tumour, instead of, as usual on the side 
 next the heart, if it checked a strong wave, would almost certainly produce 
 bursting ; yet Mr. Wardrope and others have lately performed this operation 
 with successful issue. 
 
 6. The wave would be more interrupted by the bandage in the operation 
 of bleeding, than the living pulse is. 
 
 7. The pulse of a paralytic limb often seems more affected, than mere 
 change of size in the artery will account for. The same is true, in an 
 opposite way, of the pulse in an artery leading to an inflamed part. 
 
 8. If the abdomen of a living animal be opened, the mesenteric artery, in 
 all its ramifications, is seen stiffened and raised up suddenly with every 
 pulsation, in a manner which the spreading of newly received blood in a 
 very yielding vessel does not account for. 
 
 9. In the interesting experiments of Bichat, Parry and others, to ascer- 
 tain the exact extent of the supposed dilatation and contraction of arteries, 
 during a pulse, not the slightest degree of either was discernible, even when 
 sought for with microscopes. 
 
 To explain these and other phenomena, then, it seems necessary to admit, 
 as occurring throughout the whole body, and almost simultaneously with the 
 contraction of the heart itself, such an action of the contractile fibres of the 
 arteries, so as to modify the elasticity of the arteries, and to render them rigid 
 enough, in all degrees of dilation, for the heart to produce its effects through 
 them almost as it would through tubes of metal. Dr. Young, in a paper 
 published in the Philosophical Transactions for 1809, and characterized by 
 the usual elegance and precision of his writings, has adduced experiments 
 
THE CIRCULATION. 419 
 
 and calculations, to show that waves in elastic vessels advance more quickly 
 than was before imagined : but the spreading of the pulse seems to be yet 
 more rapid than his calculation anticipates. It is evident, that when arteries, 
 In consequence of depletion, are contracted beyond the middle station of 
 their elasticity, their tension and power of quickly conveying the pulse must 
 be dependent altogether on the condition of their contractile fibres. 
 
 The careful experiments which could detect no change of size in the 
 arteries during the pulse, while they disprove the ancient belief of a con- 
 siderable tumefaction or wave passing along, or of a considerable filling and 
 emptying of arteries, like what occurs in the heart, might also be supposed 
 positively to disprove the occurrence of any general constriction of the vessels 
 on their contents but erroneously : for if a man's arterial system, con- 
 sidered as one cavity, be supposed to contain five pounds of blood (which is 
 near the truth,) and if the vessels be thought to embrace their contents, even 
 between the pulses, with force enough to have all a rounded or cylindrical 
 form, although remaining soft and yielding to the pressure of the finger ; 
 and if we suppose their coats, during the pulse to be thrown into a sudden 
 contraction, as if in obedience to an electrical shock, still, because blood is in- 
 compressible, and because just as much enters the arteries with every pulse 
 as escapes from them before the next, their bulk would not sensibly diminish 
 by the strongest conceivable action of their coats j of which action the only 
 sensible effects would be, that the soft, yielding, aad, in some places, com- 
 pressed tubes would be suddenly converted into hard or resisting cylinders ; 
 and that wherever, by any accidental pressure, an artery had been flattened, 
 it would, in regaining its cylindrical form, strike or pulsate against the com- 
 pressing body. Whether such an action as this contributes to produce the 
 arterial pulse will be considered under the head of " the pulse," after we 
 have seen how the blood moves in the capillaries and veins. 
 
 In any admissible view, however, of arterial agency, we find that the 
 arteries contribute to the circulation of the blood, but as tubes which convey 
 it, 'their own permanent tension, and, therefore, the force with which the 
 blood is pressed into the capillaries, being derived from the heart alone. 
 Even if there be a momentary arterial contraction, such as alluded to above, 
 at the instant of the pulse, it is of too short duration to have an appreciable 
 effect, and probably any effect would be counterbalanced by the same action 
 pervading the capillaries. Many physiologists have had a confused belief 
 that the arteries aided very actively in propelling the blood ; but a little 
 reflection would have shown, that as they have no vermicular or progressive 
 contraction, like the intestines, they no more propel the fluid within them, 
 than any other tubular conduits do. Although they be thus in no degree 
 instrumental in the propulsion of the blood, still, by more permanently 
 enlarging or diminishing their caliber, that is, by merely becoming larger or 
 smaller conduits, they may much influence its local distribution, and the 
 speed of its transmission.* 
 
 * It has long been a subject of dispute whether the arteries exercise any active power in 
 the circulation of the blood, and many ingenious experiments have been instituted to deter- 
 mine the question. Our author, in denying that the arteries are in any degree instrumental 
 in the propulsion of the blood, is not 'borne out by recent investigations. M. Poiseuille, 
 indeed, seems satisfactorily to have proved that the contractility of the arteries does assist 
 in the propulsion of the blood. By a series of well devised, and apparently accurate experi- 
 ments, M. Poiseuille arrived at the unexpected result, that the force of the blood in the 
 arteries will support a column of mercury of the same height with whatever part of the 
 course of the arterial circulation the column is placed in connection whether for example 
 it is connected with the origin of the carotid, or with a branch derived by repeated subdivi- 
 sion from the crural artery : and he, therefore, concludes that the force with which a mole- 
 
420 FLUIDITY IN RELATION TO ANIMALS. 
 
 The nature of this work does not allow us to record historically the vari- 
 ous errors into which even able men have fallen, in attempting to explain 
 the office of the arteries, but we shall glance at the following Dr. Monro 
 and John Hunter, two of the most able physiologists that the world ha$ 
 seen, believed that the arteries did almost as much in propelling the blood 
 as the heart itself. We need not repeat the refutation of this opinion. The 
 ingenious Hichat, again, unable to detect either momentary contraction or 
 dilatation in the arteries, thought that the blood was pushed along them by 
 
 cule of blood moves, is the same throughout the whole arterial circulation. Following out 
 these researches, he has investigated the manner in which the original impulse communi- 
 cated hy the heart, is transmitted unimpared to distant parts of the circulation, notwith- 
 standing the retarding tendency of friction, and the yielding of the parieties of the vessels. 
 His first object here was to ascertain whether the arteries are dilated by the stroke of the 
 heart, and impulse communicated to the blood, and what the amount of the dilatation may 
 be. By many physiologists a very extravagant idea used to be entertained of the^mount 
 of their dilatation; on the other hand, the latter researches of Parry, and other experi- 
 mentalists have assigned exceedingly narrow limits to it; nay, by one eminent physiologist, 
 Bichat, it has been denied altogether. M. Poiseuille has determined the point by means of 
 a very satisfactory set of experiments with an apparatus of his own invention, and has 
 ascertained that dilatation ia affected ; but that it is so small as certainly to be indistin- 
 guishable in an artery by the unaided senses. This apparatus cannot be thoroughly de- 
 scribed without a diagram ; it will be sufficient, therefore, for us to mention, that it is so 
 contrived as to contain about eight inches of the carotid artery of the horse in ^a vessel 
 filled with water and made water tight, except at one point, from which a small horizontal 
 glass-tube issues, about an eighth of an inch in diameter. At each contraction ofjthe 
 animal's heart it was found that the water in the small tube advanced two inches and 
 eight-tenths, and that it retired to its former place during the diastole of the heart. The 
 diameter of the artery was seven-twentieths of an inch. Hence it may be calculated that 
 at each pulsation its capacity was increased about a thirtieth part. 
 
 Having ascertained this fact, M. Poiseuille goes on to inquire, whether the power which 
 is expended by the blood in causing this dilatation is restored by the subsequent contrac- 
 tion of the artery. For this purpose a portion of the common carotid artery of the horse, 
 ten inches long, and seven-twentieths of an inch in diameter, taken immediately after death, 
 was connected with the end a, of the tube, (see Figure ;) a stop-cock, however, being pre- 
 viously fitted between aand b. The other end of the artery was fixed on a tube of analogous 
 construction, different in fact only in so far as the limb c d was inclined at about half a right 
 angle instead of being vertical, and the stop- cock was placed near the end d. The whole of 
 the first tube, the artery and part of the descending limb b c of the second tube was filled 
 with water, a little mercury then filled the curvature of the second tube, and the ascending 
 inclined limb of that tube above the mercury was filled with water. The stop-cock of the 
 last tube being closed, and that on the first tube being opened, mercury 
 Fig. 174. was poured into the former by its end d, till the pressure on the artery 
 
 amounted to ninety five millimetres or about 3.8 inches. The stop- 
 "t cock of the first tube was then closed and th;t on the second tube was 
 opened ; upon which the water rose insta.neously in the latter, a 
 portion flowed out at the top, and theremainder then sank a little, and 
 assumed a fixed level. On making the necessary computations, M. 
 Poiseuille found that the point to which the mercury was raised in the 
 second tube at the moment of the contraction of the artery indicated 
 an elevation of one hundred and ten millimetres or 4.4 inches. Hence 
 the power with which the arterial coatu contract upon themselves after 
 being dilated, exceeds that which is expended in dilating them. In the 
 present experiment the excess was equivalent to six-tenths of an inch 
 of mercury, or three nineteenths more than the dilating force. In three 
 other experiments, the excess of the column of mercury was 9.20, 14. 
 20, 19.20 of an inch. When repeated with the artery of an animal 
 which had been killed four days before, the excess was less than 4.20 
 It is evident, therefore, that whatever force the blood after issuing from 
 the heart loses in consequence of its acting on yielding vessels, is com- 
 pletly restored by the elastic contraction of their parietes. The me- 
 moirs of M. Poiseuille will be found in the Eepertoir General d'Ana- 
 tomie et de Physiologie, Tom, VII. 
 
 Dr. Badham in an interesting paper in the London Medical Gazette (vol. viii. p. 549) 
 has adduced some strong evidences of the existence of independent arterial action, and 
 gives a sketch of various pathological phenomena, which appears explicable on such an 
 admission, and inexplicable without it. Am. Ed. 
 
THE CIRCULATION. 421 
 
 the heart instantly through their whole extent, as a solid rod of metal or 
 wood is advanced by an -impulse at one end. Dr. Parry took nearly the 
 same view of the subject, and illustrated his idea by referring to the experi- 
 ment of moving a whole line of billiard-balls by striking the extreme one. 
 Both these authors erred by neglecting the hydrostatical truth, that pressure 
 in a fluid operates equally in all directions, and, therefore, that fluid pressed 
 into a tube tends to dilate the tube, just as powerfully as to drive the fluid 
 forward ; and they did not, advert to the fact that the progress of the blood in 
 the small arteries is not by waves or successive jets, but is nearly a uniform 
 stream. The blood could only advance, as they supposed, by the arteries 
 becoming, for an instant, absolutely rigid, in consequence of a strong action 
 of their contractile fibres. 
 
 It merits notice here, although not strictly a mechanical fact, that arteries 
 permanently increase or diminish in size when a permanent change takes 
 place in the demand for their service. The arteries of the gravid uterus, or 
 of an increasing tumour, grow with the part supplied, while on the contrary, 
 those of the stump left after amputation soon remarkably diminish. If the 
 chief artery of a limb be obliterated by any cause, as after the operation for 
 aneurism, the small collateral anastomozing branches increase in size to do 
 its duty. 
 
 It is farther remarkable, that when arteries are called upon to carry an 
 increased quantity of blood, they often become tortuous or serpentine, as 
 well as larger; and that arteries leading to parts whose actions are naturally 
 intermitting, or fluctuating, have generally the tortuous form. Of these 
 truths, the arteries leading to rapidly growing tumours, or to varicose 
 aneurisms, and the arteries of the uterus and testes, may serve as instances. 
 This bending of arteries, and the very curious divisions into many branches 
 which again re-unite, found in those leading to the brains of some animals, 
 do not seem intended to slacken the rapidity of the sanguineous current, 
 but to give the artery a greater control over the supply. 
 
 / 
 
 Passage of the blood through the capillaries. 
 
 We have seen that the heart keeps up a tension or pressure in the arteries 
 of about four pounds on the square inch of their surface j and with this force, 
 therefore, is propelling the blood into the capillaries. If these last were pas- 
 sive tubes, constantly open, such force would be sufficient to press the blood 
 through them with a certain uniform velocity; but they are vessels of great 
 and varying activity ; it is among them that the nutrition of the different tex- 
 tures of the body takes place, as of muscle, bone, membrane, &c. ; and that 
 all the secretions from the blood are performed, as of bile, gastric juice, or 
 saliva, &c. ; and to perform such varied and often fluctuating offices, they 
 require to be able to control, in all ways, the motion of the blood passing 
 through them. The capillaries of the cheek, under the influence of shame, 
 dilate instantly, and admit more blood producing what is called a blush ; 
 under the influence of anger or fear, they suddenly empty themselves, and 
 the countenance becomes pallid tears or saliva, under certain circumstances, 
 gush in a moment, and in a moment again are arrested if a person having 
 inflammation in one hand be blooded from corresponding veins in both arms 
 at the same time, twice or thrice as much blood will flow from the diseased 
 side as from the other. Similar changes occur in many other instances. 
 Now the only action of cylindrical vessels, capable of causing these pheno- 
 mena, is contraction and dilatation of their coats ; and with reference to such 
 
422 FLUIDITY IN RELATION TO ANIMALS. 
 
 caction it merits notice, that arterial branches have more of the fibrous or 
 contractile coat in proportion as they are smaller.- 
 
 A muscular capillary tube strong enough to shut itself against the arterial 
 current from the heart, is also strong enough to propel the blood to the heart 
 again through the veins, even if the resistance on the side of the veins were 
 as great as the force on the side of the arteries. For if we suppose the first 
 circular fibre of the tube to close itself completely, it would, of course, be 
 exerting the same repellent force on both sides, or as regarded both the 
 artery and vein. If, then, the series of such fibres forming the tube were to 
 contract in succession towards the vein, as the fibres of the intestinal canal 
 contract in propelling the contents of that canal, it is evident that all the 
 blood in the capillary would thereby be pressed into the vein towards the 
 heart. If after this the capillary again relaxed on the side of the artery, so 
 as to admit more blood, and again contracted towards the vein as before, it 
 would produce a forward motion of the blood, first towards the vein, and then 
 in it independently of the heart. As capillary action, however, is not visi- 
 ble, its nature has not yet been positively ascertained : some persons have 
 deemed it electrical. 
 
 It is capillary action which absorbs and moves the fluids of the classes of 
 animals which have no heart. It must also be the power which moves the 
 blood in warm-blooded monsters formed without hearts. There are cases of 
 apparent death among human beings, where the heart remains inactive for 
 days and yet a degree of circulation sufficient to preserve life is carried on 
 by the capillaries. In illustration of capillary action, we have also the absorp- 
 tion, by the lacteals, of nutriment from the alimentary canal ; and, perhaps, 
 to a certain extent, the circulation of blood in the livers of animals. In 
 this last case, the blood collected by veins from the abdominal viscera, instead 
 of going directly to the heart, is again distributed through the liver by the 
 branches of the vena portae, and is then again collected by the ordinary veins 
 of the liver, and carried to the heart : it thus moves through two sets of 
 capillaries in passing from the arteries to the heart again. 
 
 The action of the capillaries is the cause of that singular fact which pre- 
 vented the ancients from discovering the circulation of the blood, viz., the 
 empty state of the arteries after death. All the muscular parts of an animal, 
 including, therefore, the contractile coats of vessels, retain their life, or power 
 of contracting, for a considerable time after respiration has ceased as is 
 seen in the recovery of persons apparently drowned or suffocated : in the 
 leaping of a heart taken from an animal recently killed; in the actions resem- 
 bling life which can be produced, by the agency of galvanism, in a body 
 recently dead : but the fact is seen still more amply for our purpose, in the 
 total disappearance of a local inflammation after the death of the patient, 
 for inflammation involves a gorging or over-tension of the capillaries, into 
 which, when the heart has ceased to press blood, the contractile force remain- 
 ing in them, even under disease and in a dead animal, is sufficient to squeeze 
 the blood out of them, and often to remove all trace of the malady which has 
 been fatal. In ordinary cases, then, the capillaries throughout the body re- 
 main alive and active for a considerable time after breathing has ceased, work- 
 ing like innumerable little pumps, and empyting the arteries into the veins. 
 As the red blood is their proper sustenance as well as stimulus, they work as 
 long as there is any of it coming into them from the arteries behind ; except, 
 however, the capillaries of the lungs, which soon cease to act, because, after 
 breathing has ceased, they receive only black blood, and are moreover com- 
 pressed by the collapse of the chest; and all the blood accumulates behind 
 
TflE CIRCULATION. 423 
 
 them. The capillaries may continue to be filled from the arteries, either in 
 consequence of their elasticity opening them with what is called a suction 
 power, or of an absorbant powor depending on life, like that of the lacteals 
 and of the absorbents all over the body, and, perhaps, of the vessels in the 
 roots of vegetables. When death is produced by lightning, or by the poisons 
 which destroy all muscular irritability, and therefore that of capillaries, the ar- 
 teries after death are found to contain blood like the veins. In a living body, 
 if an artery be tied, the part beyond the ligature is soon emptied into the veins, 
 and becomes flat. The experiment has be made even upon the aorta itself. 
 The empty state of the arteries after death has been ascribed, by some 
 teachers, to the momentum with which they supposed the blood to be thrown 
 out from the heart in its last contraction sufficient, said they, to squirt it fairly 
 through the most distant capillaries; a doctrine exemplifying the carelessness 
 with which men sometimes receive and repeat opinions, to which their atten- 
 tion has never been fully awakened. Such an effect would not follow, even 
 if the action of a dying heart were the strongest possible ; while, in reality, 
 it is in most cases so feeble, that the pulse for sometime ceases to be per- 
 ceptible at the extremities, and the diminished circulation lets them become 
 cold. Other physiologists have taught that an artery is capable of contract- 
 ing directly upon its contents, so as to expel even the last drop ; but large 
 arteries, when emptying, do not contract roundly like an intestine ; they 
 become flat like elastic tubes of leather sucked empty, and no contractile 
 action of the vessel itself could bring its sides together in such a manner. 
 If arteries emptied themselves by their own action, the pulmonary artery 
 should be more certainly empty than the aorta, because it is shorter; yet it 
 is always full ; for the reason already stated, that the pulmonary capillaries 
 cease to act after respiration has ceased, on account of the blood in them 
 being venous or .dark blood, and therefore not life-supporting or stimulant to 
 them. 
 
 Passage of Hood through the veins. 
 
 The veins have much thinner coats than the arteries, and if taken altogether 
 have much greater capacity : for besides being larger than the corresponding 
 arteries, they, exist in many situations, as double sets, an exterior and an 
 interior : they have also very frequent inosculations or communications with 
 each other throughout their whole course. 
 
 The simple weight of the column of blood in any descending artery is just 
 sufficient to raise the blood through open capillaries to an equal height in 
 the corresponding vein, according to the hydrostatical law, that fluids attain the 
 same level in all communicating vessels ; and therefore, as the arch of the 
 aorta rises considerably above the heart, the gravitating pressure of the de- 
 scending arterial column of blood would be sufficient to lift that in the veins 
 not only up to the heart, but considerably beyond it. In addition to this in- 
 fluence of gravity on the venous current, the blood is pressed into the arteries, 
 and from them, therefore, towards the veins, with a force from the heart itself, 
 as stated above, of about four pounds to the square inch, or, in other words, 
 as if there were a column of blood eight feet higher than the heart urging the 
 current. It might be expected from the law of equal diffusion of pressure in 
 fluids, that these causes would soon produce a tension in the veins as great 
 as in the arteries : and this does not happen, only because the blood has a 
 ready escape from the veins through the right ventricle of the heart. Under 
 ordinary circumstances, there can be no greater tension in the veins than what 
 is sufficient to lift the blood to the heart and to overcome the friction ; as in 
 
424 FLUIDITY IN RELATION TO ANIMALS. 
 
 an upright leathern tube, open at top, and receiving water at its bottom from 
 a powerful forcing pump, there never can be a greater tension or pressure than 
 what corresponds to the height of the fluid column in the tube, and to the 
 friction between the fluid and tube. In Dr. Hale's experiments, already 
 alluded to, a tube connected with a vein so as to receive its blood, became 
 filled with blood to a height only of about six inches above the level of the 
 heart. As Dr. H. generally cut the vein completely across, and inserted the 
 tube into the portion leading from the capillaries, he would have discovered 
 the whole power with which the blood is pushed along the veins from the 
 capillaries, but for the free lateral communication of veins with each other, 
 which reduces the tension even in an obstruct branch, to the degree exist- 
 ing in the system generally. When, from agitation of the animal, or any 
 straining exertion, the passage of the blood into the heart was impeded, all 
 the veins became tense, and a tube inserted into the returning jugular had 
 blood running over, at a height of three feet above the heart. 
 
 If the blood did not escape from the veins, as above described, the only 
 cause which could prevent the venous tension from becoming as great as the 
 arterial, would be obstruction in the connecting capillaries : but the following 
 facts and considerations prove that these vessels, which, in the dead body, 
 allow the passage of injections, in the living body freely allow the passage 
 of blood. 1st. Magendie laid bare the chief artery and vein of a living limb, 
 and at the part, detached them from the flesh underneath, so that he could 
 apply a tight bandage round the limb without including them, and could thus 
 render them the only channels of circulation for the limb beyond the band- 
 age. He then found, that when a separate ligature was put upon the vein, 
 to prevent the return of its blood to the heart, and a puncture was made 
 beyond the ligature, the flux of blood from the puncture was rapid or slow 
 according as the heart was allowed to produce a greater or less degree of 
 tension in the artery : this tension was regulated by his compressing the 
 artery between the fingers. 2d. After a similar preparation of the parts, the 
 blood will ascend in a tube from the obstructed vein very nearly as high as 
 from the artery 3d. In the common operation of bleeding at the moment 
 of puncturing the vein, the blood often jets from it as from an artery, stain- 
 ing even the top of the bedstead. 4th. The microscope discovers in the 
 capillaries, a uniform forward motion of the blood, as if it were obeying the 
 steady pressure of the arterial tension, and not any intermitting action. 5th. 
 Disturbed action of the heart, by obstructing the passage of the blood through 
 it, is very soon attended with a tumefaction of all the veins leading to the 
 heart : the tumefaction becomes very visible about the neck and head, and 
 in the liver produces swelling and acute pain. 6th. Dr. Young, from experi- 
 ments made by him, and reported in the philosophical transactions for 1809, 
 concluded that perfectly open capillaries, of the size existing in the living 
 body, should just retard a flow of blood urged by the usual arterial tension, 
 in the degree which really occurs : a correspondence proving that they must 
 be open; and open vessels, however small, and how slowly soever they 
 transmit the blood, still, if the escape of blood from the veins were arrested, 
 would transmit the arterial tension without diminution. 7th. The fact that 
 after death the capillaries empty the arteries into the veins, proves that, under 
 certain circumstances, the venous tension may become even greater than the 
 arterial. These facts then and others that might be mentioned, prove incon- 
 testably, that the blood is pressed into the veins from the arteries and capilla- 
 ries, with force sufficient to lift it, not only to the heart again, but many feet 
 farther, viz., about as far as it would ascend in a tube rising from the tense 
 
THE CIRCULATIO.N. 425 
 
 arteries themselves. So little, however, has this important truth been under- 
 stood, that in elementary works of authority lately published, the venous 
 current is treated of as a very obscure subject; and some authors, in their 
 anxiety to explain it,, have assigned causes for it, which, as will appear here- 
 after, are positive absurdities in physics. The difficulty in the question 
 seems to have arisen from the great disparity observed between the tension 
 in the arteries and in the veins, while the reflection did not occurr, that the 
 disparity was owing to there being a free passage or outlet from the veins 
 through the heart. 
 
 The illustrious Bichat, with an inattention to facts, extraordinary in him, 
 persuaded himself that the influence of the heart ceased entirely at the capil- 
 laries and that the blood was returned through the veins by the action of the 
 capillaries, alone. How could he avoid the single reflection, that, if the pur- 
 pose of the arteries had been merely to convey the blood to the capillaries, 
 and not also to bear the force which pressed it into and through them, the 
 extraordinary strength of the arterial coats, and the great power of the heart 
 to fill them and keep up the tension described, would have been quite super- 
 fluous ? and he knew that nature does nothing in vain.* The reflection 
 applies strikingly to the pulmonary artery, of which no branch exceeds a 
 few inches in length. 
 
 The uniform current of blood along the veins, so apparent in the operation 
 of bleeding, and produced, as now explained, by the combined influence of 
 the heart and capillaries, suffers a considerable disturbance in the neigh- 
 bourhood of the heart from three causes. 1st. As there is no valve between 
 the veins and the auricles of the heart, each contraction of the right auricle 
 tends to throw the blood back into the veins, as well as forward into the 
 ventricle, and thus produces the venous pulse often felt in the neighbour- 
 hood of the chest. 2d. When the chest is expanded by inspiration, it is 
 more roomy than during the collapse of expiration, and the blood then enters 
 it more readily. 3d. While the chest is inhaling or drawing in air, that is 
 to say, expanding so as to diminish the tension or pressure of the air within 
 it, (see Pneumatics^) it is by the same action favouring the entrance of blood 
 through the veins towards the heart placed in it ; on the contrary, while it 
 is exhaling or throwing out air, it is, with equal force, resisting the entrance 
 of blood, and slackening, or even causing recoil of the inward current. This 
 favouring or resisting force, however, as will be hereafter shown, is only 
 such as to lift or support a column of blood of about half an inch in height. 
 It appears, then, that the entrance of blood into the chest fluctuates by rea- 
 son of the respiration, &c., as the entrance of a river stream into the sea 
 fluctuates by reason of the ebbing and flowing of the tide. An eye watch- 
 ing the jugular vein, under favourable circumstances, may see it tense or 
 slack in accordance with the opening and shutting of the chest. 
 
 It still remains to be ascertained whether or not veins have in themselves 
 any contractile power, such as can partially empty a lower portion into a 
 higher portion beyond an adjoining valve. If so, the valve by then bearing 
 the pressure would let more blood be easily raised from below into the por- 
 tion so relieved : and the action, without being equal to the office of com- 
 pletely emptying any portion of a vein, would still have the effect of dividing 
 
 * This incorrect and inconclusive mode of reasoning is so common that we may be per- 
 mitted to protest against it. The influence of the heart may cease with the capillaries, 
 and yet. nature has done nothing in vain. Before we would be justified in making such a 
 charge against nature we must possess an infinitely more precise knowledge of the circula- 
 tory forces and of the functions of the arterial system than we do at present. Am. Ed. 
 
426 FLUIDITY IN RELATION TO ANIMALS. 
 
 a long heavy column into a number of short columns of comparatively little 
 resistance. It is certain, at least, that the valves in the veins, by preventing 
 the falling back of blood which has once passed towards the heart, must 
 affect its flow during bodily exercise; for every time that pressure is made 
 on a vein by a swelling muscle or otherwise, the blood in the part must be 
 forced forward, and cannot return. 
 
 The veins which are surrounded by muscles are thinner anc^weaker than 
 those supported only by the skin. The external veins of the legs are 
 almost as strong as arteries. Proving, however, that the fabric of veins is 
 much weaker than that of arteries, any vein in the living body, made to 
 communicate directly with an artery, soon exhibits what is called a varicose 
 aneurism, and swells to bursting. Veins possess power, to a great extent, 
 of adapting themselves to the varying quantity of blood. 
 
 Some recent authors, as stated above, either not aware of the facts which 
 prove that -the blood is everywhere pressed into the veins with force much 
 more than sufficient to raise it to the heart again ; or, being unable, from 
 their little familiarity with mechanical science, to draw exact- conclusions 
 from the facts, or to avoid errors in their own hypotheses, have promulgated 
 the opinion that the progression of the blood in the veins is greatly owing 
 to a partial vacuum or a suction power in the heart or chest ; that is to say, 
 to the atmospheric pressure remaining constant on the body generally, while 
 it is, at intervals, lessened about the heart. Now the whole influence of 
 this effect or circumstance, as stated above, is merely a slight disturbance of 
 the uniformity of the venous current near the chest. Such a doctrine could 
 not be proposed or entertained for a moment by a person understanding the 
 principle of a common household pump; and its having been published, and 
 tolerated by certain professional men in the present time, will remain a proof 
 to posterity of the deficiency, as regards fundamental science or natural 
 philosophy, now existing in the ordinary medical education. Much ingenuity 
 has been wasted upon it, particularly by Drs. Carson and Barry, the latter 
 of whom, after making laborious experimental investigations on living ani- 
 mals, has even attempted to build upon it a superstructure of medical theory 
 and practice ! To say that the influence of the heart or chest is the power 
 which draws the blood to the heart from the general system, is as if one 
 asserted that the rising and falling of the tide at the mouth of a river is the 
 power which collects the tributary streams in the interior country. 
 
 We shall enter into a little detail on this subject, because the discussion 
 will elucidate some minor points connected with the circulation. 
 
 Presuming, then, that the reader perfectly understands the theory of 
 pumps, and therefore of atmospheric pressure, as explained under Pneu- 
 matics, he will readily understand the two following propositions, either of 
 which proves it to be a physical impossibility, that a sucking action of the 
 heart or chest can be a cause of the blood's motion along the veins. 1st. 
 The veins are pliant tubes free to collapse, and no pump can lift liquid 
 through such. 2d. The suction-power of the chest in healthy respiration is 
 too weak to lift liquid even one inch through tubes of any kind. 
 
 A practical illustration of the first proposition is afforded by putting the 
 point of a syringe into a piece of gut, or eel-skin, or vein filled with water, 
 and then trying to pump up the water. The result will be, that the fluid 
 close to the mouth of the syringe will enter it, and then the sides of the 
 pliant tube will collapse against the syringe, making an end of the experi- 
 ment. In exact proportion to the rigidity of the tube will be the distance to 
 which the influence of the syringe, will extend in it; if, for instance, half an 
 
THE CIRCULATION. 42T 
 
 ounce of pressure on the square inch of its surface be required to make it 
 collapse, then the pump will draw up one inch of water, and so for other 
 proportions. If, during the action of the syringe, the tube were allowed to 
 open freely at the bottom into a vessel of water, instead of the syringe then 
 drawing any more water from the vessel into the tube, the original contents 
 of the tube would straightway be discharged downwards into the vessel. 
 
 The explanation of all these facts is found in the pressure of the atmos- 
 phere, (see from page 158 to page 158) seeking entrance everywhere at the 
 surface of the earth, with a force of fifteen pounds per square inch, and 
 overcoming any opposing force less than this ; a pressure which is suffi- 
 cient, therefore, to push a column of water thirty-four feet in height, up 
 through a rigid tube into the vacuum of a pump, but will cause the sides of 
 the tubes to collapse, unless able to sustain its force. When nature intends 
 a tube to resist any degree of suction, the tube is made rigid in proportion ; 
 witness the wind-pine and its branches, which are the only instances in 
 the human body. jSid if tubes prepared for sucking light air only have 
 received such rigidity, how much stronger would tubes have been made for 
 sucking blood. 
 
 Some bad reasoners on this subject have believed, that if a suction power 
 exist, capable of lifting one inch of a column of liquid, any column, however 
 long, must follow the first inch when acted upon by the power ] for, say 
 they, the atmospheric pressure, by preventing a vacuum, will prevent sepa- 
 ration of the liquid. Now, in the first place, this reasoning is quite inap- 
 plicable to pliant tubes, because the ready collapse of their sides will both 
 allow the separation and prevent the vacuum ; and, in the second place, 
 with respect to rigid tubes, it is equivalent to asserting that a force just 
 capable of lifting one link of a chain, must, therefore, be able to lift any 
 number of connected links. Water, in a rigid tube, to which the air has 
 no admittance, may truly be considered as a chain, for it is held together by 
 a force of fifteen pounds per inch, pressing inwards at the two ends ; and by 
 force inferior to this, cannot lift one portion of it away from another, and, 
 therefore, cannot draw out a drop but by lifting the whole. A man cannot 
 suck any water from a rigid tube which is closed at the bottom ; and if the 
 bottom be open, and he has not power to support the whole contained fluid, 
 it will sink from his tantalized lips to stand at an elevation corresponding 
 to his suction power. 
 
 To illustrate the second proposition respecting the trifling suction power 
 really residing in the chest, we may state that a person of ordinary strength 
 using the whole power of the chest, (but not of the mouth separated, which 
 is a smaller and much more powerful pump than the chest,) cannot, through 
 a rigid tube, suck water from more than about two feet below his lips, and 
 therefore not half way so far as from the extremities of his body ; while, 
 in the opposite action of blowing outwards, as in the attempt to blow 
 through a tube which is dipping into water, he finds nearly the same limit. 
 But in ordinary breathing, instead of force corresponding to a liquid column 
 of two feet, or & fifteenth of the atmospheric pressure, the increase and di- 
 minution of air-density in the chest are measured by a column of less than 
 one inch or about a five-hundredth of the atmospheric pressure. This fact 
 is easily shown by breathing through the nose, while holding in the mouth 
 one end of a glass tube, the other end of which is immersed in water, and 
 then noting how much the water in the tube rises above the surrounding 
 level during inspiration, and sinks below it during expiration. The mouth, 
 during this experiment, may be considered as a part of the general cavity of 
 
428 FLUIDITY IN RELATION TO ANIMALS. 
 
 the chest, to and from which air is passing by the narrow openings of the 
 nostrils. In tranquil breathing, with both nostrils open, the fluctuation in 
 the tube is less than half an inch each way ; with on.e nostril closed and the 
 other a little compressed, it may amount to a whole inch ; and with hurried 
 or convulsive breathing, like that of an animal in terror and in pain, it may 
 exceed twelve inches. Although the measures thus obtained from the 
 mouth are somewhat too small for the changes in the chest itself, because 
 the chest is more remote from the opening by which the external air enters, 
 the difference is very trifling, as may be proved during such experiments, by 
 stopping the nostrils altogether, while the same respiratory efforts are con- 
 tinued ; and as is also proved by the agreement of the results witlf strict 
 calculation founded on the inertia and velocity of the air respired a calcu- 
 lation similar to that required in adjusting the index to the machine men- 
 tioned at page 215, for measuring water-currents. In common healthy 
 breathing, then, while the mouth is open, the fluctuation of pressure in the 
 chest would be measured by less than half an inch ucrotion each way of the 
 liquid column. Dr. Barry, not aware that this point could be so easily 
 determined by the bloodless experiment described above, or even by a simple 
 calculation, sought the solution by numerous trials on living animals, into 
 some part of whose chest he forced a tube. But even if farther experiments 
 had been at all necessary, those of Dr. B. could not have decided the ques- 
 tion, first, because the pain and agitation of the dying animals rendered the 
 breathing violent or unnatural; and, secondly, because his experimental 
 tube often or always became a syphon, and he, not adverting to this fact, 
 has not recorded the difference of level in the liquids at the two ends. That, 
 the external level was, for the most part, higher than the internal, is proved 
 by his having noticed, almost solely, the inhaling action of the chest, although 
 the exhaling is often a more powerful effort. 
 
 Calling an inch column of blood, then, the measure of the greatest suges- 
 cent and repellent powers of the chest during ordinary respiration, we see 
 that the force which really sends the blood from below to the heart, may 
 have to lift a column one inch shorter during inspiration, and one inch 
 longer during expiration : but this is the full and true measure and nature 
 of the influence of the inspiration on the blood's return to the heart. To 
 say, then, that the atmospheric pressure, modified by respiration, is the 
 great power which moves the venous blood, is just as if we said, that a boy, 
 standing near the fly-wheel of a steam engine of a hundred horses' power, 
 and giving it his Lilliputian thrust, alternately backward and forward, were 
 the prime mover of the machinery. 
 
 The truth explained above, that no kind of pump can lift fluid through 
 pliant tubes, free to collapse, like the veins, renders it unnecessary farther to 
 speak here of the pumping action of the heart itself, insisted on by Dr. Car- 
 son, or of that other action, mentioned in a subsequent part of this work, to 
 which, also, he attributes great influence, viz., the tendency towards a vacuum 
 external to the lungs and around the heart, produced by the disposition of 
 the lungs to collapse. It maybe remarked, however, that this last influence 
 is more considerable than the simple inspiratory action dwelt on by Dr. 
 Barry, and that it operates during expiration nearly as much as during 
 inspiration, varying in force with the degrees of expansion of the chest. It 
 is weaker in the living than in the dead body, because the rigidity of the 
 distended pulmonary arteries helps to support the weight of the living lungs. 
 
 Were it necessary to give proofs to persons unable to follow the above 
 argument, that a suction-power in the heart or chest is not the force which 
 
THE CIRCULATION. 429 
 
 draws the blood from the extreme veins, reference might be made to many 
 notorious facts quite incompatible with that supposition ; such for instance, 
 as those recorded at page 424, and others. A vein tied, fills tensely below 
 the ligature a vein cut across bleeds from the orifice which is distant from 
 the heart, and will fill a lofty tube connected with it the circulation goes on 
 in persons holding their breath the veins of fishes, which do not breathe, 
 return the blood as well as those of men, &c., &c.* 
 
 After the explanations now given, it is almost superfluous to remark that 
 absorption in animals cannot depend on atmospheric pressure, and that the 
 effect of cupping-glasses applied to extract blood, or to prevent the absorption 
 of poison in wounds, in no way depends upon the fluctuating density of the 
 air in the chest. ( Dr. Barry's reasonings upon these subjects involve the 
 same fallacies as his reasonings on the venous current. With respect to 
 absorption, they neglect the facts of fluids having weight; and with respect 
 to cupping-glasses, of which the true action is explained at page 175, they 
 are equivalent to asserting that the action of pumps drawing water from a 
 river among the hills is influenced by tides, or pumps operating at its mouth 
 in the sea. 
 
 If the fluids in animal vessels had no weight, it is true, that in absorption 
 at external atmospheric pressures of fifteen pounds per inch might force new 
 matter into a receiving orifice, at the instant during inspiration, when the 
 opposing pressure in the chest, at the other ends of the vessels, were half an 
 ounce per inch less, there would be no physical absurdity in opposing this, 
 although there are physiological facts that disprove it but when we reflect, 
 that in all vessels under the level of the heart, the weight of the contained 
 fluids causes an additional outward pressure of about half an ounce troy for 
 every perpendicular inch of fluid column, making an excess of outward pres- 
 sure at the toes, for instance, even at the more favourable time of absorption, 
 
 * The influence of inspiration of the cavity in the chest exterior of the lungs, and the ex- 
 pansive power of the heart, on the circulation of the blood in the veins, have no doubt been 
 greatly overestimated by Drs. Barry, Carson and others, but ur author appears to us to 
 have undervalued their eflfect. Their joint power is more considerable than the reader 
 might be led to suppose from the perusal of the preceding pages. 
 
 The influence of inspiration has been estimated by our author, perhaps justly, as only 
 sufficient to raise a column one inch ; if this force acted through rigid tubes of the length 
 of the veins, it would produce no movement of the contained fluid; but acting through pliant 
 tubes, it would rise one inch of the blood out of the vein nearest the heart, and if this power 
 acted alone, its effect would here cease. But the vis a tergo, produced by the propulsive 
 power of the capillaries, and, perhaps, also of fbe heart, prevents the collapse; the vein is 
 kept full, and at every inspiration this power is renewed. 
 
 The influence of the tendency towards a vacuum external to the lungs, and around the 
 heart, from the contractile disposition of the resilience of the lungs, is admitted by our 
 author to be more considerable than the int-piratory effort, and it in fact is, we think, greater 
 than is suspected. There are reasons for believing that the lungs do not entirely fill the 
 cavity in which they are contained; the influence of this space is therefore constant, though 
 greater during inspiration, and of course diminished during expiration. 
 
 The capillaries, our author has most satisfactorily shown, have a vital expansive power; 
 and though he does not assert that the heart has no such power, he denies that it can have 
 any influence on the movement of the venous blood, since it must act through pliant tubes. 
 This would be the fact if the expansion of the heart were the only moving power, but the 
 vis a tergo prevents their collapse, and the effect of the expansive power of the heart, 
 whatever that may be, is allowed to act. 
 
 While, therefore, the action of the capillaries and perhaps of the left ventricle of the 
 heart must be considered as the main forces by which the blood is propelled through the 
 veins, the expansive power of the heart respiration and the resilience of the lungs, or 
 atmospheric pressure ought to be viewed as necessary forces, though their precise power 
 cannot readily be estimated. Am. Ed. 
 
 f The effect of cupping-glasses in preventing the absorption of poisons has been shown 
 by Dr. Pennock to be owing to mechanical pressure. See the interesting experiments in 
 the American Journal of the Medical Sciences, Vol. II. Am. Ed. 
 
430 FLUIDITY IN RELATION TO ANIMALS. 
 
 of about two pounds per inch, we see that absorption must be a strong action 
 of life, able to overcome a great excess of mechanical resistance, instead of 
 a passive phenomenon obeying an excess of mechanical force. If a mere 
 balance of pressures acted at the orifices, as Dr. B. supposes, the blood and 
 other fluids would be constantly oozing out from all orifices below the heart, 
 as blood really does from an artificial opening, with force that would fill a 
 tube reaching as high as the heart. It would be good news for proprietors 
 of mines, and other persons having to raise water, if by taking off an ounce 
 or two per inch of the atmospheric pressure at the top of a full pipe, the atmo- 
 spheric pressure continuing elsewhere would then force water in at openings 
 below and cause the upward current: but in truth to make the atmosphere 
 efficient below, powerful steam-engines or other means must be used to take 
 off a pressure above, of at least half an ounce per square inch, for every inch 
 in herght the water has to rise. 
 
 Another erroneous conception of atmospheric pressure, akin to that which 
 we have been considering, is expressed in the following reasoning on the 
 progress of blood in the veins. " The atmosphere presses 15 Ibs. per square 
 inch on all thing's ; the blood therefore, in a vein which has 20 inches of sur- 
 face is pressed upon through the flesh, with a force of 20 times 15, or 300 
 Ibs., while a cross section of the vein near the heart would measure less than 
 one inch. The blood, therefore, is always running towards the heart, to 
 escape from a powerful excess of atmospheric pressure." This paradox is 
 solved by the law of fluid pressure, explained at page 131. The same rea- 
 soning would prove that an eel-skin suspended by its lip, and filled with water, 
 when exposed to the pressure of the atmosphere, should quickly be emptied; 
 and nearly the same would prove that a long sharp wedge thrown into water, 
 should be always moving in a direction away from its point; and that a ship 
 formed like the wedge, should make quick speed across the sea without either 
 oar or sail. 
 
 A knowledge of thefacte detailed under the three heads of arteries, capillaries 
 and veins, prepares us for the discussion of the following subjects. 
 
 The force of the heart. 
 
 The arterial tension of four pounds to the square inch, marked by its sup- 
 porting in a tube connected with the arteries, a column of blood eight feet 
 high, (see .page 417,) is produced by the action of the heart ; but as the 
 heart, while injecting the blood against this resistance, has moreover to over- 
 come the inertia both of the quantity injected and of the mass in the great 
 artery, first moved by the injection, as also the resisting elasticity of the 
 vessel which yields to momentary increase of pressure, the heart must act 
 with a force exceeding four pounds on the inch. And as the left ventricle 
 of the human heart, when distended, has about ten square inches of internal 
 surface, the whole force exerted by it is a matter of simple calculation. It 
 is remarkable, as there is this easy means of solving the question, that the 
 accurate Magendie, in his recent elements of physiology, should speak of it 
 as undetermined ; and should cite, as the best approximation, an estimate 
 made from the obscure circumstance of a loaded foot shaking in unison with 
 the pulse, when suspended in the cross-legged sitting -attitude.- 
 
 Some physiologists have expressed surprise that the force of the heart 
 should be so great as it is, remarking that much less would have sufficed to 
 propel the blood to the most distant capillaries; but they did not reflect that 
 the heart, besides carrying on the general circulation, has to force blood into 
 
THE CIRCULATION. 431 
 
 those parts of the flesh which, in the various positions of sitting, lying, 
 standing, &c., are for the time compressed by the whole weight of the body ; 
 for that, if it were not strong enough for this purpose, either the compressed 
 parts, deprived of their nourishment, would quickly die, or the person obliged 
 to be every moment changing position, could obtain no lengthened repose. 
 In illustration of this point, we may advert to the frequent occurrence, in 
 diseases where the power of the heart is for the time weakened, for slough- 
 ings, or bed-sores in the bearing parts, causing many cases of illness to 
 terminate fatally which would otherwise soon have terminated in health. 
 The author of tnis work has had great satisfaction in suggesting a means 
 of entirely preventing deplorable termination, namely, that which he is 
 now about to describe under the title of 
 
 The HYDROSTATIC BED for Invalids. 
 
 In many of the diseases which afflict humanity, more than half of the suf- 
 fering and danger is not really a part of the disease, but the effect or conse- 
 quence of the confinement to which the patient is subjected. Thus a fracture 
 of a bone of the arm is as serious a local injury as a fracture of one of the 
 bones of the leg ; but the former leaves the patient free to go about and amuse 
 himself, or attend to business as he wills, and to eat and drink as usual in 
 fact, hardly renders him an invalid ; while the latter imprisons the patient 
 closely upon his bed, and brings upon him, first, irksomeness of the conti- 
 nued position, and then the pains of the unequal pressures borne by the parts 
 on which the body rests. These, in many cases of confinement, disturb the 
 sleep and the appetite, and excite fever, or such constitutional irritation as 
 much to retard the cure of the original disease, and not unfrequently to pro- 
 duce new and more serious disease. That complete inaction should prove 
 hurtful to the animal system, may by all be at once conceived; the operation 
 of the continued local pressures will be understood from the following state- 
 ments. ' The health, and even life, of every part of the animal body, depend 
 on the sufficient circulation through it of fresh blood, driven in by the force 
 of the heart. Now when a man is sitting or lying, the parts of his flesh com- 
 pressed by the weight of the body, do not receive the blood so readily as at 
 other times ; and if from any cause the action of his heart has become weak, 
 the interruption will both follow more quickly and be more complete. A 
 peculiar uneasiness soon arises where the circulation is thus obstructed, im- 
 pelling the person to change of position ; and a healthy person changes as 
 regularly, and with as little reflection, as he winks to wipe and moisten his 
 eyeballs. A person weakened by disease, however, while he generally feels 
 the uneasiness sooner, as explained above, and therefore becomes what is 
 called restless, makes the changes with much fatigue; and should the sensa- 
 tions after a time become indistinct, as in the delirium of fever, in palsy, &c,, 
 or should the patient have become too weak to obey the sensation, the com- 
 pressed parts are kept so long without their natural supply of blood that they 
 lose their vitality, and become what are called sloughs or mortified parts. 
 These have afterwards to be thrown off, if the patient survive, by the pro- 
 cess of ulceration, and they leave deep holes, requiring to be filled up by 
 new flesh during a tedious convalescence. Many a fever, after a favourable 
 crisis, has terminated fatally from this occurrence of sloughing on the back 
 or sacrum ; and the same termination is common in lingering consumptions, 
 palsies, spine diseases, &c., and generally in diseases which confine the 
 patients long to bed. 
 
432 FLUIDITY IN RELATION TO ANIMALS. 
 
 It is to mitigate all, and entirely to prevent some of the evils attendant on 
 the necessity of remaining in a reclining posture, that the hydrostatic bed is 
 intended. It was first used under the following circumstances. 
 
 A lady after her confinement, which occurred prematurely, and when her 
 child had been for some time dead, passed through a combination and suc- 
 cession of l^v fever, jaundice, and slight phlegrnasia dolens of one leg. In 
 her state of extreme depression of strength and sensibility, she rested too 
 long in one posture, and the parts of the body on which she had rested all 
 suffered : a slough formed on the sacrum, another on the heeLj and in the 
 left hip, on which she had lain much, inflammation began, which terminated 
 in abscess. These evils occurred while she was using preparations of bark, 
 and other means, to invigorate the circulation, and while her ease and com- 
 fort were watched over by the affectionate assiduity of her mother with 
 numerous attendants. After the occurrence, she was placed upon the bed 
 contrived for invalids by Mr. Earle, furnished for this case with pillows of 
 down and of air of various sizes, and out of its mattrass portions were cut op- 
 posite f to the sloughing parts; and Mr. Earle himself soon afforded his 
 valuable aid. Such, however, was the reduction of the power of life, that 
 in spite of all endeavors, the mischief advanced, and about a week later, 
 during one night, the chief slough on the back was much enlarged, another 
 had formed near it, and a new abscess was proceeding in the right hip. An 
 air-pillow had pressed where these sloughs appeared. The patient was at 
 that lime so weak that she generally fainted when her wounds were dressed ; 
 she was passing days and nights of uninterrupted suffering, and as all known 
 means seemed insufficient to relieve her, her life was in imminent danger. 
 
 Under these circumstances, the idea of the hydrostatic bed occurred to met 
 Even the pressure of an air-pillow had killed her flesh ; and it was evident 
 that persons in such condition could not be saved unless they could be sup- 
 ported without sensible inequality of pressure. I then reflected, that the 
 support of water to a floating body is so uniformly diffused, that every thou- 
 sandth of an inch of the inferior surface has, as it were, its own separate 
 liquid pillar, and no part bears the load of its neighbour that a person 
 resting in a bath is nearly thus supported that this patient might be laid 
 upon the surface of a bath over which a large sheet of the water-proof India- 
 rubber cloth were previously thrown, she being rendered sufficiently buoyant 
 by a soft mattress placed beneath her thus would she repose on the face of 
 the water, like a swan on its plumage, without sensible pressure anywhere, 
 and almost as if the weight of her body were annihilated. The pressure of 
 the atmosphere on our bodies is of fifteen pounds per square inch of its sur- 
 face, but because uniformly diffused, is not felt. The pressure of a water- 
 bath or depth to cover the body, is less than half a pound per inch, even on 
 the under side where it is greatest, and is similarly unperceived. A bed 
 such as^then planned, was immediately made. A trough of convenient dimen- 
 sions (6 feet long, 2 feet 8 inches wide, and 11 inches deep, are good com- 
 mon dimensions) was lined with metal to make it water-tight; it was about 
 half filled with water, and over it was thrown a sheet of the India-rubber 
 cloth as large as would be a complete lining to it if empty. Of this sheet 
 the edges, touched with spirit varnish to prevent the water creeping round 
 by capillary attraction, were afterwards secured in a water-tight manner all 
 round to the upper border or top of the trough, shutting in the water aa 
 closely as if it had been in bottles, the only entrance left being through an 
 opening at one corner, which could be perfectly closed. Upon this beautiful 
 dry sheet a suitable mattress was laid, and constituted a bed ready to receive 
 
THE CIRCULATION. 433 
 
 its pilow and bed-clothes, and not distinguishable from a common bed but 
 by its most surpassing softness or yielding. The bed was carried to the 
 patient's house, and she was laid upon it; she was instantly relieved in a 
 remarkable degree : sweet sleep came to her; she awoke refreshed; she 
 passed the next night much better than usual ; and on the following day, 
 Mr. Earle found that all the sores had assumed a healthy appearance ; the 
 healing from that time went on rapidly, and no new sloughs were formed. 
 When the patient was first laid upon the bed, her mother asked her where 
 the down pillows, which she before had used, were to be placed; to which 
 she answered, that she knew not, for that she felt no pain to direct; in fact, 
 she needed them no more. 
 
 It may be here recalled to mind, that the human body is nearly of the 
 specific gravity of water, or of the weight of its bulk of water, and therefore, 
 as is known to swimmers, is just suspended or upheld in water without exer- 
 tion when the swimmer rests tranquilly on his back with his face upwards. 
 He then displaces water equal to his own body in weight as well as in bulk, 
 and is supported as the displaced water would have been. If his body be 
 two and a half cubical feet in bulk, (a common size,) he will just displace two 
 and a half cubic feet of water, equal in weight to his body. If, however, 
 instead of displacing the water with his mere body, he choose to have some- 
 thing around or under him which bulky with little weight, as the mattrass 
 of the bed above described, then, after his weight is forced two cubical feet 
 of that under the level of the water around, he will float with four-fifths of his 
 body above the level, and will sink much less into his floating^ mattrass than 
 a person sinks in an ordinary feather-bed. It thus appears that by choosing 
 a certain thickness of mattrass, and if unusual positions are required by 
 having different thicknesses in different parts, or by placing a bulls of folded 
 blanket or of pillow over or under the mattrass in certain situations, any 
 desired position of the body may be easily obtained. If the water be about 
 six inches deep, which in general will suffice, the person standing upon any 
 part of the bed, or sitting with the knees raised, will cause the part of the 
 mattrass on which he rests gently to touch the bottom, because a narrow end 
 of the body cannot displace water equal to the bulk of the whole, but even 
 then the person is as if standing or sitting on a soft sofa. If it be desired 
 to prevent the mattrass, when used as a seat, from touching the bottom, the 
 object may be attained by having under its middle a broad band or strap 
 fixed to one edge of the trough, and connected with the other by buttons or 
 otherwise, so as to be tightened to allow the mattrass to descend just so far, 
 and no farther. 
 
 This bed is a warm bed, owing to water being nearly an absolute non- 
 conductor of heat from above downwards, and owing to its allowing no pas- 
 sage of cold air from below. From this last fact, however, less of the per- 
 spiration, sensible and insensible, is carried off by the air than in a common 
 bed, and unless the patient can leave the bed daily to let it be aired like a 
 common bed, there will be a necessity for ventilation to prevent the perspi- 
 ration from being condensed on the water-sheet below. This ventilation is 
 perfectly obtained by placing under the mattrass, arranged like the bars of a 
 gridiron, small flexible tubes of tinned wire, wound spirally, with their ends 
 open to the atmosphere, either directly or through two larger tubes crossing 
 and connecting their extremities near the ends of the mattrass, and then 
 issuing at the corners of the bed from under the clothes. This bed is in 
 itself as dry as a bed can be, for the India-rubber cloth (of which bottles can 
 be made) is quite impermeable to water, and the maker is now preparing 
 
 28 
 
434 FLUIDITY IN RELATION TO ANIMALS. 
 
 cloth expressly for this purpose. Then, as Sir Humphrey Davy recommended 
 that his safety lamp should be double, some persons may prefer a double 
 sheet, to obviate the possibility of accident. Unlike any other bed that ever 
 was contrived, it allows the patient, when capable of only feeble efforts, to 
 change his position, almost like a person swimming, and so to take a de- 
 gree of exercise, affording the kind of relief which, in constrained positions, 
 is obtained by occasional stretching, or which an invalid seeks by driving out 
 in a soft-springed carriage. It exceedingly facilitates turning for the pur- 
 pose of dressing wounds, for by raising one side of the mattrass or depress- 
 ing the other, or merely by the patient's extending a limb to one side, he is 
 gently rolled over, nearly as if he were simply suspended in water; and it is 
 possible even to dress wounds, apply poultcies, or place vessels under any 
 part of the body, without moving the body at all : for there are some inches 
 of yielding water under the body, and the elastic mattrass may at any part 
 be pushed down, leaving vacant space there, without the support being less- 
 ened for the other parts. Then, with all the advantages which other inva- 
 lid beds posses, and with those which are entirely its own, it may yet be 
 made so cheaply, that even in hospitals, where economy must prevail, it may 
 at once be adopted for many of the bed-ridden. Mr. Earle within a few days 
 of seeing the first one, had others made for patients in St. Bartholomew's 
 Hospital, and has been as much pleased with the results of them as of the 
 first. The bed has since been introduced into St. George's Hospital by Mr. 
 Keate, and elsewhere,. The author has now seen enough of the effects of 
 this bed to make him feel it a duty at once to publish a notice of it. With 
 it, evidently, *the fatal termination called sloughing, now so common, of fe- 
 vers, and other diseases need never occur again. And not only will it pre- 
 vent that 'termination, but by alleviating the distress through the earlier 
 stages, it may prevent many cases from even reaching the degree of danger. 
 Then it is peculiarly applicable to cases of fractured bones, and other surgical 
 injuries; to palsies, diseases of the hip joint, and spine; and universally, 
 where persons are obliged to pass much time in bed. And in all cases of 
 curvature of the spine, either actually existing or threatened, it affords a 
 means of laying a patient in any desired position, and with any degree of 
 pressure incessantly urging any part of the spine back to its place. If used 
 without the mattrass, it becomes a warm or a cold bath, not allowing the 
 body, however, to be touched by the water, and in India, it might be made 
 a cool bed for persons sick or sound, during the heats which there prevent 
 sleep and endanger health. There are numerous other professional adaptions 
 and modifications of it, which will readily occur to practitioners sufficiently 
 versed in the department of natural philosophy (hydrostatics) to which it be- 
 longs. Before reflection, a person might suppose a resemblance between it 
 and an air-bed or pillow, calling this a water-bed or pillow; but the prin- 
 ciples of the two are perfectly distinct or opposite. An air-pillow supports 
 by the tension of the surface which encloses the air, and is therefore like a 
 hammock or the tight sacking under the straw mattrass of a common bed, 
 and really is a hard pillow; but in the hydrostatic bed, there is no tense 
 surface or web at all ; the patient is floating upon the water, on which a 
 loose sheet is lying, merely to keep the mattrass dry, and every point of his 
 body is supported by the water immediately beneath it. To recall the dif- 
 ference here described, and which is of g'reat importance, the bed is better 
 described by the appellations of hydrostatic-bed, or floating -led, than of 
 water-bed. 
 
 The author has given no exclusive right or privilege to any person to 
 
THE CIRCULATION. 435 
 
 make this bed. He has hitherto employed the carpenter nearest to him, 
 Mr. Smith, 253 Tottenham-court Road, at the back of Bedford Square; 
 Mackintosh & Co., 58 Charing Cross, the manufacturers of the cloth; and 
 Mr. Williams, 25 Cleveland Street, Fitzroy Square; but any carpenter or 
 upholsterer may learn to supply them, and he gives free permission to all. 
 He hopes, for the sake of the poor, that a trough, without metallic lining, 
 and with a cheaper water-proof cloth, may be found to answer satisfactorily.. 
 
 The principle of the hydrostatic bed, is applicable, also, to couches for 
 invalids, and with certain considerable modifications, to the construction, 
 also, of chairs ; and there are other means than the water-proof sheet of 
 adapting the hydrostatic principle for all but the subject has already 
 occupied its full share of this volume. 
 
 The preceding paragraphs are intended as much to direct in the choice and 
 use of common beds for the sick, as to announce and describe the hydro- 
 static bed for the cases in which it may be required. At present, the medi- 
 cal attendant generally leaves whatever regards the bed to the judgment of 
 friends or nurses ; but evidently, he who has been led to reflect how much 
 the course and event of a maladay may depend on the patient's being sup- 
 ported, so that no pain shall arise from local pressure, and as little muscular 
 weariness as possible from constrained position, will deem the bed-manage- 
 ment worthy of his own attention, and will be able more judiciously both to 
 choose and to use beds. There is a bed constructed of spiral springs, which 
 may be made so as to diffuse the support more equally than any except the 
 hydrostatic bed ; and bad professional men generally been acquainted with 
 it, it would have been more used than it is, and would have received various 
 modifications, of which it is susceptible, for medical purposes. It has long 
 been known, chiefly, however, as a mechanical curiosity, or an object of 
 luxury, and was introduced into this country about seventy years ago by 
 Mr. Merlin ; but it has been so little known, that a few years ago an English 
 tradesman thought he might appropriate the manufacture by taking a patent 
 for it. It is now made by upholsterers generally, and the same principle is 
 applied in the construction of sofas, chairs, and carriage cushions. 
 
 The velocity of the circulating blood. 
 
 This has been much overrated. 1st. By assuming that the ventricles of 
 the heart are both completely filled from the auricles and emptied towards 
 the arteries at each pulsation : an assumption disproved by inspections of 
 the exposed heart of a living body, and by the fact of the valves between the 
 auricles and ventricles not closing so perfectly as quite to prevent regurgi- 
 tation. 2d. By supposing the issue of blood from a wounded artery or vein 
 to be the measure of the usual velocity. Now it would be as reasonable to 
 suppose the issue of water from a wounded pipe connected with any reser- 
 voir to be the measure of a continued current in that pipe, although, in truth, 
 the issue would be the same even if the water in the pipe were usually at 
 rest. 3d. By supposing the frequency of the pulse to be a measure. Now 
 we know, that in diseases of debility, and in animals bleeding to death, the 
 pulse usually becomes more frequent as it becomes more feeble, and as there 
 is less blood moving : viz., the heart very partially discharging its contents 
 at each contraction. 4th, and lastly. By supposing the strength of the 
 pulse to be the measure. Now we find that the pulse in an artery just tied, 
 and in which, consequently, there is no current at all, is scarcely weaker 
 than in an open artery. The common fact of a person's feet remaining 
 stone-cold for hours, although the arteries leading to them pulsate nearly 
 
436 FLUIDITY IN RELATION TO ANIMALS. 
 
 as usual, is a proof that exceedingly little blood is passing through the 
 capillaries at the time, and that the strength of the pulse, therefore, is no 
 measure of the speed of the blood. 
 
 The ventricles of the heart appear, under common circumstances, to throw 
 out about an ounce and a half of blood at every contraction or about seven 
 pounds per minute. Now if the body contain about twenty pounds alto- 
 gether, as seems to be the case, the whole would circulate twenty times in 
 an hour. This would give an average velocity of about eight inches per 
 second in the aorta, but gradually less in the smaller arteries, because when- 
 ever a vascular channel subdivides, the branches taken collectively have 
 considerably greater area than the trunk from which they arise, and the 
 current diminishes in a corresponding proportion, just as the speed of a 
 river is always less in the parts of the channel which are deeper and broader. 
 The velocity in the extreme capillaries is found to be often less than one 
 inch per minute. In the veins, the blood must move more slowly than in 
 corresponding arteries, in proportion as the veins are more capacious than 
 the arteries. 
 
 The pulse. 
 
 The opinion which the ancients held, that the arteries contained vital 
 spirits or air and not blood, rendered the pulse, to them, a very mysterious 
 phenomenon; and many curious hypotheses were framed to explain it. 
 These it would now be unprofitable to detail. Even Harvey's grand dis- 
 covery of the circulation, however, has not rendered the subject so simple 
 as might have been anticipated. The following opinions now exist, or have 
 lately existed with respect to the pulse. 
 
 1st. The great majority of physiologists have believed that a tumefaction 
 is produced in the aorta by each jet of blood from the heart, and spreads 
 afterwards as a wave into all the arterial branches. 2d. Many have supposed 
 an extensive contractile action of the arteries themselves, corresponding to 
 that of the heart. 3d. Bichat, unable by any means to detect the slightest 
 change of diameter in the arteries during pulsation, but perceiving that in 
 many situations they were at the time somewhat lengthened, so that straight 
 portions became bent, and portions originally bent, were bent still more, held 
 that this locomotion or changing of place in the arteries was the cause. 
 4th. Others have supposed the impulse of the heart's contraction to be trans- 
 mitted through the fluid blood, somewhat as sound is transmitted through 
 bodies generally, or as a blow struck on one end of a log of wood, is felt 
 dfstinctly by a hand applied to the other, although there be no 7 visible loco- 
 motion. 5th. Dr. Young, in the paper in the Philosophical Transactions 
 already alluded to, has shown that a sudden rush forward of the blood in 
 the artery, such as might be produced by injection at one end of a rigid 
 tube, would be felt by a finger applied to the artery, quite as distinctly as 
 a tumefaction ; and he deems this occurrence to be a chief cause of the 
 pulse. Dr. Parry, in his work on the pulse, points to this almost as exclusively 
 the cause. 
 
 Now the truth is, that the pulse in the living body does not depend 
 exclusively upon any one of the particulars just noticed, but has. all of them 
 as elements ; and its fluctuations and varieties depend upon proportions in 
 which these elements are combined. We shall review them again to prove 
 this. 
 
 1st. At each jet of blood thrown into the aorta, a tumefaction or wave 
 must spread from the heart to the extremities ; for it is evident, that if blood 
 
THE CIRCULATION. 437 
 
 be at all pushed into the arterial system, it either must dilate it, or cause an 
 equal quantity to be expelled at the same instant from the distant extremities : 
 now as the passage of blood through the capillaries appears perfectly uniform, 
 there must be an intermediate dilatation. Dr. Parry and others should not 
 have denied this dilatation because they could not see it: for even if its ad- 
 vancing front were more considerable than it is, it passes with such velocity 
 that like a cannon-ball crossing before the face, it would not be perceived. 
 
 2d. Contraction of the arterial coats certainly does not take place in the 
 manner and to the extent supposed by some, who have spoken of it as resem- 
 bling the contraction of the heart itself, and as what might be a substitute for 
 the action of the heart in propelling the blood; but as shown at page 414, 
 the rigidity of tube which in all degrees of arterial dilatation causes the pulse 
 to be transmitted so quickly, can depend on nothing but a contractile action 
 of the fibres. There are some reasons for doubting whether this rigidity 
 may not increase at the moment of the pulse. 
 
 od. Unless the arterial tubes were absolutely inelastic, which they are far 
 from being, they must be lengthened a little by a sudden injection of blood, 
 and, therefore, at all the curvatures particularly, there must be a degree of 
 the locomotion described by Bi^hat, often sensible to a finger applied. 
 
 4th. That a tangible shock is conveyed through a fluid without any appa- 
 rent accumulation of the fluid or change of velocity, and much in the manner 
 of sound, is proved, by the facts, that we may discover the working of a 
 water-pump at very great distances, through iron pipes connected with it, and 
 even through elastic pipes of leathe^, as those of a common fire-engine, from 
 which the water is spouting, nevertheless, in a uniform stream. The pulse 
 in a tied artery, in which there is no current or rushing wave, must be chiefly 
 from this cause and from the locomotion of the artery. 
 
 5th. That any additional quantity of fluid injected into elastic vessels 
 already full, must spread all over with & forward rush, affecting the finger 
 of an examiner, as described above, is also most certain. As the heart, how- 
 ever, often beats without discharging much of its blood, and as in many 
 arteries from inaction of the capillaries, or pressure of the blood for a time 
 makes little or no progress, while the pulse, however, remains very distinct, 
 the pulse in such cases must be produced independently of the forward rush. 
 An animal intestine prepared, and filled with water or air, and laid upon a 
 table or a full vein in the living body, carries a rapid and distinct pulse to 
 a great distance when gently tapped by the finger. The cause of the sensa- 
 tion, then, cannot be the simple forward rush without tumefaction, described 
 by Dr. Young and Dr. Parry. 
 
 In whatever proportions these particulars combine to form the pulse, its 
 force will be proportioned to the size of the artery. Hence as an artery lead- 
 ing to an inflamed part becomes of greater calibre, its pulse also becomes 
 stronger. 
 
 It is a remark respecting the pulse, appearing to the author worthy of deep 
 consideration, that if the purpose of the heart and arteries were merely the 
 propulsion and conveyance of the blood, their structure and action would 
 form most signal deviations from the ascertained rules of fitness in mechanics. 
 In machines of human contrivance, it is one of the most important maxims 
 u to avoid shocks, or jerking motions j" and in former parts of this work, we 
 have described fly-wheels, air-vessels, springs, &c., as means of accomplishing 
 this object, and thereby of preventing the tearing and straining of parts which 
 would else happen. In the human body, also we have to describe the ad- 
 mirable elasticity of the spine, of the arch of the foot, of the cartilages of joints, 
 
438 FLUIDITY IN RELATION TO ANIMALS. 
 
 &c., as contrivances answering the same end ; and to remark that, in other 
 cavities than the heart, which are alternately filled and emptied like it, as the 
 stomach, bladder, uterus, &c., there is a smooth and gradual action. The 
 heart alone is the rugged anomaly, which, from before birth unto the dying 
 moment, throbs unceasingly, and sends the bounding pulse of life to every 
 part; and which, moreover, instead of being secured and tied down to its 
 place, is attached at the extremity of the aorta, like a weight at the end of 
 an elastic branch of a tree, and every time that it fills the aorta, is thrown 
 with violence, by the consequent sudden tendency of that vessel to become 
 straighter, against the ribs, in the place where the hand applied, feels it so 
 distinctly beating. 
 
 Now one use of the pulsation of the heart probably is, by the agitation and 
 churning which the blood suffers in passing through it, to keep in complete 
 mixture all the heterogeneous parts of the blood, which so readily separate 
 when left to repose: but this cannot be the only use, for the object might 
 have been more simply detained; and we may conclude that the phenomenon 
 has relation to some important laws of life still hidden from us. The cause 
 commonly assigned for the heart' scon traction is the peculiar stimulus of the 
 blood ; yet if we reflect that the heart will beat after removal from the body 
 and when it contains only air, and that during life it beats with extraordinary 
 regularity, whether the state of the circulation allow it to empty itself at each 
 beat or not, we perceive that the case is more obscure. We cannot con- 
 template this subject attentively without perceiving a strong analogy between 
 the action of the heart and some electrical phenomena in which there are 
 successive accumulations and exhaustion of power ; and, recollecting the 
 important relations which late researches have shown to exist between elec- 
 tricity and certain other actions of life, the inquiry becomes very interesting. 
 Galvanism can excite the muscles to their usual actions; it powerfully affects 
 the secretions and the digestive function; and the breathing in asthma; strong 
 animal passion seems to produce electrical excitement : and certain animals 
 have the faculty of stunning their enemies by an electrical discharge. The 
 pulse, then, in its sudden, strong, and regular recurrence, may be a kindred 
 phenomenon. In this view, there would be less difficulty in supposing a 
 momentary stiffening or slight contraction of the whole arterial system, such 
 as the sudden rising of the mesenteric arterial tree so readily suggests : if 
 there be such, however, it is still closely connected with, and proportioned 
 to, the action of the heart; for it occurs only with that action, it indicates 
 any disturbance in the action, and as death approaches, it ceases in the 
 remote extremities first. 
 
 The preceding considerations exhibit the pulse as a complex subject, and 
 one on which professional opinions are not yet settled. By showing its close 
 relation to the powers of life, they also prove it to be an object of high im- 
 portance to the medical practitioner. This last truth has scarcely been ques- 
 tioned but by persons either utterly uninformed or singularly deficient in the 
 power of tactile discernment ; yet, because no simple and good analysis of 
 the pulse, and detail of its relation to morbid states, has been made and pub- 
 lished, the degrees of skill acquired by individual practitioners with respect 
 to it at'fi very various, and in a great measure accidental. Some practitioners 
 try the pulse merely for form's sake, because patients expect it ; many ex- 
 amine it only to count its frequency; but others read in it, with confidence, 
 much of the history and probabilities of the disorder, and decide on the treat- 
 ment accordingly. Few who have attended to the subject at all, can confound 
 the pulses of such diseases, as acute rheumatism, gastric inflammation, the 
 
THE CIRCULATION. '439 
 
 fits of ague, &c. The author remembers to have conversed with a Chinese 
 practitioner who had only the scanty medical information of his countrymen, 
 but who judged by the pulse with a singular penetration. 
 
 The changing circumstances in the state of the circulatory system, con- 
 nected with health and disease, and discoverable by a finger watching the 
 pulse, seem to be chiefly the following ; and the epithets added in italics, 
 are those which seem best to indicate the sensations perceived. The artery 
 at the wrist is that generally chosen for examination, because it is not like 
 others imbedded in soft parts, having only the skin over it, and nothing be- 
 tween it and the bone below. 
 
 1st. The number of the contractions of the heart in a given time, and 
 the regularity of their recurrence. Pulse, frequent, slow, intermittent, equal, 
 regular, of varying force. 
 
 2d. The degree of the heart's contraction, or the quantity of blood ejected 
 .at each time ; and the corresponding state of the capillaries as to the quan- 
 tity of blood passing through them. Pulse, full, long, labouring, bound- 
 ing, feeble.. 
 
 3d. The force of the heart's action, with the correspondent arterial tension 
 or rigidity. Pulse, hard, sharp, strong, wiry, weak, soft, yielding. 
 
 4th. The suddenness of the individual contractions of the heart, and the 
 rigidity of the vessels in conveying the shock. Pulse, quick, tardy. 
 
 5th. The size of the artery for the time, whether larger or smaller than 
 usual. Pulse, large, small. 
 
 Superficial as is this sketch, it may show that a good treatise on the sub- 
 ject of the pulse, as connected with disease, is yet a desideratum in medi- 
 cine. The sort of empirical but useful tact which many persons acquire, 
 is not fitted to satisfy the physician who reasons deeply, and whose mind 
 should have always present to it the various constituents of the pulse, and 
 all the important circumstances of health or disease relategl to its indications. 
 The laboured treatise of Solano, Bordue, Boerhaave, &c., may treat of 
 what were clear ideas to their authors, but by not referring the physical 
 causes of many varieties, they become so obscure to others, that many of 
 the divisions and denominations appear altogether fanciful. Dr. Young's 
 excellent paper in the Phiosophical Transactions, details important facts, as 
 far as it goes, but it was not intended to point out all the pathological rela- 
 tions. Dr. Y., guided by general principle, asserted a progressive motion 
 of the pulse, while other authorities were holding it to be quite simultaneous 
 over the whole system. He might have mentioned in proof, that careful 
 examination can practically detect a succession of beats at different distances, 
 particularly at the four stations; 1st, of the heart; 2d, in the lip; 3d, at 
 the wrist; 4th, at the ankle: but the interval of time, even between the 
 extremes, being only a small part of a second, persons will often fail -to 
 make their first experiment satisfactorily. Dr. Parry's treatise on the pulse 
 which is the last one of note, although having excellencies, errs in attri- 
 buting the phenomenon to one cause so exclusively in denying arterial 
 dilatation, because it was not discovered by his mode of searching for it, in 
 supposing that a liquid column in an elastic tube, can be made to advance 
 like a solid rod, or line of billiard-balls. The too common neglect of me- 
 chanical philosophy by medical, men is signally proved, by our finding in 
 works of authority, published at the present day, such statements as that the 
 arterial pulse may be more frequent or less frequent than the beatings of the 
 heart. Dr. Good (Study of Medicine'} says, that there may be various fre- 
 quency of pulse in various parts of the body at the same time : Richerand 
 
440 " FLUIDITY IN RELATION TO ANIMALS. 
 
 (Physiologic) says, the pulse is more frequent in the artery leading to a 
 \vhitlow than at the same time elsewhere; and many practitioners share 
 these notions. What a satire on the medical profession is this disagreement 
 on a point which, to common observers, seems, obove all others, to occupy 
 the attention of the attendant on the sick ! 
 
 Having now explained the circulation of the blood in general, we proceed to 
 consider some cases where mechanical circumstances modify it. 
 
 Circulation in the head. 
 
 The head may be considered as an air-tight vessel or cavity of bone, con- 
 taining chiefly brain arid blood, and having openings occupied by blood- 
 vessels, leading to and from the heart. The atmospheric pressure, there- 
 fore, always keeps the head full, as it keeps the top of a syphon full ; and 
 because the substance of the brain itself does not more than water, sensibly 
 change in bulk by any ordinary degrees of pressure, there must always be 
 the same quantity of blood in the head, how much soever the quantity may 
 vary in the body generally. Regard to this important truth, a knowledge of 
 which has followed the discovery of the true nature of atmospheric pressure, 
 enables us to explain many hitherto obscure facts, both in health and disease ; 
 as the following instances will show. 
 
 If, from any cause, the arteries in the he-ad become too full of blood, in 
 the same proportion the veins must become too empty or, if the veins 
 become too full, the arteries must be too empty ; and in either case, the cir- 
 culation in the head will be in a corresponding degree impeded, because, 
 when one part of a channel is narrowed or diminished, the current through- 
 out the whole is slackened. Now, as insensibility supervenes when the 
 supply of fresh blood to the brain is interrupted, and death follows if the 
 interruption continue long, it seems evident that in many of the cases of 
 apoplexy, where, on inspection, there is found nothing but a fulness of the 
 arterial or of the venous system of the head, death has happened merely 
 because the circulation was arrested in this way. In other parts of the 
 body, not circumstanced like the brain, an excess of blood in one set of 
 vessels may happen without inducing deficiency in another, and therefore 
 with perfect impunity to the individual. 
 
 Simple increase of pressure produced by the blood on the brain, provided 
 the proper balance exist between the quantity in veins and arteries, has no 
 injurious effect. This is proved by the safe descent of a person in a diving- 
 bell, where, at thirty-four feet under the surface of the water, the body is 
 bearing an additional pressure of fifteen pounds on a square inch (see page 
 163,) which pressure through the blood-vessels affects the brain as much as 
 any other part. On the other hand, when a man climbs a mountain, or is 
 lifted up in a balloon, the brain is less pressed than usual ; but the proper 
 balance in artery and vein being maintained, no inconvenience is felt. The 
 inhabitants of some of the valleys among the Andes are as far above the sea 
 as they would be at the top of Mont Blanc, where the atmosphere presses 
 only half as much as on the sea-shore; but they enjoy good health. 
 
 As the box of the cranium encloses the brain so as to leave no vacant 
 space, it is evident, that when the heart injects blood with unusual violence, 
 the strain at first is borne chiefly by the cranium, and not by the coats of 
 the blood-vessels. Hence, the arteries of the brain need not to be, and are 
 not, nearly so strong as those of other parts of the body. 
 
THE CIRCULATION. 441 
 
 The veins of the brain are also peculiar. Common veins in the head 
 would, for the reasons above given, collapse by any sudden tension of the 
 arteries there, and if they did, insensibility or death would ensue, on account 
 of the consequent stoppage of the circulation. The chief channels, there- 
 fore, for the refluent blood, instead of being common compressilile veins, are 
 what have been called sinuses, or grooves in the bone itself, with -exceed- 
 ingly strong membranous coverings, supported so that the channels become 
 in strength, and as to maintenance of their capacity, a little inferior to com- 
 plete channels of bone. This singular deviation in the structure of the 
 cerebral veins from what is found elsewhere, and without which deviation, 
 animal existence could not be continued, is one of those particulars which 
 powerfully affect the contemplative mind, as proofs of the designing intelli- 
 gence which has planned this glorious universe. 
 
 From not adverting sufficiently to the fact now explained, of the cranium 
 being a vessel always full, and which will hold only a certain quantity, 
 misconception has prevailed among medical men with respect to many of 
 the affections of the brain. 
 
 It has been said^for instance, that the substance of the brain cannot bear 
 pressure with impunity, for that stupor immediately follows pressure, how- 
 ever produced. Now the truth is, that pressure produces stupor only when 
 it interferes with the circulation. In wounds with loss of a large piece of 
 the cranium, the brain will bear very rough handling, because if compressed 
 at one part, it may bulge at another, and leave the circulation free ; but if the 
 wound be small, pressure made through it instantly affects the whole brain, 
 and the blood is prevented from entering from the heart. Let one reflect, for 
 an instant, on what happens to the foetal head during parturition how often 
 it escapes elongated and bent, almost as if it were of soft clay yet the child 
 lives and thrives, and the natural form is soon recovered. The reason is, 
 that the fretal skull is soft, and pressure in one part is compensated for by 
 a bulging or extension in another, and the blood is not expelled. 
 
 Water in the head, again, is said to kill by this fatal pressure on the tender 
 brain ; but, in reality, it kills by keeping out of the blood, and so mechanically 
 arresting the circulation. Accordingly we see, that where the fontanelle still 
 remains open, or where the sutures or joinings of the skull will yield, water 
 may accumulate to a great degree without causing much disturbance. 
 
 A tumour in the brain, which would be of no consequence if the brain 
 were unconfined, soon becomes fatal by occupying room in the skull, and to 
 the extent of its size excluding or checking the supply of blood. 
 
 If the substance of the brain at all increase and diminish in bulk, as muscles, 
 &c., under certain circumstances, do, in the body below, all such changes 
 must produce a considerable effect on the cerebral circulation and functions. 
 
 Effects of position on the circulation. 
 
 While a man is in a standing attitude, the heart and arteries have to 
 send the blood up the head against gravity; but in the horizontal position, 
 the blood, if equally propelled, must arrive with greater force, because gravity 
 then does not resist. Hence headache, no other symptom arising from ful- 
 ness of blood in the arteries of the head, is often relieved by the upright 
 position, and is increased by lying down. 
 
 Many people who have had a slight degree of toothache during the day, 
 find it intolerable when they lie down at night, and are relieved again by 
 rising and walking about. Commonly they suppose that it is the cold of the 
 
442 FLUIDITY IN RELATION TO ANIMALS. 
 
 night which then lulls the pain ; but it is in fact the change of position. 
 The authoKknew a lady who was obliged to sleep for months in the sitting 
 posture, because she had a tic doloureux in the face whenever she lay down; 
 and another who was under the same necessity for a considerable period after 
 an inflammatory affection of the brain, because if her head fell low during 
 sleep she was immediately assailed by a terrific dream of swords driven into 
 the brain. 
 
 Delirium in fever is sometimes checked at once by elevating the head. 
 On account of the great relief thus obtained, some continental practitioners 
 had proposed to support the patients occasionally in an upright posture. 
 
 Apoplexy has often been brought on by a man bending his head down in 
 the act of tying his shoe, or of pulling on his boot. 
 
 Children and professed tumblers being much in the habit of placing their 
 bodies in all positions, feel no inconvenience from having the head down- 
 wards ; apparently, because arteries and veins usually become strong enough 
 to bear the pressure to which they are habitually exposed; but to many old 
 people, accustomed to keep the head always up, the attempt would be fatal. 
 
 Ulcers on the legs are often obstinate and will bleed, because the veins 
 about them are too weak to support the lofty columns of blood above. Hence 
 the frequent counsel given in such cases to keep the feet raised upon a 
 chair, and the utility of certain modes of bandaging. 
 
 Many inflammations of the legs and feet become exceedingly painful when 
 the limbs are in a hanging position, and the pain is relieved by laying them 
 horizontally. 
 
 Many anasarcous or dropsical affections of the legs increase towards night, 
 because, during the dependent position of the legs through the day, the 
 absorbents have not power to lift the fluid. The swelling disappears again 
 before morning. 
 
 When the heart has to send blood upwards, it requires to act more strongly 
 than when the body is horizontal, and the pulse increases five or six beats 
 in the minute; hence the common rule to make a patient with haemorrhage 
 lie in the horizontal position, that the heart may become tranquil and allow 
 the bleeding to cease. 
 
 Fainting from diminished arterial tension. 
 
 Painting, which is a temporary cessation of the action of the heart, and 
 hence, as explained above, of the action of the brain for want of blood, is 
 produced by several causes, and among others, by any occurrence which 
 renders the blood-vessels about the heart suddenly less full of tense than 
 usual. It would appear that the heart being accustomed, when it contracts, 
 to a certain degree of resistance, has its action disturbed when the resistance 
 is much diminished. 
 
 Thus haemorrhage, from any cause, by lessening the general tension of the 
 sanguiferous system, often causes fainting. The state is relieved by lying 
 down ; probably because the still remaining weaker action of the heart is 
 sufficient to send blood to the head along a horizontal course, until the gradual 
 contraction of the whole vascular system reproduces the tension necessary to 
 perfect action. A small quantity of blood taken away suddenly, affects the 
 circulation as much as a larger quantity taken gradually, apparently because 
 a certain space of time i? required for the gradual lessening of the vessels. 
 
 The operation of tapping for dropsy in the abdomen would often bring on 
 fainting, but for the precaution of tightening a broad bandage upon the body 
 
THE CIRCULATION. 443 
 
 as the water flows. The reason is, that the sudden removal of a large 
 quantity of fluid which had been compressing all the abdominal vessels, and 
 keeping them perhaps only half full of blood, allows them again suddenly 
 to receive their natural quantity, arid thus produces a relaxation of the other 
 parts of the vascular system. 
 
 Sudden parturition often causes faintness for the same reasons. 
 
 Even rising up suddenly from a horizontal position will cause an approach 
 to fainting in weak people, or in those who have been long bed-ridden : pro- 
 bably because the heart having for a time been accustomed to send blood 
 only in a horizontal direction to the head, does not at an instant exert the 
 additional power required to lift an upright column with equal force ; 
 besides, that the blood does not then return to the heart by the veins, from 
 the inferior parts of the body so readily as before. 
 
 These various facts, now easily understood, form the reason of a rule 
 which is a great modern improvement in the practice of the healing art, viz., 
 in bleeding for the cure of inflammation, to take the blood away as quickly 
 as possible. This subject deserves a little farther consideration. 
 
 A great proportion of dangerous diseases involve inflammation of some 
 vital organ ; an inflammation consists chiefly, as already stated at page 422, 
 of a gorging or over-distension of the capillary vessels in the part. The 
 nature of the capillaries, again, is such, (page 422) that when not maintained 
 constantly full by the pressure of the heart behind them, they gradually by 
 their own action, empty themselves towards the veins as is seen in the 
 disappearance of a local inflammation soon after the death of the person, or 
 in the fact of the arteries being emptied of blood after breathing ceases, &c. 
 Now ever since medicine deserved the name of an art, practitioners have 
 accounted the lancet their sheet-anchor in inflammatory disease ; but it is 
 only in late times, since the circulation of the blood was understood, that 
 they have known the rationale of the remedy, viz., that it acts by diminishing 
 vascular tension, and hence the action of the heart, and so allowing the small 
 vessels to empty themselves by their own force, and to recover sufficiently 
 to resist the return of an excessive load. It is still more lately that they 
 have understood how much more suddenly and completely the disease is 
 cured by abstraction of a small quantity of blood so rapidly as to produce 
 fainting, than of a much larger quantity so slowly that only weakness fol- 
 lows. Judicious treatment now cures inflammation much more certainly 
 and completely than was done formerly, yet with much smaller loss of the 
 precious blood, and with less danger of those diseases of weakness, or of 
 that complete breaking up of the constitution, which often follow great 
 depletion. To induce faintness, large openings are to be made into the 
 veins sometimes into two veins at once, and the patient is kept in the 
 upright attitude, Often thus an inflamed eye, which was as red as scarlet 
 before bleeding, in a few minutes is rendered nearly of the natural appear- 
 ance ; and intense internal inflammations, as of the brain, lungs, bowels, 
 &c., which, if neglected, would be shortly fatal, are removed in the same 
 manner. In all these cases, the faintness seems to be almost equally effica- 
 cious, whether it happens after the loss of ten ounces of blood, or of fifty; 
 or even, as sometimes occurs, when it happens without bleeding at all, after 
 merely tying the arm in preparation. 
 
 Reflection upon these circumstances led the author to think that, in certain 
 cases, the beneficial effects of blood-letting might be attainable by the simple 
 means of extensive dry cupping, alluded to at page 176 ; that is to say, by 
 diminishing the atmospherical pressure on a considerable part of the body, 
 
444 FLUIDITY IN RELATION TO ANIMALS. 
 
 on the principle of the cupping-glass used very gently, and thus suddenly 
 removing for a time from about the heart, a quantity of blood, sufficient by 
 its absence to produce faintness. The results of trial have been such as to 
 give great interest to the inquiry, and the author's leisure will be devoted 
 to the prosecution of it. An air-tight case of copper or tin-plate, or of air- 
 tight cloth kept extended by hoops, being put upon a limb, and made close 
 by a suitable collar tied at the same time round its mouth and the limb, 
 on part of the air being then extracted by a suitable syringe, in an instant 
 the vessels all over the limb become gently distended with blood; and as the 
 blood is suddenly taken from the centre of the body, faintness is produced, 
 just as by bleeding from a vein. The excess of blood may be retained in 
 the limb as long as desired ; for the circulation is not impeded. To produce 
 a powerful effect with a slight diminution of pressure, more than one limb 
 must be operated upon at the same time. 
 
 An instrument resembling the contrivance now described, was proposed 
 about twenty years ago by a non-professional person, as a means of drawing 
 all sorts of diseases out of the body through the pores of the skin. He 
 enclosed a leg in an air-tight case ; he then admitted steam to heat the limb, 
 and relax the pores of the skin, as he said, and then he worked an air-pump 
 to draw out the disease. He called the engine the air-pump vapour bath. 
 In various cases where its true action was desirable, although not understood 
 by the proposer, nor judiciously managed, it proved beneficial. 
 
 The operation of applying tourniquets or bandages round the limbs, so as 
 to influence the transmission of the blood, affects the action of the heart. It 
 is said sometimes to have prevented the accession of the ague. It is a means 
 akin to those above described. 
 
 Because arteries are stronger and more tense than veins, a bandage may 
 be put round a limb tight enough to close the veins but not the arteries, and 
 the limb will then swell beyond the ligature. By thus putting tight elastic 
 bandages round all the limbs at once, and immersing them in warm water to 
 favour the dilatation of their vessels, so much blood may be suddenly detained 
 in them as to cause the person to faint. Such a means, therefore, might 
 also be used remedially. 
 
 In the same way, a tight handkerchief, or stock round the neck, will often 
 retain the venous blood in the head, and cause apoplexy. Strong pressure 
 made on a jugular vein kills as certainly as if made on the windpipe. 
 
 When a hernia, or other tumor, is strangulated, it swells in the manner 
 above described, and if not relieved, soon mortifies. 
 
 Diffused pressure, like that made by rolling a bandage round a whole limb, 
 or by immersing the limb in fluid, must affect the circulation. The veins 
 will be more compressed than the arteries, by reason of the distending force 
 in them being less. Varicose veins, therefore, are usefully supported by a 
 bandage or laced stocking. The reason why this manner of supporting 
 assists so powerfully in the healing of ulcers on the legs, may be, that the 
 support affects the capillaries and absorbents, as well as the larger vessels. 
 
 Poultices, by their weight, produce a soft compression of the parts on 
 which they are applied ; and in certain cases, may benefit by mechanically 
 squeezing the excess of blood out of the weakened vessels. 
 
 The author has relieved the chronic inflammation of a sprained ankle, by 
 ordering the foot and leg, covered with an oiled-silk stocking, to be enclosed 
 in a boot strong enough to support the pressure of quicksilver, which was 
 then poured into the boot. The effect is a pressure by the fluid metal on 
 the weak vessels, of one pound to the square inch, for every two inches of the 
 
RESPIRATION AND VOICE. 445 
 
 depth of metal above the part. A height of four or five inches gives the 
 relief expected. A much greater elevation would stop the circulation alto- 
 gether. No bandage can press with uniformity approaching to this action 
 of a fluid. 
 
 The effect of continued pressure, in removing tumours of various kinds, 
 is explicable on the same principle. The author doubts not that in such 
 cases, pressure properly managed, would prove a more valuable remedy than 
 is at present generally supposed. The elastic steel half-hoop with one 
 cushion before and another behind, lately introduced for the relief of hernia, 
 affords an admirable mode, in certain cases, of producing a uniform pressure 
 of the nature spoken of, and of any desired force. 
 
 When a man stands in a bath, with the water up to his chin, there is a 
 pressure of the water upon his body, proportioned everywhere to the depth, 
 (see page 130.) This pressure must produce a considerable effect on the 
 blood-vessels of the lower parts of the body. We see in this that a bath 
 must propel the blood from all other parts of the body, towards the cavity 
 of the chest, which the pressure cannot reach. It is this effect which in 
 part causes the feeling of thoracic oppression experienced by persons on 
 first plunging into water, which feeling is usually attributed altogether to 
 the cold. 
 
 The old practice of placing a patient in a pit, and surrounding his body 
 with earth and sand, must have had a mechanical action of the kind now 
 contemplated, in addition to any other influence. 
 
 Transfusion of blood from a vein of a healthy person into that of one 
 fainting or dying from haemorrhage, is an operation the converse of some of 
 those mentioned above. It has been frequently performed with success. 
 The cases to which it seems best fitted, are those of flooding after parturition, 
 and of wound ; and there can be no doubt that many of the lives lost from 
 these so frequently recurring causes, might be saved by its adoption. The 
 blood to be injected is received into a vessel, as in common bleeding, from 
 which vessel, by a fit syringe, (to be described in a future page,) it is trans- 
 ferred, as it flows, into an opened vein of the patient. The admission of air 
 with the blood would be fatal, and has therefore to be most carefully guarded 
 against. The last interesting report upon this subject is that of Dr. Blundell, 
 in his Physiological Essays. 
 
 RESPIRATION AND VOICE, 
 
 Tlie doctrines of fluidity ', illustrating and illustrated by the animal reppi- 
 
 ration and voice. 
 
 As the motion of a windmill depends altogether on the breeze to which its 
 vanes are exposed, so does the motion and the life of that most wonderful of 
 structures, the animal body, depend on the supply of air for its breathing. 
 If this supply be withheld but for a few moments, painful convulsions ensue; 
 and if for a still longer period, the body, however perfect and beautiful, is 
 made a lifeless corpse, soon to putrify and be decomposed. 
 
 The mechanical nature of air, as to its lightness, elasticity, &c., and the 
 fact of its forming an ocean around the earth of about fifty miles high, are 
 now well understood, and have been fully explained under pneumatics ; but 
 the precise nature of its life-sustaining action has yet to be elucidated by 
 farther research of chemists and physiologists. Thus far, however, we 
 
446 FLUIDITY IN RELATION TO ANIMALS. 
 
 know that the ingredient called oxygen, constituting a fifth of the atmo- 
 sphere, is the most essential part that air, by being breathed once is rendered 
 unfit for farther respiration at the time and that a man requires about a 
 gallon per minute. The enterprising Mr. Spalding, who introduced the use 
 of the diving-bell, descended for the last time with a companion on the coast 
 of Ireland, when owing to the signal cord becoming entangled round the 
 great rope of the bell, which had turned in descending, he could not make 
 their want of air known above, and both were found dead when the bell was 
 drawn up soon after, although the water had not touched them. Of a hun- 
 dred and forty-six Englishmen, who, in the year, 1750, were made prisoners 
 at Calcutta, and where thrown into the close dungeon, since called the black- 
 hole, only twenty-three survived the few hours of their confinement, and one 
 of the most appalling recitals of human suffering existing on record, is what 
 these persons had afterwards to make. 
 
 We know generally of the life-supporting action of air, that it consists in 
 some change operated by the air on the blood; and we know that the func- 
 tion of respiration has merely to bring air and blood together in the cavity of 
 the chest, that this change may take place. The blood while in the chest is 
 moving along a part of its circle, in vessels of extreme minuteness and thin- 
 ness, and the air at each inspiration rushes in among these, so that every 
 globule of blood passes within its influence. And the blood, which, after 
 having served the purposes of the body, arrives at this part of its course, 
 black and impure, immediately after its exposure to the air enters the left 
 chamber of the heart, of a beautiful scarlet colour, and thence departs to 
 carry new life to the general system. . 
 
 The minute vessels through which the circulating blood is strained in the 
 chest, do not hang loose in the cavity, but are supported by running through 
 spungy masses, called the lungs, which consist chiefly of these vessels and 
 of thin membrane formed into cells. The cells at every inspiration, receive 
 fresh air through the cartilaginous windpipe which branches into them, and 
 at every inspiration they return the changed air by the same channels to the 
 atmosphere. The lungs of a child, before birth, are perfectly collapsed, or 
 without the least air in their structure, and hence are dense enough to sink 
 in water ; but after breathing, they retain a portion of air, and will float. 
 This fact has been accounted a test of whether a child had been born dead 
 or alive; but because putrefaction, &c., will cause air to be in lungs which 
 have never breathed, the criterion may be fallacious. 
 
 The chest is a large cavity, of form approaching to that of a common bee- 
 hive, bounded laterally by the encircling ribs, behind the spine, and before 
 by the sternum, and divided below, from the abdomen or belly, by a strong 
 membranous and muscular expansion, called the diaphragm. The ribs, in 
 the natural state, hang obliquely downwards from their posterior attachments, 
 and on being raised in front, they widen or increase the size of the cavity, as 
 already explained at p. 403. The cavity is farther enlarged by the descent of 
 the diaphragm, which may be regarded as both the floor of the chest and the 
 roof of the abdomen, and which being convex upwards like a dome, by con- 
 tracting itself to a more flat condition, sinks out of, and enlarges the chest, 
 while it descends into, and diminishes the abdomen, or at least causes 
 protrusion of its sides. 
 
 Now on the chest being enlarged by the rising of the ribs and descent of 
 the diaphragm, or by either singly, the air rushes into it through the mouth 
 and windpipe, exactly as air rushes into a common bellows through its pipe, 
 when the valve is shut and the two boards are drawn apart; and air is again 
 
RESPIRATION AND VOICE., 447 
 
 expelled from the lungs by the contraction of the chest, as from the bellows 
 by the approximation of the boards. Into both cavities air enter, because 
 with the enlarging dimensions, the air which was within dilates, and 
 becomes less powerfully tense or resisting against the external pressure of 
 the atmosphere, and so allows more air to rush in to restore the equilibrium. 
 The air is expelled again by the contraction of the cavities, because by being 
 compressed, its elastic force or tension becomes greater than that of the ex- 
 ternal air, which it therefore easily repels, and so in part escapes. By im- 
 mersing in water an india rubber bottle, and then opening and shutting it, 
 the entrance and exit of fluid in this manner may be rendered very apparent. 
 
 That the air admitted into the chest should have the fullest action on the 
 blood passing there, it was necessary that the spongy mass of lungs in which 
 the blood-vessels ramify, should occupy the whole of the cavity, and be 
 equally distributed. Now while the equable distribution is effected by the 
 uniform elasticity or resilience which belongs to the structure of the lung the 
 complete filling of the cavity is obtained, not by general attachment between 
 the lungs and the ribs or sides of the chest, as might be expected, but by the 
 following means, equally simple, and still more perfect. The spongy mass 
 of the lungs, is completely covered by a strong adherent membrane, called the 
 pleura, through which air cannot pass ; between this membrane and a similar 
 lining of the chest there is no air or empty space, and therefore in the raising 
 and falling of the ribs during respiration, this membrane remains always in 
 contact with the lining of the ribs, just as a bladder put into a bellows as a 
 lining, with its mouth secured around the nozzel, is filled and emptied, and 
 remains in contact with the interior of the bellows, in all the states of dilata- 
 tion, as if there were attachments in a thousand places. This construction 
 allows the lungs to have a singular freedom of play during all the motions 
 of the body ; a freedom further provided for by their being divided into five 
 portions or lobes, which slide upon one another : of these, three occupy the 
 right side of the chest, and two with the heart occupy the left. The right 
 and left sides of the chest are rendered cavities quite distinct from each other 
 by the mediastinum, a strong membranous partition. The mechanical dis- 
 position of the contents of the chest, as now described, is productive of 
 certain consequences which it is important to understand; for instance, 
 
 If a wound be made in one side of the chest so as to admit air, the lungs of 
 that side collapse in obedience to their weight and elasticity ; and as the chest 
 afterwards enlarges and diminishes in respiration, air more easily enters and 
 leaves the space around the collapsed lung, through the wound, than it can 
 enter or leave the lung itself through the windpipe \ because, in the first case, 
 it has no force to overcome, and in the second, the elasticity, weight and 
 inertia of the lung oppose Thus the lungs of the wounded side become col- 
 lapsed and useless. If such a wound, therefore, were made in both sides of 
 the chest at once, even without hurting any part within, the person, unless 
 assisted, would die of suffocation. The kind of resistance required in such a 
 case, is first to press the ribs down so as to empty the chest of air as much 
 as possible, and then to keep the wounds close or covered while the ribs rise 
 again ; the air, of course, will then enter by the natural road, the only one left, 
 to fill the chest, and will distend the lung, and reach the blood in the pulmo- 
 nary vessels as usual. Then by straining with the muscles or the chest, as 
 in the action of blowing, and at the same time preventing the breath from 
 escaping by the mouth or nose, all the air which had entered by any wound 
 in the chest may be expelled. In Benjamin Bell's system of surgery, which 
 was long the manual practitioners, counsel on this head was given the very 
 
448 FLUIP1TY IN RELATION TO ANIMALS. 
 
 contrary of that required, and, of course, any patient treated according to it 
 must have been lost. 
 
 In cases of dangerous haemorrhage from a lung, caused by a wound in the 
 side, the proper practice is to allow the lung to collapse, as now explained, 
 that the haemorrhage may be checked : and when the danger is past, the 
 treatment above described is to be adopted to restore the natural play of the 
 lung. Life may be supported for a long time by the lung in one side of the 
 chest. 
 
 In cases of haemoptysis, or spontaneous bleeding from the lungs, a disease 
 so often fatal, life might sometime be "saved or prolonged by making an 
 opening between two of the ribs, and allowing the lung to collapse. The 
 affected lung is often pointed out by the circumstances ; and the opening, 
 when properly made, would be no more dangerous than in the case where, 
 by a similar opening, water or pus is discharged from the chest. 
 
 The same operation has been tried as a forlorn hope in pulmonary con- 
 sumption. This disease is often limited to the lung of one side, and as the 
 alternate stretching and collapse of the deceased lung during respiration, 
 together with the contact of the air, powerfully prevent an ulcer there from 
 healing, or inflammation from subsiding, a new chance of recovery is given 
 by allowing the deceased lung to collapse and remain at rest. Some cases 
 are recorded where cure is said to have followed this operation, and certainly 
 where the circumstances are favourable for it, and where death must ensue 
 unless it can save, it is worth trying. 
 
 When ribs are fractured, it is the practice to put a bandage round the chest, 
 so as for the time to prevent almost entirely the respiratory motion of the 
 ribs, and the breathing is then performed chiefly by the rising and fulling of 
 the diaphragm or floor of the chest as above described. Although a person 
 with broken ribs is wisely for a time subjected to the unnatural restraint, it 
 is surely the height of folly to inflict the same on healthy beings, as is yet, 
 however, so commonly done among young women, and often to the destruc- 
 tion of their health, by the fashion of bracing the body in tight stays. 
 
 The force of a healthy chest's action in blowing is equal, as stated in last 
 section, to be about one pound on the inch of its surface ; that is to say, the 
 chest can condense its contained air with that force, and can, therefore, blow, 
 through a tube, the mouth of which is two feet under the surface of the 
 water. In the opposite action of sucking or drawing in air, the power, is 
 nearly the same. In both actions it is possible to use the cavity of the 
 mouth separately from that of the chest ; and the mouth, being smaller, with 
 stronger muscles about it in proportion to its size, it can act more strongly. 
 Some men can suck with the mouth so as to make nearly a perfect vacuum, 
 or to lift water nearly thirty feet. An expert operator with the blow-pipe 
 can keep up an uninterrupted blast by shutting the mouth behind, while he 
 inhales, and replenishing it as is required in the intervals. 
 
 In coughing, the glottis or top of the windpipe, by a curious sympathy 
 of parts, is first closed for an instant, during which the chest is compressing 
 and condensing its contained air, and on the glottis being then opened, a 
 slight explosion as it were, of the compressed air takes place, and blows out 
 any irritating matter that may be in the air-passages ; just, only with 
 inferior force, as the burst from the chamber of an air-gun discharges its 
 bullet. This shutting of the glottis to allow the compression of the air, and 
 the subsequent opening to allow the discharge, may recur at every minute 
 intervals, and many times for one fill of the chest, as is instanced in hooping- 
 cough. The action of coughing is often produced by irritation from a cause 
 
RESPIRATION AND VOICE. 449 
 
 which cannot be removed by coughing, as inflammation of the chest, or 
 tubercles ; or even by irritation in a distant part, as when children are 
 teething, or when the stomach is overloaded. 
 
 Sneezing is a phenomenon resembling cough, only the chest empties itself 
 at one throe, and chiefly through the nose, instead of through the mouth, 
 as in coughing. The irritation that produces sneezing is generally in the 
 nose; but, as in the case of cough, sneezing may occur from distant sympa- 
 thies : witness that from worms in the bowels. 
 
 Laughing consists of quickly repeated expulsions of air from the chest, 
 the glottis being at the time in a condition to produce voice ; but there is not 
 between the gusts, as in coughing, complete closure of the glottis. 
 
 Crying differs from laughing almost solely in the circumstances of the 
 intervals between the gusts of air being longer. Children laugh and cry 
 in the same breath, and it is often difficult to mark the moment of change. 
 
 Hiccup is the sudden stopping, by a closure of the glottis, of a strong 
 inspiration at its commencement. If the inspiratory effort be afterwards 
 continued, it may cause air from the atmosphere, or half-digested food from 
 the stomach, to enter the oesophagus. 
 
 In straining to lift weights, or to make any powerful effort, the air is shut 
 up in the lungs, that there may be steadiness and firmness of the person. 
 At such a time, by the compression and condensation of air around the heart 
 and large blood-vessels, the blood is determined violently outwards from the 
 chest, and often rises to the head, with force that produces giddiness, or even 
 apoplexy, and the eye will sometimes become suddenly bloodshot from a 
 small vessel giving way; and leech-bites will break out afresh. The force 
 of this pressure outwards is measured, as already stated, by a column of 
 about two feet of blood ; and this is, therefore, the measure of the additional 
 arterial and venous tension in the body generally. 
 
 Suffocation is the name given to what happens when the supply of air to 
 the lungs is, any way, prevented. The blood, not then refreshed by the 
 approach of the air, rises in the brain unfit for its purpose, and confusion of 
 thought is immediately produced, soon followed by convulsion and death. 
 
 When this happens from mechanical obstruction at the narrow entrance 
 of the windpipe, as in croup, by the tenacious films thrown off from the 
 inflamed lining of the air-passages, life may be saved by making a new 
 entrance for air through the windpipe, lower down in the neck, and keeping 
 it free by a little tube inserted, until the obstruction above be removed. 
 Where children die with croup, it is frequently not from the violence of the 
 constitutional disease, but from detached films thus accidentally sticking in 
 the narrow entrance of the air-passage. 
 
 In the cases of strangling and hanging, the tight binding of the rope or 
 ligature crushes inwards the cartilaginous rings of the windpipe, and shuts 
 the air-passage. It may also cause apoplexy, by arresting the passage of 
 blood to and from the head : and there may be dislocation of the cervical 
 vertebrae of the spine. 
 
 In drowning, communication with the atmosphere is cut off altogether 
 by the supernatent water. If, during submersion, the chest expands, it can 
 receive water only, instead of air. The nerves and muscles, however, at the 
 entrance of the windpipe, are so irritable, as to be immediately excited by 
 the contact of any unusual matter, and, for a considerable time, they keep 
 the passage shut against the liquid seeking entrance. It is partly on this 
 account that the body of a person, after submersion in water and apparent 
 death, may, often, if recovered within a moderate time, be restored to life. 
 
 29 
 
450 FLUIDITY IN RELATION TO ANIMALS. 
 
 The apparatus of the Humane Society for the recovery of persons appa- 
 rently drowned, includes a bellows for producing artificial respiration. This 
 bellows resembles a common bellows, except that its flap or valve, instead 
 of being internal, is external like a large flute-key, and has a spring to close 
 it, obedient to the finger of the operator. The bellows receives its charge 
 of fresh air on being expanded, while the valve is open ; it sends the charge 
 into the lungs on being compressed while the valve is shut ; it withdraws 
 the charge again on being expanded with the valve shut ; and the impure 
 air is thrown out to the atmosphere on its being compressed with the valve 
 open. These changes, repeated and continued, produce the artificial respi- 
 ration required. It is most important to remark here, that if air be injected 
 into the lungs, either in too large quantity or very suddenly, instead of 
 recalling or sustaining life, it is as certain a means of killing as a dagger 
 driven through the heart. This truth has been but lately known, and igno- 
 rance of it has probably decided the fate of many persons, treated with a 
 view to recovery after submersion. The operator should reflect that he is 
 dilating the delicate air-cells of the lungs with the force of an hydraulic 
 press; and that if he does so very suddenly, although to a small extent, he 
 still may rupture many small blood-vessels, before they can empty them- 
 selves so as to yield. In a bellows for the purpose of artificial respiration, 
 there should be the means of checking its opening to suit the capacity of the 
 patient's chest, and there should be a cock in the pipe or nozzle to regulate 
 the speed of the passing air. 
 
 In addition to the artificial breathing for the recovery of suspended ani- 
 mation, it is often necessary to restore natural warmth of the body, to rub 
 the limbs in aid of the circulation, to administer stimulants by the mouth, 
 to excite by galvanism, &c. 
 
 It seems to be an error, and probably often a fatal error, in the present 
 mode of treating persons apparently drowned, to use cold instead of warm 
 air for the artificial respiration. Thus while the important object of restoring 
 the temperature of life is sought by all external means, the great inconsis- 
 tency is committed of blowing cold air upon an internal surface of the body 
 more extensive than the external ; and until that reciprocal action of the 
 air and blood begins, which constitutes the slow combustion of natural 
 respiration, every bellows-full of cold air admitted, brings back with it a 
 portion of the remaining central warmth, and may thus chill so as to make 
 the recovery impossible : as a fire which has fallen very low may be imme- 
 diately extinguished by the same action of a bellows, which a little before 
 would have made it blaze. Air might easily be heated for the purpose of 
 respiration by pouring boiling water into a vessel containing it, and then 
 connecting the bellows with that vessel by a fit pipe, or by making the 
 bellows draw through a pipe partially immersed in hot water/: a quart of 
 boiling water has heat enough in it to warm many gallons of air to blood- 
 heat. This plan would not only avoid the mischiefs arising from the cold 
 air, but by affording the means of applying warmth even higher than that 
 of life, it might probably furnish the most useful of all stimulants to the 
 parts about the heart. A healthy man can breathe with impunity air that 
 is much hotter than boiling water. 
 
 Late physiological investigations have shown that the breathing, or me- 
 chanical action of the chest in respiration, is so dependent upon the influence 
 of the brain, as to be disturbed and even stopped when the brain is embar- 
 rassed ] they have shown, farther, that the action of the heart is dependent on 
 the breathing, but not on the brain, except as the cause of the breathing 
 
THE VOICE AND SPEECH. 451 
 
 for that respiration kept up artificially, will preserve the circulation and the 
 life for a considerable time after the brain has altogether ceased to act, or 
 even has been removed from the body. Now, some interesting experiments 
 of Mr. Brodie have proved, that certain poisons are fatal, merely because 
 they suspend for a time the action of the brain through which suspension, 
 the actions, first of the chest, and then of the heart, cease, and death ensues : 
 but that in such cases, if the action of the chest be maintained artificially, 
 tho circulation and life of the body will be for a time continued, and the 
 brain may gradually recover from the effect of the poison, so as to resume 
 its office. Thus certain cases of poisoning, which formerly would have been 
 fatal may now end in recovery. 
 
 An important application of this discovery is to the treatment of cases of 
 convulsion, particularly those occurring from teething or other irritations in 
 infancy. The respiration ceases in these cases often only because the action 
 of the brain is suspended ; and if the respiration be continued artificially, the 
 circulation and life will also continue for a time, during which the brain may 
 recover itself, either spontaneously, or in consequence of remedies employed, 
 and life may be saved. The chest of an infant is comparatively so small, 
 that it may be filled from the mouth and windpipe of a grown person, with 
 air which has not descended to that person's lungs, and therefore has not been 
 rendered unfit by respiration; and on the little chest being afterwards com- 
 pressed by the hand, the air will return. The air may be blown directly in- 
 to the child's mouth through a thin handkerchief laid over the mouth, or it 
 may pass through a tube inserted into the nostril or treachea : to prevent it 
 from passing into the stomach, the larynx must be pressed against the oeso- 
 phagus during its entrance. Let all who try this remedy, keep present to 
 their minds the danger of inflating too much. 
 
 Any medicated air is generally inhaled by a patient from an oiled-silk, 
 or other air-tight bag, or from a light gasometer (see page 211.) When the 
 compound nature of our atmosphere was first discovered, great advantages 
 were anticipated to medicine from the use of pneumatic or aerial mixtures. 
 These expectations have not been realized, but the subject still remains 
 highly deserving of research. 
 
 THE VOICE AND SPEECH. 
 
 The chest and air-passages, with certain additional parts, constitute the 
 organs of voice and speech. 
 
 An inquirer into the constitution of the universe around him, meets with 
 few things calculated more to surprise him than th^t faculty in the human 
 mind by which it can associate the ideas of objects with any arbitrary signs, 
 so closely that the ideas are afterwards excited by the signs almost as vividly 
 as by the objects themselves. The inhabitants of China, for instance, have 
 contrived many thousand grotesque characters, and determined what object 
 each should recall ; when a Chinese by study becomes familiar with these, 
 he may have his bodily eye poring over pages of crooked and unseemly 
 scratches, while his mental eye through them sees only a pleasing succession 
 of the most beautiful imagery of nature; and the characters may be rendered 
 intelligible to the deaf and dumb man, as well as to him who speaks ; and 
 they serve as a media of thought and communication through many provinces 
 and countries of which the spoken languages have no common resemblance. 
 
452 FLUIDITY IN RELATION TO ANIMALS. 
 
 But if the ready resemblance of visible signs be wonderful, which have 
 a permanent existence, and which often may have some resemblance in form 
 to the things signified, how much more wonderful is it that an audible sign, 
 that is, a passing sound or fugitive breath, called by man a word, should 
 serve as well; and that by a succession of mere sounds, having so little 
 natural connection with the things signified, that they are totally different in 
 different countries, and are changing with fashion from age to age, any train 
 of thought may be made to pass through the minds of an audience, so as to 
 excite and to leave impressions almost as vivid as from realities ! Such, 
 however, is the fact, and it is greatly owing to this and to a correspondent 
 faculty of producing at will a sufficient number of distinguishable sounds, 
 that man owes his elevation above the brutes of the field. His godlike 
 powers of intellect would have remained dormant and unknown, had he 
 wanted the faculty of comparing invisible thoughts with those of his fellow 
 men, and of arranging and recording them by means of signs. Written 
 language is a double remove from the objects themselves, being visible signs 
 not of things, but of the audible signs. 
 
 The admirable apparatus by which man is enabled to produce a sufficient 
 variety of sounds to answer his purposes, passes generally under the denomi- 
 nation of the organs of speech; because the act of using sounds which have 
 meanings assigned to them is called speech. It consists of the chest for con- 
 taining air; of the larynx or cartilaginous box, with its narrow aperture called 
 the glottis, at the top of the windpipe, for producing the voice, and varying 
 its pitch; and of the short tube of the mouth, with the tongue and lips, for 
 farther modifying the voice. 
 
 In the chapter on acoustics, we explained that sound is the name given to 
 the effect produced upon the ear by certain tremblings conveyed to it gener- 
 ally through the medium of the air; and we explained how air, forced from 
 the human lungs through the opening at the top of the windpipe, causes the 
 elastic lips of that opening to vibrate, and to excite the tremblings. We have 
 now to show that this sound, in passing forward from the top of the wind- 
 pipe, may be modified at the will of the individual, in a great variety of ways 
 a variety, however, which is still very simple. 
 
 The modifications of voice easily made, and easily distinguishable by the 
 ear, and therefore fit elements of language, are about fifty in number; but 
 no single language contains more than about half of them. They are divisi- 
 ble into two very distinct and nearly equal classes, called, for reasons now to 
 be explained, vowels and consonants. 
 
 Those of the first class are the simple voices issuing through the open mouth, 
 and influenced only by the degrees in which the mouth is opened and elon- 
 gated. They may be continued as long as there is breath to issue from the 
 chest, and it is for this reason that they are named vowels or calling sounds, 
 The Roman letters, A, E, I, 0, U, as generally pronounced on the continent 
 of Europe, and of which the sounds correspond nearly to those aw, a, e, o y 
 and oo } of English writing, indicate the most easily distinguishable vowels. 
 Sounds passing through the mouth in its most natural state of relaxation, is 
 heard as the modifications expressed by the Italian E, (or the a of the English 
 wdrd care ;) if the mouth be then widened, the sound becomes A (as in the 
 English word bar ;) if the mouth be narrowed, we hear the I (or the e of the 
 English tedious ;) if the mouth be elongated and at the same time widened, 
 we hear the (as in the English word bore;] and if more elongated but 
 narrowed, we hear the U (as in the English rude.) The possible number of 
 vowels, however, is as great as the possible degrees in which the dimensions 
 
THE VOICE AND SPEECH. 453 
 
 of the mouth may be altered. About twenty of them are sufficiently distin- 
 guishable, but few languages comprehend more than twelve. Modern art 
 can produce the vowel sounds mechanically by means of tubes of certain 
 dimensions. 
 
 The alphabets of Europe are very faulty in not all using the same charac- 
 ters for the same sounds, and in not having, according to the true intent of 
 an alphabet, a character for each distinct sound. In English, one letter is 
 used for several sounds, as A, in water, far, fat, fate, where it indicates four 
 sounds perfectly distinct. In repeating the English alphabet, the A is pro- 
 nounced as the broad E of the Italians and of continental Europe, and the E 
 as the I; and the I (in tide, for instance,) as the dipthong AI of more cor- 
 rect alphabets ; and the U (in mw.se,) as the dipthong IU. In consequence 
 of the changes which the English have made in the meaning of the Roman 
 letters, they now experience increased difficulty in learning modern conti- 
 nental languages; and their own pronunciation of the ancient languages, to 
 all but themselves, is ridiculous, and almost unintelligible. The same cause 
 renders the pronunciation of English difficult to foreigners, and thus restricts 
 much in other countries, the cultivation of English literature. 
 
 To explain the second class of the modifications of sound, called conso- 
 nants, we may remark, that while any continued or vowel sound'is passing 
 through the mouth, if it be interrupted, whether by a complete closure of 
 the mouth, or only by an approximation of parts, the effect on the ear of a 
 listener is very remarkable, and is so exceedingly different, according to the 
 situation in the mouth where the interruption occurs, and to the manner in 
 which it occurs, that many most distinct modifications thence arise. Thus 
 any continued sound, as A, if arrested by a closure of the mouth at the ex- 
 ternal confine or lips, is heard to terminate with the modification which we 
 choose to express by the letter P, that is, the syllable AP has been pro- 
 nounced ; but if, under similar circumstances, the closure be made towards 
 the back of the mouth by the tongue rising against the palate, we hear the 
 modification expressed by the letter K, and the syllable AK has been pro- 
 nounced ; and if the closure be made in the middle of the mouth by the tip 
 of the tongue rising against the roof, the sound expressed by T, is produced 
 and the syllable AT is heard and so of others. It is to be remarked, also, 
 that the ear is equally sensible of the peculiarities whether the closure pre- 
 cedes the continued sound or follows it : that is to say, whether the syllables 
 pronounced are as above, AP, AT, AK, or on the contrary, PA, TA, KA. 
 The modifications of which we are now speaking, appear, then, not so 
 much to be sounds, as distinguishable manners of beginning and ending 
 sounds j and it is because they are thus only perceivable in connection with 
 .vocal sounds that they are called consonants. 
 
 Now in the mouth, considered as a vocal tube, there are three situations, 
 in which interruption of voice or breath may most conveniently be made, 
 and there are six modes of making it at each , so that eighteen distinct inter- 
 ruptive modifications or consonants hence arise. These we shall now describe. 
 
 The three great oral positions, as they may be called, are, 
 
 1st. At the external confine of the mouth, or lips, giving the labial arti- 
 culations, of which is an example. 
 
 2d. In the middle of the mouth, where the tip of the tongue approaches 
 the palate behind the teeth, producing the palatal articulations, of which 
 T is an example. 
 
454 FLUIDITY IN RELATION TO ANIMALS. 
 
 3d. Near the back of the mouth, where the body of the mouth approaches 
 the palate, giving th guttural articulations, of which K is an example. 
 
 And the six modes in which the voice or breath may be affected in passing 
 through each of the three positions of the mouth, are, 
 
 1st. A sudden and complete stoppage, producing what may be called a 
 mute articulation : viz., P, in the labial position; T, in the palatal ; and K, 
 in the guttural. (See here the general table of articulations next page 
 which table may be considered as representing the tube of the mouth, with 
 the letters so placed as to show in what situations in the mouth the sounds 
 represented by them are severally produced.) A mute may also be made by 
 stopping the breath exactly at the teeth, viz., a dental-mute; but it is 
 hardly distinguishable from the palatal mute, produced just behind it, and 
 being less perfect is not used. Some awkward speakers, substituting it for 
 the proper mute, are said to speak thick. 
 
 2d. A sudden shutting, as in the last case, but the voice being allowed 
 to continue until the part of the mouth behind the closure be distended with 
 air. This produces the semi-mules, B, D, and G, (as heard in the syllables 
 AB, AD, AG,) for the three positions. There might be a dental half-mute, 
 but it is not more used than the dental-mute, and for the same reasons. If 
 the sides of the tongue be depressed, after it has taken the position required 
 for T or D, the sound L is produced j and the letter is, in the table, placed 
 below D, although the sound, from being continuable, is not in any sense a 
 mute, 
 
 3d. The positions closed as for the mutes, while sound is allowed to pass 
 by the nose. Thus arise the semi-voiocls or nasals, M, N, NO, for the 
 three positions, NG (as in Icing) is a simple sound, although our imperfect 
 alphabet has no single letter for it. The nasal sound of the French language, 
 which gives it so great a peculiarity, approximates to the English NG, but 
 differs from it in sound being allowed to pass by the mouth, as well as by 
 the nose. It is pointed at by the small n in the table, and like the other 
 sounds which do not occur in the English language, is here pointed in the 
 Italic character. 
 
 4th. Breath only (or whisper) allowed to pass at the three oral positions, 
 nearly closed Hence come the sounds which we call aspirates, viz., F, for 
 the labial position, TH and S, for the palatal, and CH (heard in the Scottish 
 word loch,) for the guttural ; the TH and CH, are simple sounds, although 
 each expressed in Britain by two letters. The TH is heard in the word 
 lath, and is the sound expressed by the single letter 6 of the Greeks. The 
 CH is heard in the German icli, and is the % of the Greeks. The soft aspi- 
 rate TH is more easily made by pressing the tongue gently against the 
 teeth, and allowing the breath to pass all round, than by the true palatal 
 approximation of parts, and the soft dental aspirate, therefore, is used in 
 preference to the palatal. The letter S is ( the hard palatal aspirate, and 
 differs from the soft palatal aspirate TH, in the breath being made to 
 issue with greater force, and only by a narrow space over the centre of a 
 rigid tongue, instead of on all sides of a soft tongue, as for TH. French 
 people on first attempting to pronounce TH, substitute for it the D, or the 
 S, or the Z (which is nearly related to S, as explained below.) The author 
 has found it easy to enable them to pronounce the TH at once, and perfectly, 
 by explaining its nature as above. If we depress the sides of the tongue while 
 pronouncing S, we produce the simple sound expressed by the English double 
 
THE VOICE AND SPEECH. 
 
 455 
 
 letter S H, just as by depressing the sides, of the tongue while making D, 
 we produce L. 
 
 5th. Using voice in the same manner as breath or whisper is used for the 
 aspirates. This produces the sound called vocal aspirates, viz., V, TH, Z, 
 J, and GIL TH vocal aspirate, is heard in lathe, as contrasted with the 
 simple aspirate in bath; Z comes from the S position, only with sound 
 instead of breath ; SH pronounced with voice, becomes the J of the French 
 in the word je, or the sound heard in the middle of the English word vision. 
 GH is a simple sound in German, but not in English. 
 
 Table of Articulations. 
 
 i 
 1 ' S 
 
 P 
 
 B 
 
 M 
 
 F 
 
 Y 
 
 pr 
 
 . 
 
 . 
 
 . 
 
 th 
 
 th 
 
 , 
 
 T 
 
 D 
 
 N 
 
 S 
 
 Z 
 
 R 
 
 
 
 L 
 
 
 
 sh 
 
 J 
 
 
 
 K 
 
 G 
 
 ng 
 
 ch 
 
 ah 
 
 ghr 
 
 
 
 n 
 
 H 
 
 
 
 LABIAL. 
 
 Dental. 
 PALATAL. 
 
 {with the edges of 
 the tongue depressed. 
 
 GUTTURAL. 
 
 6th. Shaking the approaching parts in the three positions. We thus 
 make vibratory sounds, of which the middle position gives the common R, 
 the only one of them used in England. Some bad .speakers of English, how- 
 ever, make the labial vibratory by shaking the P in such words i\ property ; 
 and many use the guttural, which is the burr of Northumberland, and the 
 common affectation in Parisian speech, termed parler gras, or yrasscyer. 
 
 Additional Remarks. 
 
 The sound of H is an aspirate produced even behind the situation of the 
 guttural aspirate ch ; it is, indeed, merely a forcible passing of the breath 
 through the very back part of the mouth or throat. 
 
456 FLUIDITY IN RELATION TO ANIMALS. 
 
 CH, in such words fts chain, means T before sh. 
 
 J, as heard in the English name John, is a compound sound, viz., D be- 
 fore the simple J of the table, which is the S of vision. 
 
 LL. The liquid or double LL of the French as heard in the word jo?7fc, 
 is merely L with the letter Y begun to be pronounced after it. It is heard 
 in the English words billiard and halyard, and would be their terminating 
 liquid were the syllable ard not pronounced. The double LL of the Welch, 
 as in the name of Lloyd, has the first L pronounced as an aspirate, that is, 
 as a whisper, and the second in the ordinary way. 
 
 GN. The soft G N of the Italians and French, is the English N with Y 
 begun to be pronounced after it. It is heard in our word lanyard; and in 
 the Italian words pegnio bagnio ; and in the French word craignent. 
 
 C, in English, stands always either for" S or K, as in the words certain 
 and car, and has no sound proper to itself. 
 
 Q, in English, expresses the sound of the letter K, with U following it : 
 and yet, uselessly, U is always written after Q. 
 
 X, in English, means either KS, as in the word axle, or GZ as in the word 
 example. 
 
 The consonants are best heard by sounding them with voice before them : 
 'that is to say, by making them rather terminate a syllable than begin it : 
 pronouncing B, I), G, thus eb, ed, eg, rather than their common alphabetical 
 names be, de, ge. 
 
 The labial sounds may be made either by the two lips, or by one lip and 
 the opposite teeth. F may be pronounced, for instance, by the lips only, or 
 by the lips and teeth : and some persons awkwardly make" it by the under 
 teeth and upper lip. 
 
 The letters Y and I, in most modern languages, stand for nearly the same 
 sound. In English, for instance, bullion and minion might be written 
 bullyon and mint/on, without suggesting a change of pronunciation. In the 
 words, yard, you, yes, &c., the Y is a short I, very closely joined to the 
 following sound. W is also thus a short U, as perceived in the words icar, 
 
 Wi &C. 
 
 The author believes the analysis of articulation to be the best basis for a 
 system of short-hand written characters. He has tried such a system, and 
 found it exceedingly convenient. 
 
 Lisping is chiefly the habitual substitution of the aspirate Til for the S 
 and SH. 
 
 Whispering is articulation without voice; that is to say, articulation while 
 breath only is passing. 
 
 Stuttering, stammering or hesitation of speech, are terms implying an 
 interrupted articulation, accompanied generally with more or less of strain- 
 ing and distortion of feature. It is remarkable with respect to this defect, 
 that when the present work was first published, scientific or regular medicine 
 had taught as yet no certain cure for it, although the frequent success of non- 
 professional, and often ignorant individuals, by a mode of treatment which 
 they solemnly bound their patients not to divulge, proved the cure, in certain 
 cases, to be both possible and not difficult. The author's attention had been 
 drawn to the subject some years before, by an interesting case submitted to 
 him of stuttering connected with other disease; and it was in analysing the 
 subject with a view to the treatment of that case, that he framed the analysis 
 of articulation contained in the preceding pages, and drew up a part of the 
 additional observations which are now to follow. A cure was obtained; 
 but as the case possessed a favourable peculiarity in the powerful mind of 
 
THE VOICE AND SPEECH. 457 
 
 the individual, to which the author attributed great importance, as he had 
 little leisure from his ordinary professional duties, to pursue the subject, or 
 to ascertain in what respects his plan might differ from that employed by 
 the most sudcessful of the practitioners who concealed their proceedings, he 
 gave his remarks in former editions of this work, merely as continued elu- 
 cidation of the subject of speech. He is now, however, enabled to state, that 
 his analysis has completely detected the nature of the morbid affection, and 
 that it directs simple and effectual means of relief. He declined meddling 
 with many cases offered to him after the original publication of his work, 
 from the impression that the cure in the instance mentioned above, was 
 owitg at least as much to the ingenuity and perseverance of the patient, as 
 to his suggestions, and, therefore, that his professional superintendence of 
 the discipline required for ordinary cases would demand care and attention 
 which he could not spare ; but subsequent experience has proved to him 
 that the business is altogether very simple and easy, and as regards children, 
 may be managed by any intelligent instructor of youth who chooses to de- 
 vote attention to it, while grown individuals will often be able to relieve 
 themselves by the study of the present section ; and he hopes that in very 
 few cases will the counsel of a person familiar with the anatomy and dictions 
 of the organs be found to fail. 
 
 Command over the organs of speech is acquired in the same way as over 
 all the other muscular organs of the body ; those, for instance, used in walk- 
 ing, skating, fencing, performing on musical instruments, &c. : that is to say, 
 at first, a distinct act of volition is required for every individual movement ; 
 but the law of association or habit of rendering the actions easier with each 
 successive repetition, they are at last formed -into connected tribes or trains, 
 which appear as obedient to a single wish as the separate elements originally 
 were. A child at first exerts as distinct and powerful a volition to pro- 
 nounce the syllable pa, as after some practice to double the syllable and 
 make it papa ; or after still more practice, to pronounce the longest and 
 hardest word of the language : nay, at last, where there is strong and 
 healthy power of association, complete sentences, and even rounded periods 
 of eloquence, are poured out like single words, the mind of the speaker seem- 
 ing at liberty, after each sentence or period is begun, to meditate and prepare 
 that which is next to follow. As the faculties of locomotion and of speech 
 are acquired in infancy and early childhood, persons no more recollect how 
 they gradually acquired them than how their limbs grew; but the progress, 
 described above, may be watched by any individual of mature years in his 
 own person, while he is learning such an art as that of playing on a musical 
 instrument. He will find, that at first, every finger which is moved to pro- 
 duce a note, obeys a distinct thought and volition ; that soon short trains of 
 connected notes become obedient to the will almost like a single note ; that 
 then by degrees, longer arid longer trains or passages become familiar, until 
 at last the instrument is obedient to the practised player, as voice is to the 
 singer or speech to the orator.' 
 
 There is a great original diversity among individuals as to their powers of 
 muscular association, and therefore, also, as to their aptitude for acquiring 
 the various faculties of which we have been speaking. Thus some children 
 walk well before a year, others require a much longer time, and some never 
 succeed perfectly until they have had lessons from the dancing-master or 
 drill serjeant. So, again, many people, by ear and imitation alone, learn 
 easily to play on musical instruments ; but others must begin by studying 
 the written notes, and the precise fingering by which each note is produced 
 
458 FLUIDITY IN RELATION TO ANIMALS. 
 
 on the instrument; and many, unless the notes be constantly before thein 
 cannot play at all. So, again, all persons may be said to learn to speak at 
 first by ear and imitation ; ^ut many grow up to a certain age with defects, 
 which judicious lessons from parents or other tutors are required to remove; 
 and there are some as stutterers, who, owing to a naturally weak or irregu- 
 lar association, or to some accident in early life, which has strongly affected 
 their nervous system, retain defects which no ordinary teaching can correct. 
 It appears, then, that an analysis and scale of articulate sounds, with minute 
 description of the organic actions required to produce them, like the scale 
 which we possess for music, in the gamut and rules for fingering, should 
 give nearly the same assistance to the speaker which the gamut gives \& the 
 player. The table and analysis contained in the preceding pages is intended 
 to supply this information. It is constructed from minute consideration of 
 the organs of speech while in action. It agrees in many respects with the 
 common grammatical divisions of elementary sounds, but in others it pursues 
 the analysis in a different way, and considerably farther. A person who 
 understands it well, will have, while, he speaks, an intelligent perception of 
 what he is doing, in addition to the parrot-like faculty of habit, or of repeat- 
 ing by rote, and will thus command any desired sound by two powers instead 
 of one. And, as a musician, when his musical memory fails him, finds 
 help by thinking of his written notes and their relation to his instrument, 
 so may a stutterer, when hesitating at any sound, receive benefit by thinking 
 of the letter which represents it, and of the position of the organs required 
 for that letter. Then, by frequent practice in making the particular combi- 
 nations of sound which are difficult to him, he may strengthen the useful 
 habit, and ultimately overcome his defect. 
 
 The most common case of stuttering, however, is not, as has been almost 
 universally believed, where the individual has a difficulty in respect to some 
 particular letter or articulation, by the disobedience of the parts of the mouth 
 which should form it to the will or power of association, but where the spas- 
 modic interruption occurs altogether behind or beyond the mouth, viz., in 
 the glottis, so as to affect all the articulations equally. To a person ignorant 
 of anatomy, and therefore knowing not what or where the glottis is, it may 
 be sufficient explanation to say, that it is the slit or narrow opening at the 
 top of the windpipe, by which the air passes, to and from the lungs, being 
 situated just behind the root of the tongue. It is that which is felt to close 
 suddenly in hiccup, arresting the ingress of air, and that which closes, to 
 prevent the egress of air from the chest of a person lifting a heavy weight or 
 making any straining exertion ; it is that, also, by the repeated shutting of 
 which a person divides the sound in pronouncing several times, in distinct 
 and rapid succession, any vowel, as o, o, o, o. Now the glottis, during 
 common speech, needs never to be closed, and an ordinary stutterer is instant- 
 ly cured, if, by having his attention properly directed to it, he can keep it open. 
 Had the edges or thin lips of the glottis been visible, like the external lips of 
 the mouth, the nature of stuttering would not so long have remained a mystery, 
 and the effort necessary to the cure would have been suggested to the most 
 careless observer : but because they were hidden, and professional men had 
 not detected in how far they were concerned, and the patient himself had 
 only a vague feeling of some difficulty, which, after straining, grimace, gesti- 
 culation, and sometimes almost general convulsions of the body, gave way, 
 the uncertainty with respect to the subject has remained. Even many per- 
 sons who, by attention and much labor, had overcome the defect in themselves, 
 as Demosthenes did, have not been able to describe to others the nature of 
 
THE VOICE AND SPEECH. 459 
 
 their efforts, so as to ensure imitation ; and evidently the quacks who have 
 succeeded in relieving many cases, but in many also have failed, or have 
 given only temporary relief, have not really understood what precise end in 
 the action of the organs their imperfect directions were accomplishing. 
 
 Now, a stutterer, understanding of anatomy only what is stated above, will 
 comprehend what he is to aim at, by being farther told, that when any con- 
 tinued sound is issuing from his mouth, as when he is humming a single note 
 or tune the glottis is necessarily open, and therefore, that when he chooses 
 to begin pronouncing or droning what we have already described to be the 
 simplest of vocal sounds, namely the vowel e, and in its less distinct modifi- 
 cation, as heard in the English word certain or in the French word que (to 
 do what at once no stutterer has difficulty,) he thereby opens the glottis, and 
 renders the pronunciation of any other sound easy: or if, when speaking or 
 reading, he joins his words together, nearly as a person joins them in singing, 
 (and this may be done without its being at all noted as a peculiarity of speech, 
 for many persons do it in their ordinary conversation,) the voice never stops, 
 the glottis never closes, and there is of course no stutter. The author has 
 given merely this explanation or lesson, with examples, to persons who be- 
 fore would have required half an hour to read a page, but who immediately 
 afterwards read it quite smoothly; and who then, on transferring the lesson 
 to the speech, by continued practice and attention, obtained the same facility 
 with respect to it. There are many persons not accounted peculiar in their 
 speech, who in seeking words to express themselves, or while coming to a 
 decision, often rest between their words on the simple sound of e mentioned 
 
 above, saying, for instance, hesitatingly, " e. . . . . . I e think 
 
 e I shall," the sound never ceasing until the end of a sentence, 
 
 however long it may be delayed. Now a stutterer, who, to open his glottis 
 at the begiuing of a phrase, or 'in the middle after any interruption, uses 
 such a sound, would not even at first be more remarkable than a drawling 
 speaker, and he would only require to drawl for a little while, until practice 
 facilitated his command of the other sounds. Although producing the simple 
 sound mentioned is a means of opening the glottis, which by stutterers is 
 found very generally to answer, there are cases in which such other means 
 may be more suitable, as the intelligent preceptor will soon discover. Were 
 it possible to divide the nerves of the muscles which close the glottis, with- 
 out at the same time destroying the faculty of producing voice, such an 
 operation would be an immediate and certain cure of stuttering. 
 
 While the spasmodic closure of the glottis, as above described, is the com- 
 mon cause of stuttering, there are also cases in which the cause is a spasmodic 
 prolongation of some of the aspirates or semivowel sounds, as of s m I, &c., 
 Fortunately, however, the substitution of the simple sound is equally the 
 cure for all. 
 
 While the cure of many stutterers has been accomplished by their own 
 efforts, after the study of what is written in this section, for others, and par- 
 ticularly for young people, the following have been found to be farther useful 
 rules or forms of direction ; and a commentary upon them making them 
 perfectly intelligible, would seem to comprehend all that can be communicated 
 upon the subject 1. Familiarize yourself with the idea of a continued sound 
 as of the roar of the sea or waterfall, or the note of an organ-pipe, and feel 
 that your speech is to be as uninterrupted. 2d. Then never stutter more, but 
 substitute always the simple continued sound for any threatened defect, and 
 rest upon it until power be felt to overcome the difficulty. 3d. Never repeat 
 words or syllables. 4. The simple sound must become the first syllable 
 
460 FLUIDITY IN RELATION TO ANIMALS. 
 
 (closely joined) of every difficult word, until the morbid habit be weakened. 
 The object of all these directions is to enable the patient, first to substitute 
 universally the drawl for the stutter, and then ; as soon as possible, to discard 
 the drawl too. 
 
 The view given above of the nature of stuttering and its cure, explains the 
 following facts^ which to many persons have hitherto appeared extraordinary. 
 Stutterers often can sing well, and without the least interruption j for the 
 tune being continued the glottis does not close. Many stutterers also can 
 read poetry well, or any declamatory composition, in which the uninterrupted 
 tone is almost as remarkable as jn singing. A person who draws a deep 
 breath before beginning to speak, as he cannot long retain the air, and the 
 glottis must be open to let it escape, is to a degree secured against the occur- 
 rence of stuttering. The secret remedy of an American quack, who years 
 ago got much money from Englishmen, was the direction thus to fill the chest 
 before beginning to speak. A Dr. McCormac, also, who published a work 
 on this subject, founded on the erroneous idea, that stuttering was an effort 
 to speak while inhaling air, instead of while exhaling, gave the same direc- 
 tion. The cause of stuttering being a weak and easily disturbed association 
 of certain muscular actions, we have the reason why any degree of anxiety 
 or dread as to speaking well, exceedingly increases the defect ; and why many 
 stutterers, who cannot make themselves intelligible in society, still, when 
 alone can speak and read as perfectly as any other person. This explains 
 also why many stutterers, who have gone to live for a time at the houses of 
 pretended curers of their defect, have felt themselves singularly relieved from 
 the moment of enteringthe house; because knowing that they were expected 
 to speak ill, they had no fear of disagreeably attracting attention, and therefore 
 had their powers much more at command. These persons, on returning to 
 the world, have generally stuttered as badly as ever, but many of the asserted 
 cures of stuttering, with certificates obtained from the parties at the time have 
 been of the nature now described. The cause of stuttering being so simple, 
 as above described, one rule given and explained may, in certain cases, in- 
 stantly cure the defect, however aggravated, as has been observed in not a 
 few instances; and this explains also why an ignorant pretender may occa- 
 sionally succeed in curing, by a given rule of which he knows not the reason, 
 and which he cannot modify to the peculiarities of other cases. The same 
 view of the subject explains why the speech of a stutterer has been correctly 
 compared to the escape of liquid from a bottle with a long, narrow neck, 
 coming " either by hurried gushes or not at all :" for when the glottis -is 
 once opened, and the stutterer feels that he has the power of utterance, he is 
 glad to hurry out as many words as he can, before the interruption recurs. 
 
 The study of the table of articulations lead to the immediate correction of 
 many minor defects in utterance, and is calculated to facilitate the acquire- 
 ment of foreign languages. A lisping person, for instance, is cured at once, 
 by being told that the tongue must not touch the teeth in pronouncing the 
 letter S; and a Frenchman who deems it impossible for him to pronounce 
 the English sound of TH, discovers that he cannot avoid doing so if he rests 
 his tongue softly against his teeth, opened a little, and then forces breath or 
 sound to pass between the tongue and teeth. 
 
 Several of the modern languages of Europe consist of nearly the same ele- 
 mentary or radical words, and differ among themselves chiefly by the preva- 
 lence in each of certain terminations and of one or other of the related and 
 convertible sounds classified in the analysis given above. A student, there- 
 fore, who, by analytical investigation, or considerable practice, has become 
 
THE VOICE AND SPEECH. 461 
 
 impressed with the peculiar genius of a language, may invent, or determine 
 by analogy, even "before minute study, the majority of those words belonging 
 to each which have sprung from a common origin. This remark is so true 
 with respect to the languages of Italy, Spain, Portugal, and even France, that 
 to persons familiar with them, they are at last listened to rather as the same 
 language spoken by different individuals, than as languages in themselves 
 different. 
 
 Ventriloquism is the name commonly given to the art by which an indi- 
 vidual can assume characters of voice and speech which are not natural to 
 him, and thus, although alone, can imitate closely a conversation hfila 
 between two or more persons. 
 
 The most remarkable diversity is obtained by speaking during inspiration, 
 instead of, as usual, during expiration. The voice so produced is more feeble 
 than the ordinary voice, and when accompanied by other circumstances fa- 
 vouring the illusion, it may suggest very completely the idea of a boy calling 
 from the bottom of a pit, or from the interior of a chimney, &c. An unsus- 
 pecting peasant may be tricked into unloading his hay-wagon by an expert 
 ventriloquist, who makes him believe that there is a poor child packed under 
 the heap and ready to be smothered there. 
 
 A person, by a little practice, may acquire the power of producing, with- 
 out the slightest apparent motion of the lips or countenance, all the articu- 
 lations except the labial, and of them the F, V and M may be tolerably imi- 
 tated by parts behind ; hence by avoiding words in which P and B occur, 
 such person may speak without visible movements of the organs, and if he 
 assume the attitude of a listener, he may make the deception of ventriloquism 
 complete. The idea which some authors have had (see Good's Study of 
 Medicine, <c.) that the articulations of the ventriloquist are not produced by 
 the tongue and mouth, as in common speech, is altogether an error. The art, 
 carried to a certain degree, is not very difficult, as any person may ascertain 
 who tries it after considering minutely the nature of common speech. 
 
 There are also striking varieties of voice producable by speaking with a 
 more acute or grave pitch than usual, and with different degrees of contrac- 
 tion of the mouth ; but these may be more properly called imitations than 
 ventriloquism. 
 
 The variety of effect in sound which the human organs are capable of pro- 
 ducing is truly surprising. There are adepts in the art of imitations, who 
 not only mimic the speech of all ages and condition of the human race, but 
 the songs of birds, the cries of animals, and even not a few of the sounds pro- 
 ducing among inanimate things. Many of these performances become in the 
 highest degree ludicrous, and furnish favourite amusements in our theatres. 
 A Mr. Henderson, of London, about the end of the eighteenth century, used 
 to kill his calf, as he called it, to crowded houses every night. After drop- 
 ping a screen between him and the audience, he caused to issue from behind 
 it all the sound, even to the minutest particular, which may be heard while 
 a calf is falling a victim in the slaughter-house; the conversation of the 
 butchers, the struggling and bellowing and quick breathing of the frightened 
 animal, the whetting of the knife, the plunge, the gush, the agony ; and, 
 revolting as the occasion is in itself, the imitation was so true to nature, that 
 thousands eagerly went to see the art of the mimic. 
 
 The following cases of inanimate sound may be closely imitated by the 
 mouth : The working of a grindstone, including the noise of the water into 
 which it dips, the rough attrition of the steel upon it, and the various changes 
 occuring with the change of pressure; the working of a saw cutting wood; 
 
462 FLUIDITY IX RELATION TO ANIMALS. 
 
 the uncorking of a bottle, and the gurgling noise of decanting its contents; 
 the sound of air rushing into a room in a winter night by a crevice or 
 key-hole and many others. 
 
 It has already been explained, that voice depends on the vibration of tho 
 two edges or lips of the slit-like opening of the glottis, by which the air 
 passes to and from the chest. The number of vibrations in a given time, or 
 the pitch of voice depends, of course, on the length and tension of these 
 edges. The length is varied by the position sof the arytrenoid cartilages, and 
 the tension by the action of small muscles which act on these ; and the 
 cavity of the mouth is enlarged or lessened to accord with the number of 
 vibrations, by the rising or falling of the tongue and larynx which form its 
 bottom. The peculiarities of individual voices must depend chiefly on the 
 size and firmness of the cartilaginious box of the larynx, the strength of the 
 muscles of the chest which force the air through the glottis, and the pliancy 
 of the moving parts. 
 
 The glottis is smaller in women than in men, and hence their pitch of 
 voice is higher : with reference to music, the difference is generally of an 
 octave, or eight notes. 
 
 The voice of a boy, in regard to pitch, is generally the same as that of a 
 woman ; but at the age of puberty, the sounding organs in the male enlarge 
 suddenly, and render the voice stronger than before, and by nearly an octave 
 graver. The voice of a eunuch is the voi-cc of the boy continued, because 
 the change called puberty does not take place in him. 
 
 Complete loss of voice, for longer or shorter periods, is often experienced 
 by persons while in feeble states of health. The vibrating, and, therefore, 
 sounding edges of the glottis, which are usually kept tense by the operation 
 of certain muscles, on these ceasing to act, owing to the state of their nerves, 
 will not vibrate as required, and the voice is lost. Slight colds suffice in 
 many people to produce this effect : in others of morbidly sensitive or deli- 
 cate nervous temperament, it follows fatigue, or any other, cause of debility. 
 Articulation is not destroyed by loss of voice; and whispering answers 
 passably the end of vocal speech. 
 
 No intelligent mind can meditate on human speech and its influence in the 
 world, without being roused to vivid admiration. But for speech, the most 
 gifted individuals who have lived, had they existed at all, could have been 
 little superior in their worldly state to the leading oxen of our herds, or to 
 leading monkeys in the woods. As regarded the rest of mankind, Homer 
 and Newton would have lived in vain. At the present day, among the 
 natives of Australasia, where language may be said scarcely yet to exist, 
 human nature is seen thus brutishly debased; while, on the other hand, in 
 the history of the world, we may trace, as a consequence of more perfect 
 speech, all the progress which has been made in arts and civilization. By 
 language, fathers have communicated their gathered experience and reflec- 
 tions to their children, who in their turn become fathers, have transmitted 
 them to succeeding children, with new accumulation; and when, in the 
 course of ages, the precious store had increased, until mere memory could 
 retain no more, the art of writing arose, making language visible and per- 
 manent, and enlarging without limit the receptacles of knowledge; and then 
 the art of printing came, which now rolls the still swelling flood into every 
 hamlet and every hut. Language thus, at the present moment of the world's 
 existence, may be said to bind the whole human race of uncounted millions 
 into one gigantic rational being, whose memory reaches to the beginnings 
 of written record, and retains imperishably the important events that have 
 
THE DIGESTION. 463 
 
 occurred ; wholl judgment, analysing the treasures of memory, has already 
 discovered many of the sublime and unchanging laws of nature, and has 
 built on them the arts of life, and through them, piercing far into futurity, 
 sees distinctly many events that are to come; and whose eyes, and ears, and 
 observant mind are, at this moment, in every corner of the earth, watching 
 and recording new phenomena, for the purpose of still better comprehending 
 the magnificence and simplicity and beauty of creation. 
 
 THE DIGESTION. 
 
 The doctrines of fluidity, illustrating and illustrated by certain phenomena 
 
 of digestion. 
 
 The animal body may be seen at first, in the maternal ovary, as a single, 
 speck of mucus ; but from possessing life wonderful life the little nucleus, 
 placed in new circumstances, begins to gather itself substance from around, 
 and it increases in bulk. For a certain time it remains attached to the body 
 of its parent, and draws the material of its increase from its parent's blood ; 
 but after that time it is alone and entirely dependent on its own resources. 
 Then we see brought into play that extraordinary apparatus now to be 
 described under the name of the digestive or assimilating organs ; which, 
 under the direction of a nervous energy, can, out of almost any kind's of dead 
 animal or vegetable matter build up the beautiful living body to perfect 
 maturity of size, and form, and faculty. And it is not only while their 
 bodies are growing that animals require to take in and assimilate new matter, 
 but also after maturity, in order to repair the waste of constant action. 
 Supply of fuel and water to the steam-engine is not more necessary than of 
 aliments to the living body. 
 
 Some of the less perfect animals take in sustenance almost like vegetables, 
 by absorbent tubes that open on their surface; but by far the greater part 
 receive it first into an interior cavity, where it undergoes certain preparation, 
 and is then offered to internal absorbents, which drink up what is required 
 and carry it into the circulating blood. This internal cavity is called a 
 stomach. Its form and appendages differ exceedingly in different animals, 
 according to the nature of the substances which serve for their sustenance, 
 and to various other circumstances. 
 
 In man, the process of digestion has the following steps. The food is 
 first received by the mouth. It is there broken or torn into small portions 
 by the cutting and grinding wedges, called teeth, with which the Jaws are 
 armed; at the same time a fluid called saliva is mixed with it, poured out 
 from glands around, so as to reduce it into a pulpy mass : this mass is then 
 pushed backwards by the tongue to enter the long tube called the gullet or 
 oesophagus, which by successive contraction of circular fibres, propels it 
 down to the pouch of the stomach, placed under the edge of the left ribs. 
 From the internal surface of the stomach a liquor oozes, called the gastric 
 juice, the most general solvent in nature, and which, attacking the received 
 food, soon reduces it, of whatever kind, to the state of a pultaceous mass, 
 named cliyme ; in this state it enters the narrow intestinal canal which is 
 continued from the stomach, where it almost immediately receives a mixture 
 of bile and pancreatic juice poured out from the liver and pancreas. After 
 this mixture, as it gradually passes on, a chemical decomposition and separa- 
 tion of parts takes place, and the pure nutriment of the body assumes the 
 
464 FLUIDITY IN RELATION TO ANIMALS. 
 
 state of a milky fluid floating among refuse. This milky fliHd, called chyle, 
 is taken up all along the canal by the numberless absorbent mouths of the 
 vessels called lacteah, and is then carried to the thoracic duct, and by it into 
 the blood to supply the waste. The intestinal canal is about six times as 
 long as the body, affording, therefore, a very extensive surface from which 
 absorbtion may take place. That remnant of the chyme which the absorb- 
 ents refuse, mixed with various depositions or secretions, continues its 
 journey onwards, and is periodically discharged. 
 
 Much of the process which we have now described is mechanical, as will 
 appear immediately; other parts of it are chemical, such as the solution of 
 the food by the gastric juice, the separation of the milky chyle, &c.; and 
 parts are vital, such as the afflux, just when wanted, of saliva, gastric juice, 
 bile, &c., and the muscular and absorbent actions. He who neglects the 
 study of any one of these three classes of particulars, must have a very 
 incomplete acquaintance with the function. We proceed now to explain the 
 mechanical or physical circumstances connected with digestion. 
 
 The abdomen may be considered as a vessel full of liquid, in which, there- 
 fore, there is pressure in all directions, increasing with the depth, (see 
 hydrostatics,) and increased also by^the action of the surrounding muscles 
 which form the sides of the cavity. 
 
 The justness of this view of the abdomen becomes evident, when we con- 
 sider that only moistened or semifluid food descends into the stomach, that 
 drink follows, and that gastric and other juices are poured out to mix with 
 the food as it passes on to occupy the long intestinal canal; and that then 
 the intestines externally are perfectly smooth, and are moistened by the 
 constant secretion of lubricating serum, so that they slide among each other, 
 without sensible impediment from friction. The abdomen, therefore, is in 
 fact a roundish smooth vessel filled with a thick fluid, which is farther con- 
 tained in a perfectly pliant and smooth-coated tube. 
 
 Thus any part of the contents of the stomach and bowels, in a living man. 
 is supported like water in surrounding water, and therefore, if the whole 
 contents be of equal specific gravity, no part can descend or advance by its 
 weight. Neither can any general pressure, nor contraction of the surround- 
 ing parietes, hasten, except at the moment of expulsion, the motion of any 
 contained matter as has, however, often been supposed ; nor can it help to 
 empty one part into another the stomach, for instance, or the gall-bladder, 
 into the small intestine. 
 
 For the same reason, however, the very slightest contractile action of any 
 containing part is sufficient to dislodge its contents gravity as a resistance 
 being neutralized by the surrounding fluid. And when the gall-bladder, or 
 stomach, or any part of the intestinal tube, becomes so full as to put the 
 elasticity of the coats ever so little upon the stretch, this circumstance alone, 
 unless some muscular action oppose, will cause a discharge of the contents. 
 The natural action of the intestinal canal is a successive contraction of its 
 circular fibres from above downwards propelling the contents, just as if a 
 small ring or tube were put round the canal and pushed forwards. 
 
 These considerations make evident the common error of supposing that 
 vomiting can by the sudden compression of the abdominal viscera mechani- 
 cally emulge or clear the obstructed biliary ducts. If general pressure of the 
 abdomen could produce this and similar effects, a descent in the diving-bell 
 should be a powerful remedy in human maladies; for nearly fifteen pounds 
 
THE DIGESTION. 465 
 
 on the inch are added to the ordinary abdominal pressure, at a depth of 
 thirty feet in water. 
 
 We hence see also the kind of error into which our predecessors fell so 
 generally, when they attributed much of the digestive power of the stomach 
 to its simple pressure upon the food. The idea probably arose from the 
 contemplation of the stomach or gizzard of a fowl, which is a powerful gristly 
 substance, answering the purpose almost of a mouth and teeth, as well as of 
 a stomach. 
 
 It is an error also to suppose that quicksilver, which is sometimes swal- 
 lowed to remove obstruction, runs through the bowels simply by its weight. 
 On first entering the loose small intestine, it must drag the part containing 
 it to the bottom of the abdomen, and in that situation, the whole intestine 
 must pass round it, nearly as a rope passes through a ring fixed to the floor. 
 "When the mercury arrives at the part of the intestine called the caecum, 
 where the farther course lies upward along the fixed arch of the colon, it pro- 
 bably can be dislodged only by the patient's lying down. Any useful opera- 
 tion of quicksilver in such cases, may be in its stimulating the bowels, by 
 dragging or displacing them, in the*manner above described. 
 
 When the abdominal muscles, which are the containing sides of the cavity, 
 become tense, whether from unusual fulness of the cavity, or from their 
 own action in any of the straining exertions, a variety of important me- 
 chanical effects ensue. Thus,. 
 
 A full stomach produces tension and projection of the belly projection 
 of the diaphragm into the chest, causing hurried breathing, and impeding 
 speech and singing expulsion of blood from the abdominal vessels, aftd, 
 therefore, congestions elsewhere, as in the arteries of the head, sometimes 
 producing apoplexy. 
 
 Abdominal fulness, as in dropsy, tympanitis, corpulency, pregnancy,&c., 
 produces most of the effects now mentioned in an aggravated degree. If 
 dropsy be allowed to proceed too far without tapping, the patient will die of 
 suffocation from the rise of the diaphragm. The external veins of the legs 
 and abdomen of a dropsical person are generally turgid, because the blood is 
 pressed into them out of the abdominal cavity, and because the passage of 
 blood through the abdomen is impeded. In tympanitis, or windy dropsy, 
 as it has been called, the viscera hang down in the abdominal cavity, while 
 the air occupies the upper part. In common dropsy the viscera float about 
 and are supported. 
 
 Straining or strong action of the abdominal muscle, and therefore, also 
 pressure on the abdominal contents, occur with almost every considerable 
 bodily exertion ; for the abdominal muscles are the antagonists of the great 
 muscles on the back and about the spine, and must always come into play 
 with them to give firmness and rigidity to the trunk of the body.. This 
 may be seen remarkably in the actions of lifting, running, wrestling, &c. 
 As the abdominal muscles cannot act in a continued way and strongly, unless 
 the ribs, from which they arise, become nearly fixed, the ribs are supported 
 during exertion by the intercostal muscles, and by the air in the chest, then 
 confined by the closure of the air-passages : hence there is generally compres- 
 sion in the chest also when the abdomen is compressed, and the blood is 
 squeezed towards the extremities from both cavities at once. It is important 
 to remark also, that in what are called the strong actions of the chest, as 
 coughing, sneezing, blowing, &c., the abdominal muscles are at least as 
 
 30 
 
466 FLUIDITY IN RELATION TO ANIMALS. 
 
 active as the pectoral : by pulling down the ribs to which they are attached, 
 they narrow the chest, and by compressing the abdominal contents, and thus 
 raising up the diaphragm, they shorten the chest. 
 
 The following cases exemplify the effects of straining The lifting of a 
 great weight, or making any great exertion, drives the blood up to the head ; 
 as is marked by the sudden redness of the face. Coughing or vomiting will 
 cause closed leech-bites to bleed afresh, and sometimes will overcome the 
 action of the sphincter of the bladder or rectum : coughing will also produce 
 vomiting, Straining to empty the bladder, rectum, or womb, or the effort 
 of vomiting, will cause the rupture of a blood-vessel in the white of the eye, 
 with consequent effusion of blood there. Apoplexy often happens under 
 the same circumstances, from the breaking of a vessel in the brain. The 
 rupture of a varicose vein, or of aneurism, generally happens during exer- 
 tion. And during exertion, the protrusion is likely to occur at any weak 
 part of the abdominal cavity, of some portion of its contents, producing what 
 is called hernia or rupture. 
 
 Vomiting is produced, not by the forcible contraction of the stomach, as 
 was long supposed, but chiefly by the action of the parietes of the abdomen. 
 This is proved by the fact that the stomach has been removed from a living 
 animal, and a sheep's bladder containing liquid has been substituted for it, 
 in connection with the gullet above and the intestines below ; and on then 
 injecting an emetic drug^into the veins of the animal vomiting has taken 
 place, as if the stomach had been there and unhurt.* From this we see why, 
 to prevent regurgitation of the food, during exertion, the upper orfice of the 
 stomach requires to be almost as strongly closed as the sphincters below. 
 
 A small pump in this application called the stomach-pump has lately 
 been used in medical practice, for removing poisons from the stomach in 
 cases where the action of vomiting could not be excited. It has already saved 
 many lives. It resembles the common small syringe, except that there are 
 two appertures near the end, instead of one, which, owing to the valves in 
 them, opening different ways, become what are called a sucking and a forcing 
 passage. When the object is to extract from the stomach, the pump is 
 worked while its sucking orifice is in connection with an elastic tube passed 
 into the stomach, and the discharged matter passes by the forcing orifice. 
 "When it is desired, on the contrary, to throw the cleansing water or liquid 
 into the stomach, the connection of the tube is reversed. 
 
 As a pump may not be always procurable when the occasion for it arises, 
 the profession should be aware that in many cases a simple tube will answer 
 the purpose as well, if not better. Such a tube being introduced, and the 
 body of the patient being so placed that the tube forms a downward channel 
 from the stomach, all fluid matter will escape from the stomach by the tube, 
 as water escapes from a funnel by its pipe ; and if the outer end of the tube 
 
 * The mechanism of vomiting is still a moot point in physiology. Mr. Haighton, a cele- 
 brated English physiologist, opened several animals during the effort of vomiting, and he 
 asserts that he distinctly saw the contractions of the stomach. The more recent experiments 
 of M. Magendie, which were repeated in the presence of a committee of the French Insti- 
 tute, are, however, entirely contradictory of those of Mr. Haighton, and seem to show that 
 the stomach is entirely quiescent in the act of vomiting. M. Maingault, nevertheless, has 
 been led to results opposed to those of M. Magendie, and he is supported by Professor 
 Portal and M. Bourdon, both of whom appeal to experiments, and to some pathological 
 facts, which are very imposing. It appears to us probable that vomiting is usually the 
 joint effect of the contraction of the stomach, and of the diaphragm and abdominal muscles, 
 though either is of itself occasionally sufficient for that purpose. Am. Ed. 
 
THE DIGESTION. 467 
 
 be kept immersed in liquid, there will be during the discharge a syphon action 
 of considerable force. On then changing the posture of the body, water may 
 be poured in through the tube to wash the stomach, and may, by the same 
 channel, be again discharged. Such a tube, made long enough, might, if 
 desired, be rendered a complete bent syphon, the necessary preliminary 
 suction being produced by a syringe, or by an assistant, who acts through 
 an interposed vessel. 
 
 But there is still an easier mode than either of these now described, of dis- 
 lodging poison from a torpid stomach, viz., merely to place the patient so 
 that the mouth shall be considerably lower than the stomach, as when the 
 body lies across a chair or a sofa, with the face near the floor, and then, if 
 necessary, to press on the stomach with the hand. The cardiac orifice opens 
 readily in such a case, and the stomach is emptied like any other inverted 
 vessel. 
 
 Useful as the pump may prove, upon occasions, in evacuating the stomach, 
 its more ancient office of injecting the enema is still the more important and 
 recent experience seems to show that such injection may become a remedy 
 of more extensive utility than had yet been suspected. From an erroneous 
 opinion, that what had been called the valve of the csecum acts as a perfect 
 valve, allowing passage downwards only, few practitioners have ventured to 
 order much liquid to be injected, for fear of overstretching the lower part of 
 the intestine; and the possibility of thus, by injection, relieving disease situ- 
 ated above the supposed valve, has scarcely been contemplated. It is now 
 ascertained, however, that fluid may be safely thrown in even until it reach 
 the stomach. Perhaps few, if any cases of obstruction of the bowels, could 
 resist the gentle force of penetrating water, so that a mechanical remedy of 
 certain effect may, in many cases, be substituted for the drastic purgatives 
 and pernicious bleedings now used, and often used in vain. From what has 
 been said above of the abdomen and the intestinal canal, it appears that an 
 injection tends to spread itself with singular uniformity over the whole. This 
 tendency may be rendered obvious to sight by throwing a sheep's intestine, 
 recently extracted, into a bucket of water, and then pumping water in at one 
 end; a stream will issue strongly at the other end, although several feet dis- 
 tant, almost immediately, and without any intermediate part having become 
 very sensibly tense. Of course, in the living body, in cases of spasms or 
 obstruction, the liquid must be thrown in against resistance very gradually. 
 
 That case is called intro-susception of the bowel, in which an upper portion 
 falls, or is received into a portion below, (as one part of the finger of a glove 
 may be received into another part,) and the receiving portion of the bowel, 
 mistaking the received for descending food, holds it fast. This occurrence 
 forms a complete obstruction, and generally proves fatal. Many infants with 
 irritable bowels, die of it. Now a copious enema, such as we have described 
 above, is almost a certain cure. The liquid advances until it reaches the 
 part where the portion of gut has been swallowed by gut below ; and as it 
 cannot pass without pushing the intro-suscepted portion back to liberty, it 
 effects the cure.* 
 
 The perpetual syringe or little valved pump, of wl^ch we have been 
 speaking as lately used in applications to the animal body, can inject or with- 
 
 * It should be remarked, however, that this measure can succeed only whilst the intro- 
 susception is recent; at least before inflammation has occurred and adhesions formed 
 between the intro-suscepted portion and that portion of the bowel in which it is received. 
 Common or atmospheric air, from its great elasticity, lightness, &c., is the best fluid for 
 the injection. Am. Ed. 
 
468 
 
 FLUIDITY IN RELATION TO ANIMALS. 
 
 Fig. 175. 
 
 Fig. 176. 
 
 draw any quantity, and is therefore very superior, for almost every purpose, 
 to the large old syringes which had no valves, and which, without being 
 removed could inject only once their fill. With well-adapted additional 
 apparatus, the same instrument will answer for many purposes, as for throw- 
 ing up the enema, clearing the stomach, transfusion of blood ex- 
 hausting cupping-glasses, relieving the over-distended breasts ; 
 for the lotio vesicae, vaginae vel urethrse, &c. No surgical 
 apparatus is now complete without one. The annexed out- 
 line represents such a syringe. The aperature c is rendered a 
 sucking orifice, by a valve at it, opening inwards ; and a is 
 the forcing orifice, in consequence of having its valve opening 
 outwards : b is the piston, with its handle. The valves may 
 be variously made, or a single double-way cock may be used 
 instead of both. Convenient dimensions for the syringe are 
 from four to six inches for the length, and from three-quarters 
 of an inch to an inch and a quarter for the diameter. 
 
 For a case of diseased rectum, where it was necessary to 
 use an enena daily, or oftener, the enema funnel, represented 
 here, (from the funnel-shaped mouth a downwards, and exclusive of the part 
 above a) was contrived, and was found more manageable by the patient than 
 any other instrument. If the tube a b be about two feet long, (it may be of 
 metal, oiled silk, caoutchouc cloth, &c.,) the liquid column contained in it 
 suffices to overcome the ordinary abdominal resistance ; 
 but if a very short tube be used, the open funnel a must 
 be converted into a close vessel, as represented here by 
 the dotted line above the funnel, having a bladder, or 
 other air-tight bag, d, connected with it, and a bottle- 
 neck and cork, or acock, at c, for admitting the enema. 
 On pouring in the liquid at c, the air in the vessel at c a 
 is forced into the bag, and on then closing the opening 
 at c, and compressing the bag, it is evident, that any 
 desired degree of injecting pressure may be exerted on 
 the enema. This apparatus is both cheaper and more 
 simple than a syringe, and is equally effectual; and 
 the bag never being wetted, lasts long : b is a cock kept 
 shut until the moment of injection. The principle of 
 substituting, in an injecting apparatus, the pressure of a 
 liquid column for that of a piston, was first suggested in 
 this work; and yet since the publication of the work, 
 more than one patent has been taken for it, for parties 
 seeking to convert it to their profit. 
 By viewing the abdomen in the true light of a vessel or bag filled with 
 liquid which is seeking to escape in all directions, we have the explanation 
 of several circumstances connected with hernia or rupture ; in which accident, 
 the containing sides of the abdomen in some part have given way, allowing 
 a portion of the viscera to escape, so as to form a tumour under the skin. 
 
 Hernia may be produced by all causes which strain or weaken the muscles ; 
 as by leaping, lifting, great weights, coughing and sneezing, lying with the 
 belly across a bench or yard, as sailors do on ship-board, over distention of 
 the belly by eating or drinking, corpulency, dropsy, pregnancy; debility of 
 muscle from dissipation, &c. 
 
 The reason that a rupture increases so rapidly after it has once begun, is, 
 that the protruding part is truly a fluid wedge, of which, therefore, the opening 
 
 'ci 
 
PELVIC APPARATUS. 469 
 
 force increases with the diameter. (See Hydrostatics.) This shows the 
 singular importance of arresting the accident at its very commencement. 
 The truss used to repress rupture were described at page 412. 
 
 In attempting to return any part of the abdominal contents which may have 
 escaped as rupture, we should recollect, that a soft uniform compression or 
 squeezing exerted upon the tumour by the hands of the operator, if greater 
 than the internal pressure of the abdomen, is slowly pushing back again any 
 fluid matter that can ooze inward from the tumour ; and by thus gradually 
 lessening the size of the tumour, may effect the desired object, without the 
 adoption of the last resource, of cutting parts to widen the inlet. When, in 
 such a case, the operator sees clearly with the mind's eye what is passing 
 under his fingers, his efforts may often be successf^L where a less intelligent 
 individual would fail. No man practices medicinelnong, whatever his nomi- 
 nal department, without having opportunities of saving life, or of preventing 
 a serious operation, by judicious management of recent hernia. The barba- 
 rous old fashion of lifting the patient by the heels and shaking him, that the 
 weight of the bowels might drag back again the part which had escaped, was 
 founded on ignorance of the fact, that the weight of the bowels in all posi- 
 tions of the body, is supported almost entirely, not by their attachments, but 
 by the surrounding parts. 
 
 The function of digestion or assimilation sketched in the preceding para- 
 graphs, by which the animal body assumes foreign matters from around, and 
 converts them into his own substances, is a subject of study little inviting in 
 some of its details, but taken altogether is one of the most wonderful which 
 can engage the human attention. It points directly to the curious and yet 
 unanswered question what is LIFE? The student of nature may analyze 
 with all his art those minute portions of matter called seech and ova, which 
 he knows to be the rudiments of future creatures, and the links by which 
 endless generations of living creatures hang to existence : but he cannot dis- 
 entangle and display apart their mysterious LIFE! that something under the 
 influence of which each little germ, when placed in due circumstances, swells 
 out, to fill an invisible mould of maturity which determines its forms and pro- 
 portions. One such substance thus expands into a beauteous rose-bush ; 
 another into an noble oak; a third into an eagle; a fourth into an elephant 
 yea, in the same way, out of the rude materials of broken seeds and roots, 
 and leaves of plants, and bits of animal flesh, is built up the human frame 
 itself, whether of the active man, combining gracefulness with strength or of 
 the gentler woman, with beauty around her as light. How passing strange, 
 that such should be the origin of the bright eye, whose glance pierces as if 
 the invisible soul were shot with it of the lips which pour forth sweetest 
 eloquence of the larynx, whose vibrating fills the surrounding air with 
 music ; and, more wonderful than all, of that mass shut up within the body 
 fortress of the skull, whose delicate and curious texture is the abode of the 
 soul, with its reason which contemplates, and its sensibility which delights 
 n these and endless other miracles of creation. 
 
 PELVIC APPARATUS. 
 
 The Secretion of the Kidneys, &c. 
 
 OF the large quantity of fluid daily taken into the human body, much 
 escapes with the breath, as is provided by the visible condensation of it in 
 frosty air ; or on any cold polished surface held near the mouth ; part escapes 
 
470 FLUIDITY IN RELATION TO ANIMALS. 
 
 by the skin in perspiration ; but the greatest part after having answered the 
 purposes of the constitution, is separated from the blood by the two secreting 
 organs, called the kidneys, and from them by fit channels, is carried off, 
 holding in solution various other matters, which the system does not require. 
 The kidneys are situated in the loins, one on each side of the spine ; and 
 the constant drain of liquid from them passes down by two membranous 
 canals called ureters into the bladder, from which the liquid is again expelled 
 through the urethra, at considerable intervals, according to the rapidity of 
 accumulation. 
 
 The bladder is a curious membranous and muscular reservoir, of which 
 the fibres can contract so as to expel the last drop, and yet can yield so as 
 to admit a quart or more. 
 
 The passage of fluid downwards through the ureters from the kindeys to 
 the bladder resembles, in some respects, the passage of blood in the veins. 
 Authors have erroneously supposed that the weight of the fluid suffices to 
 cause its descent : but the bladder and ureters are enclosed in a common 
 cavity with the intestinal canal ; and while this is full of a semi-fluid mass 
 of greater specific gravity than the urine, the latter is not only supported by 
 the surrounding pressure, as water would be supported by water, but is forced 
 upwards ur resisted, as water would be in honey or treacle : in descending, 
 therefore, it obeys some other force than gravity, namely, that of the secret- 
 ing vessels and heart. 
 
 The ureters, bladder and urethra are the seats of some of the most distress- 
 ing diseases, to which the human frame is liable. Two classes of these 
 being relievable chiefly by mechanical means, require to be shortly consi- 
 dered here. They are, obstructions in the urethra ; and concretions, or stones, 
 as they are called, in the bladder. 
 
 Obstructions or strictures in the urethra are generally consequences of an 
 inflammation, which has destroyed the dilatability of a part of the canal. 
 They appear as if a thread or a bit of tape were tied round the canal, so as 
 to narrow its caliber. Constant irritation, which destroys the general health, 
 fits of fever, broken rest, and even death from total suppression of urine, 
 have been common consequences of stricture. 
 
 Until within a recent period, the treatment of such obstructions was pur- 
 sued very generally according to a blind routine. The attempt was made 
 either to bore them open by wedges, called bougies, often of doubtful and 
 tedious operation, or to destroy them by caustic passed down to them in the 
 end of a bougie, which caustic often hurt the part of the canal anterior to 
 them, or eat out false passages about the stricture, or open blood-vessels 
 so as to cause dangerous haemorrhage. 
 
 Struck by the defective state of this branch of the healing art, the author 
 of the present work, while abroad, and situated where he had interesting 
 opportunities of observation, had bestowed considerable attention upon it, 
 and he then contrived and tried several new means of relief. These were 
 afterwards brought more extensively into use and improved, and others were 
 added, by his brother Dr. James Arnott, superintendent surgeon in the ser- 
 vice of the Hon. East India Company, who gave a minute account of them 
 in a treatise on urethral diseases, and a supplement published in the years 
 1818 and 1820, They have become, perhaps, still better known in Francee 
 than in England, through the work of Dr. Ducamp, which describes them, 
 and which, having been submitted to the French Institute, and most favoura- 
 bly reported upon by the appointed authorities, soon became a standard trea- 
 tise in the country; in France, also, the philosophy of mechanics had been 
 
PELVIC APPARATUS. 471 
 
 studied by surgeons more generally than in England. It is painful to be 
 obliged to add, that Dr Ducamp as regarded these instruments and the 
 views of disease and treatment which had suggested them, concealed the fact 
 of his being only a translator. The imposition was not discovered at the 
 time of his death, which happened two years afterwards, hastened appa- 
 rently by the fatigues of the extensive practice which the report of the 
 Acadamy brought upon him. The auther has had so much pleasing inter- 
 course with enlightened and honourable Frenchmen, that it pains him to 
 have this fact to relate. 
 
 The objects aimed at by the new means were, to ascertain the exact con- 
 dition of the diseased canal to facilitate the passing of instruments in cases 
 of difficulty and to effect a permanent cure. The following seven of these 
 means may here be particularized : 
 
 1st. An examining sound ; being a bougie with the point formed of a softer 
 tenacious material ; in which fibres of cotton or silk are mixed to prevent 
 any portion from being broken off or detached during use. This sound, 
 pressed against the obstruction, takes a correct impression of its anterior face, 
 and shows the magnitude and exact position of the still remaining opening. 
 
 2nd An expanding or dilator sound which is a small tube with a dilatable 
 Button at its extremity. The button consists of a little bag, which is passed 
 through the structure empty, and is filled with fluid after it has passed. It 
 readily discovers any other strictures beyond the first ; and ; to a certain degree, 
 the state of each. 
 
 3d. A conducting canula or tube, open at both ends. It is passed down 
 to the stricture, for the purpose of supporting and directing small bougies 
 seeking entrances through very narrow strictures, or of guarding the caustic 
 bougie in its approach to the place of its action. 
 
 4th. In cases where the attempt to open the passage has failed by all com- 
 mon means, a conducting tube is first introduced, and through it six or more 
 small bougies are passed side by side, so as to probe the whole face of the 
 stricture at the same time. It is thus scarcely possible that the opening 
 should not be found. 
 
 5th. Were even this means to fail, the conducting tube may be filled with 
 water, under any degree of pressure, which water will either open the pass- 
 age for the small bougies, or will itself act as the sharpest and most insinu- 
 ating of all instruments. The stricture, by which ever means opened will 
 then allow the urine to escape. As patients might fear that water forced 
 towards a bladder already too full would ? only^increase the evil, J. Arnott 
 waited for more numerous proofs of the utility and safety of the practice, 
 before strongly recommending it : Dr. Amussat, of Paris, has since pub- 
 lished. a statement of numerous cases of retention thus relieved. 
 
 6th. A dilator for widening the stricture, after a small instrument can be 
 passed through it. It is intended as a substitute for the bougies and sounds 
 of former times. The chief objections to these last are, the painful friction, 
 the danger of making false passages, the tediousness and imperfection of the 
 cure, and that they cannot dilate any part of the canal beyond the size of its 
 orifice, through which they have to pass, and which during health, is the 
 narrowest part of the canal. * 
 
 The dilator consists of a tube of thin membrane introduced while empty 
 into the stricture, on a ball-pointed wire and then filled with fluid by a. 
 syringe, so to dilate the stricture, with any degree of force, from the mere 
 filling of the part to the strain of the hydrostatic press, sufficient to tear the 
 strongest texture that disease can form. The dilating tube is about two inches 
 
472 FLUIDITY IN KELATION TO ANIMALS. 
 
 long, and its end next to the operator is fixed to the point of a small catheter, 
 through which the distending fluid is injected. The tube is formed of thin 
 silk riband of various sizes, with the edges joined. It is lined with prepared 
 gut of the cat or dog, which is almost as thin as gold-beater's skin, although 
 very strong and water-tight; and it is covered with the same to give the 
 smoothest and softest possible external surface. When complete and en- 
 closing its blunt wire, it is still much less bulky than the bougie which would 
 be required for the same case. Thus, it passes easily; it cannot tear the canal 
 or make false passages ; it can enter through a small orifice, and then dilate 
 to any desired extent ; and its greatest advantage is, that by swelling so as 
 to follow the yielding of the stricture, it can eifect at one application, what 
 only a succession of hard bougies, during long treatment, could accomplish. 
 In one day it has often removed disease which had resisted other means for 
 months or even years. 
 
 Some practitioners and critics, not understanding the law of fluid pressure 
 (explained at p. 128,) objected at first to the dilator, that a little water or air 
 pressed into it by a syringe, would be unable to overcome much resistance. 
 Had they seen the instrument lifting so readily as it does, a heavy weight 
 laid upon it, or snapping a strong ligature tied round it they would. not have 
 had this prejudice. It was objected, also, that the instrument would do mis- 
 chief by dilating the urethra before and behind the stricture more than th*e 
 stricture itself; now its dimensions being determined and fixed by those of 
 its silken tunic, it never can distend beyond the diameter chosen, and, there- 
 fore, if of the proper size, it can only press on the stricture itself. It was 
 also said, that this instrument requires, in the operator, greater manual dex- 
 terity and acquaintance with mechanical philosophy than many surgeons 
 possess ; but this is merely saying that the arts are progressive, and that the 
 accomplished surgeon of the present day is more dexterous and intelligent 
 than his predecessors of the last centuary. It is not accounted a reason why 
 the delicate apparatus of the oculist should fall into disuse, that all surgeons 
 are not able to apply it. 
 
 Some attempts had been made before, to construct a dilator of fluid pres- 
 sure, but they produced nothing of value. For urethral purposes, a simple 
 gut or intestine is worse than useless, for, being yielding in its texture, the 
 surgeon can never know truly the size of his instrument, and therefore may 
 do much mischief by it. Dr. Ducamp, in speaking of the dilator, allows that 
 he did not first invent it, but then, from ignorance of what constitutes its true 
 value, he takes praise to himself for simplifying and improving it, by throw- 
 ing away the silk, and using the gut only. A variety of metallic dilators 
 have been contrived and used by English surgeons since the publication of 
 Arnott's Treatise on Strictures, but although manageable with less trouble 
 than the fluid dilator, they want its chief merits. 
 
 The dilator is applicable to many other purposes in surgery besides that 
 now mentioned, as for removing stricture of the gullet, and of the rectum, 
 for checking haemorrhage in deep wound^, for dilating wounds as a tent, &c. 
 And the operation of lithotomy was saved to a gentleman, whom Sir Astley 
 Cooper and the auther of this work were attending together, by the dilator 
 opening a fistula in perineq^ so that a large stone was extracted without cut- 
 ting. The dilator has also served in removing stones from the female bladder. 
 
 7th. Another improved means for the treatment of stricture, described in 
 the treatise, is a mode of apply ing caustic for its entire destruction, but so as 
 not to touch any other part of the canal. Formerly the caustic was applied 
 to the face or anterior part of the stricture, and, therefore, had almost always 
 
PELVIC APPARATUS. 473 
 
 to destroy a portion of the healthy canal before it could reach the narrowest 
 fibres : the extent of such portion depending on the distance from these 
 fibres of the part where the lining of the canal began to be drawn inwards by 
 them. This explains why tfot unfrequently a hundred applications of caustic 
 were made in a single case, and why, during such treatment, false passages 
 were often bored, and other mischiefs produced. Now by applying the caustic 
 within the stricture at once a single application generally suffices. To ac- 
 complish this, a ring of caustic is placed (as described in the Treatise, and 
 in the Cases,) on a bougie of peculiar construction, about an inch from its 
 extremity ; and the bougie being then passed down to the stricture through 
 a tube or conductor, and the point being passed beyond the stricture, the 
 caustic is guided to the very spot where it is desired to act.* 
 
 * Dr. Ducamp incurred a singular risk in giving himself out as the first proposer of the 
 instruments and practice described above; for he was already known as a translator of 
 English medical books, and the Treatise on Strictures of J. Arnott had been held up to 
 public attention two years before by the various medical reviews, in terms such as the fol- 
 lowing: " We have carefully perused this little volume,*and are of opinion that it is by 
 far the best systematic work on the subject in the English language." It is a judicious 
 compilation, interwoven with much original and acute observation ; and it gives publicity 
 to instruments which promise to be of essential benefit to operative surgery." Medico- 
 Chirurgical Review, January, 1819. 
 
 Perhaps Dr. Ducamp imagined that the slight alteration proposed by him in the con- 
 struction of three of the new instruments, might be a shield to him when detected; but as 
 the chief merit was in the analysis of the subject which suggested such instruments, and 
 not in the mere mechanical fulfilment of intentions, even a considerable improvement in 
 the instruments would not have been a sufficient excuse. His changes, however, were either 
 trifling or retrograde. His metallic dilating sound is less perfect than metallic sounds con- 
 trived by J. A., but not described, because the fluid dilating sound was found to be pre- 
 ferable. His porte-caustique is defective in not distending the stricture at the. moment of 
 applying the caustic ; and his mode of making a dilator without the silken tunic, renders 
 it not only a useless, but a dangerous instrument: indeed, such as obliged him to use the 
 caustic in almost every case. His silence with respect to the liquid probe favors the con- 
 clusion that he did not understand it, although Dr. Amussat of Paris has since used it with 
 much success : and the same remark applies to the double catheter (see Arnott's cases,) 
 or sonde a double current, as it has been called' by those who have since used it in Paris. 
 
 The following are extracts from the report made by the commissioners of the French 
 Institute, Doctors Deschamps and Percy, in May, 1822, on the subject of Ducamp's work 
 entitled Traite des retentions d'urine. 
 
 "This treatise concerning a most important malady, because one of the most common 
 and painful which affects humanity, has appeared to us to merit more than ordinary 
 attention. 
 
 "When, some years ago, your same commissioners had to express their opinion of an- 
 other work on this subjct, they commended the zeal and industry of its estimable author 
 (Dr. Petit;) but they could not conceal that there were still imperfections in his modes of 
 treatment; and also that they were almost entirely either borrowed or imitated from the 
 English. 
 
 "The work of Dr. Ducamp now leaves us, however, nothing more to desire, and we have 
 no longer reason, as regards this subject, to envy our neighbours. Although a volume of 
 moderate size, it is incomparably more complete and full of matter than the bulky trea- 
 tises lately published in other countries. 
 
 " * * Ducamp leaves all these authors far behind him, whether as to the soundness of 
 his doctrines, the superiority of his trials, or the invention of instruments. 
 
 "He takes a print or model of the stricture by an instrument of his invention, called 
 Sonde Exploratrice. (Arnott's examining sound, page 471.) 
 
 " For introducing bougies in difficult cases, he uses an elastic gum tube, which he calls 
 conductor. (Described above, page 471.) 
 
 "Mr. D. has invented for measuring the length of strictures, &c., an instrument which, 
 when introduced, enlarges beyond the stricture. (The dilating sound, page 471.) 
 
 " The nitrate of silver, or common caustic, is what he uses for destroying strictures, but 
 he employs it in a new manner, which appears to us to give it new powers, and to deprive 
 it of all its former dangers. * * He carries the caustic into the stricture by means of his 
 porte caustique. (See above, page 473. No. 7 of Mr. Arnott.) 
 
 " To enlarge the canal at the morbid part of its true caliber, he uses an instrument 
 
 which he names a dilatateur. (Dilator, page 471.) He does not conceal that this instru- 
 
474 FLUIDITY IN RELATION TO ANIMALS. 
 
 Stone in the bladder is another disease relievable chiefly by mechanical 
 
 means 
 
 
 
 The urine as secreted in the kidneys, contains dissolved in it, a variety of 
 substances which, under certain circumstances, separate and assume the solid 
 form, as sugar separates in small crystals from cooling syrup, or salt from 
 cooling brine : and it is thus that those minute grains are produced which 
 we call urinary gravel. A single particle of gravel remaining by an acci- 
 dent in the bladder, soon attracts to itself more matter of the same kind, and 
 becomes the nucleus or centre of an increasing mass, which is what we call 
 the stone in the bladder. 
 
 In a second Tract by the author's brother, published in 1820,* the follow 
 ing paragraph appears : 
 
 " From the severe suffering of the patient labouring under stone in the 
 bladder, and the remedy being an operation so painful and dangerous, that 
 many wear out their lives in, certain misery, rather than submit to it, it has 
 arisen that no part of surgery has excited more attention, either in the medical 
 profession or out of it.")" No very important change in the treatment of this 
 disease has now been made for upwards of a century ; and, indeed, it has 
 appeared to be the opinion of modern surgeons, that the manner of operating 
 practised in Cheselden, about a century ago, and which has been called the 
 * glory of English surgery' was so nearly perfect as to leave little room for 
 improvement. The hopes which the rapid progress of chemistry, and the 
 grand discoveries relating to stone of Scheele, Wollaston, Fourcroy, and 
 others some time ago gave birth to, that we should be able to dissolve stone 
 by lithontriptics, and thus save the horrors of lithotomy, had again died 
 away, and the researches of many ingenious men who have been, and still 
 are employed about the question, have for their end, more to prevent the 
 formation of stone by remedies and regimen, than to improve the manner of 
 removing it when once formed. I trust, however, notwithstanding the sup- 
 posed exhausted nature of the subject, that the following essay will prove 
 that much was still possible in the improvement of this department of the 
 healing art." 
 
 The publication from which the above paragraph is taken, and the " Trea- 
 tise" which preceded it, in both of which new instruments and new pro- 
 cesses were described, and interesting facts were detailed, aroused the public 
 attention in England to the possibility of improving the treatment of stone j 
 and about the same time, a similar spirit awoke with more decided effects in 
 France. The results have now become of great importance to humanity. 
 In the medical publications since that time, cases soon began to be re- 
 corded in lithotomy superseded by new means, and lately such cases form 
 the majority. We shall now briefly animadvert to the principal of these 
 
 ment had been imagined before him, but he has the merit of perfecting it, and of reducing 
 to practice what before had only existed as a project. 
 
 " In rendering justice to the able men who have preceded Ducamp, we must still 
 
 say, that no one has displayed so much industry, dexterity, and talent, and we think that 
 he has high claims to the confidence of patients and the gratitude of the profession, and 
 that his work merits the eulogium of the Academy. 
 
 (Signed "DESCHAMPS, PERCY, Reporters. 
 CUVIER, Secretary. 
 
 * Cases illustrative of the Treatment of Urethral Obstructions and of Stone. By James 
 Arnott. Longman and Co., 1820. 
 
 f The Catalogue of authors who have written upon stone occupies in Plocquet's Litera- 
 tura Mtdica, no less than twenty-nine very closely printed quarto pages. 
 
PELVIC APPARATUS. 475 
 
 means, intending, however, only to interest the reader in a manner that may 
 lead him to the persual of the original works, where more minute information 
 is to be found. They shall be named in the order in which they have come 
 into use. 
 
 The dilator, as applied to the treatment of stone, has already been spoken 
 of in the preceding pages. 
 
 The double catheter. This instrument, with its applications to causes of 
 stone and other affections of the bladder, is described in Arnott's Cases. It 
 has two channels, by one of which a fluid may pass into the bladder, while 
 by the other there is a returning current mixed with urine. It is equipped 
 with two pliant tubes, of which one leads, from a supplying reservoir, and 
 the other to a waste vessel. It will soothe irritation of the bladder, whether 
 arising from stone or not, by keeping the acrid urine in a dilated state, or by 
 applying bland and medicated liquids directly to the internal surface of the 
 bladder. Not being larger than a common catheter, it may be worn for any 
 period as the common catheter now is. It need prevent no sedentary occu- 
 pation, and may be used during sleep. It will act powerfully to dilate a 
 contracted bladder, if the reservoirs be placed high, and the fluid be caused to 
 distend with the pressure of a lofty column. It also affords by far the best 
 means of admitting to the bladder any solvent of stone. Even pure water is 
 a weak solvent of most animal calculi, as is proved by placing them in a run- 
 ning stream ; but the living bladder bears with impunity a diluted acid or 
 alkali. 
 
 The syphon catheter (also first described in Arnott's Cases') is merely a 
 catheter of a-length that will allow its external part to descend, so as to con- 
 stitute the long leg of a syphon. (See Pneumatics.') Its outer extremity 
 is turned up a little, or has a portion of soft animal gut tied upon it to act 
 as a valve, for preventing the entrance of air. The most useful application 
 of this instrument is to keep the bladder empty after operations, until the 
 healing process has made a certain advance. The diffusion of urine among 
 the surrounding parts after lithotomy, particularly after the high operation, 
 is often a cause of death ; and the syphon catheter, by providing a channel 
 by which the urine must immediately pass away as secreted, obviates the 
 clanger. This instrument is sometimes useful in very irritable bladders, by 
 preventing the repeated distensions of the bladder, with the consequent 
 excruciating contraction. It has also relieved in the deplorable case of the 
 bladder torn or opened by sloughing in parturition, as it can keep the unhap- 
 py patient quite dry. 
 
 A forceps, calculated to pass through a tube into the bladder, and to open 
 there, for the purpose of seizing any small stone or other solid object offered 
 to it, was described long ago in the Armamentum Chiruryicum of Scultetus, 
 but was again forgotten until John Hunter's investigations led him to a 
 second invention of it. Such an instrument had for a considerable time passed 
 under the appellation of Hunter's urethra or bladder forceps, answering for 
 extracting small stones, and therefore, if used in time, preventing occasion- 
 ally, the necessity of lithotomy. Soon after the publication of Arnott's 
 Essay, it was modified and much more extensively used by Sir Astley Cooper 
 and other surgeons in England. 
 
 But a new and intense interest has now been excited with respect to the 
 forceps, as a means for removing stone, by the discovery also an old disco- 
 very revived that a straight tube may be passed to the bladder, as a conductor 
 instead of the bent tubes or catheters commonly used. A door is thus, as it 
 were, opened directly into the bladder, through which a stone might even be 
 
476 FLUIDITY IN RELATION TO ANIMALS. 
 
 seen, if light were directed upon it, and through which it easily may be caught 
 and broken to pieces, and brought away without doing injury to the living 
 parts. Dr. Civiale, of Paris, had the merit first of contriving good instru- 
 ments for this operation, and of himself operating with complete success in 
 many cases.. But the praise of carrying the operation of Lithotrity (sfone- 
 wearing-down,') as it is now named, to its present state of perfection, is shared 
 by various other ingenious surgeons, as Gruithuisen (who first used the 
 straight sound,) Amussat, Leroy, Heurteloup, (who proposed the mode of 
 percussion,) &c. The operator introduces a strong forceps, which seizes 
 and holds fast the stone, he then weakens the stone by boring it in various 
 directions with a simple drill, which passes through the handle of the forceps, 
 and is turned rapidly by a drill-bow acting on its external end, or with a drill 
 of which the point can be bent to one side, so as to excavate to any desired 
 extent ; after which weakening, the stone is crushed, either by the forceps 
 which first held it, or by another instrument called brisecoque, made on pur- 
 pose ; or without boring at all, Heurteloup and others at once break the stone 
 to pieces by blows of a small hammer acting on a sliding limb of the forceps. 
 
 Dr. Dartfin, in his Zoonomia, published in 1790, proposed to seize stones 
 by forceps passed into the bladder, and then to break them down or destroy 
 them mechanically ; but the supposed necessity of working through a long 
 bent tube prevented trials from being made. The author of this work also 
 showed some years ago, before any of the above-described improvements 
 were made (see Cases, page 93,) that it was possible to pass a bag into the 
 living bladder, and to enclose a stone there, so that any solvent might be 
 injected into the bag, and again withdrawn without coming into contact with 
 the bladder. This was shown rather to excite attention to the possibility of 
 operating with the living bladder with great precision, than to recommend 
 that precise means of destroying stone. 
 
 To all the ingenious instruments above spoken of for breaking down the 
 stone, there is still this objection, that it is broken into such fragments, that 
 many of them require to be afterwards treated as distinct stones, and thus 
 the painful operation has to be repeated again and again, and whole months 
 may pass before the operation be completed. The author deems it possible 
 to make a forceps of several claws or ribs, which should surround the stone 
 so loosely as to leave it freedom of motion within the claws, like a loose 
 kernal in a shell, and so that on making the forceps itself whirl backwards 
 and forwards, like the drill in Civiale's apparatus, the stone might be quickly 
 rubbed to dust by the friction or file action of the roughened interior of the 
 claws. The bladder would be filled during the operation, with water, or even 
 air, to secure plenty of room for the turning instrument : or a slender exter- 
 nal forceps might be used as a guard, to prevent contact of the bladder with 
 the moving instrument. Out of the body, a stone harder than urinary 
 calculus, placed in such a cage with rough interior, and subjected to the 
 action described, is soon reduced to dust. There are various ways of making 
 a forceps or cage for this operation, which will readily suggest themselves 
 to persons knowing what has already been achieved in this department of 
 practice, and having the ingenuity likely to engage them in such a pursuit. 
 
 The high operation of lithotomy possesses over the common lateral ope- 
 ration such advantages as the following : thinness of the parts cut through 
 distance of the knife from important arteries stones of very large size may 
 be more easily extracted the prostate gland is not wounded. But the high 
 operation has not become general, because there was difficulty in avoid- 
 ing the peritoneum while making the opening into the bladder there was 
 
PELVIC APPARATUS. 477 
 
 danger of effusion of urine among the cut parts, after the operation and 
 where the bladder was contracted, the incision had to be very deep. Now 
 these objections are obviated by, 1st, the double catheter, which will dilate 
 the contracted bladder ; 2d, by the syphon catheter, which will prevent the 
 effusion of urine; and 3d, by the jointed sliding sound, (see Cases, page 
 104,) which will ensure the accurate cutting in the desired place. Had we 
 possessed then, for the removal of stone, no less hazardous means than 
 cutting, the high operations with the new securities might have been best. 
 When a catheter has to be retained in the bladder after any operation, in 
 cases where, if it slipped out, it might with difficulty be replaced, something 
 should be passed through it like a small spring forceps to expand and 
 become an internal button preventing its escape. (See Cases, page 94.) 
 
 UTERINE PHENOMENA. 
 
 Although so many of the uterine phenomena are mechanical, there are 
 few of them which could be treated of with advantage, except in connection 
 with particulars, of which the consideration does not belong to a work like 
 this. We shall, however, cite the following particulars as examples. 
 
 The protection given to the tender foetus by the liquor amnii in which it 
 floats, is such, that a blow from without is expended on the surrounding 
 water, and cannot reach the foetus. 
 
 The head of the foetus, because ossification begins in it first, becomes of 
 greater specific gravity than the other parts of the body, and therefore gene- 
 rally lies at the bottom of its liquid bed. It is thus ready to appear first in 
 parturition, according to the safest course of delivery 
 
 The membranes distended by the liquor amnii descend before the head, 
 as a soft but powerful wedge preparing the way, according to the principle 
 explained in a previous page. 
 
 We have spoken, at page 168, under the name of pneumatic tractor, of a 
 circular piece of leather or similar soft substances, kept extended by included 
 solid rings or radii, as being adapted to some purposes of surgery. Now it 
 seems peculiarly adapted to a purpose of obstetric surgery, viz., as a substi- 
 tute for the steel forceps, in the hands of men who are deficient in manual 
 dexterity, whether from inexperience or natural inaptitude. The forceps, to 
 be well and safely used, requires address, which even the naturally dexterous 
 man cannot possess without a certain degree of continued practical familiarity 
 with it, and except in large towns, a man must be very unfortunate in his 
 practice who often requires it : hence the really small number of persons, who 
 use it well. The consideration of the tractor as a substitute for it belongs 
 properly to the present section : but as the true mode of action of the tractor 
 is not very readily conceived, by persons who either have never been in- 
 structed in the general laws of physics, or who have ceased to be familiar 
 with them, such persons are advised to read this paragraph in continuation 
 of that at page 168, and to weigh well the following remarks. A tractor of 
 three inches in diameter, would act upon any body, to lift or draw it, with a 
 force of about a hundred pounds with more therefore, than is ever required 
 or allowable in obstetric practice. In lifting a stone, the tractor does act 
 as if it were glued or nailed to the stone, but merely bears or takes off the 
 atmospheric pressure from one part, and allows the pressure on the opposite 
 side, not then counterbalanced, to push the stone in the direction of the trac- 
 tor ; so when placed upon the head, it would not pull by the skin, in the 
 
478 FLUIDITY IN RELATION TO ANIMALS. 
 
 manner of a very strong adhesive plaster applied there, as uninformed per- 
 sons would be apt to suppose, but by taking off a certain atmospheric pres- 
 sure from the part of the head on which it rested, it would allow the pressure 
 on the other side or behind to urge the head forward on its way. Of course 
 the forwarding pressure in such a case would not operate on the head directly, 
 but through the intervening parietes and contents of the maternal abdomen. 
 It would be much better to have a gentle and diffused action of the tractor 
 over a large surface, than an intense action on a small surface, and therefore 
 a tractor for the purpose now contemplated should not be very small, and 
 should have a little air underneath it in a slight depression or cavity at its 
 centre. The forceps must be more effective than the tractor for rectifying 
 malposition of the head, and diminishing its transverse diameter ; but the 
 tractor will answer both these purposes in a degree greater than many would 
 expect.* The author proposes to publish on this matter, and on some other 
 strictly professional subjects which are lightly touched upon in the present 
 general work, such a practical detail, as for the dilator, syphon catheter, &c., 
 is found in his brother's " Treatise" and " Cases." 
 
 Conclusion. 
 
 It is. almost, superfluous to remark here, that, for the practice of general 
 and obstetric surgery, learning and judgment are of little avail unless accom- 
 panied by manual dexterity : and it is one of the improvements yet to be 
 made in our system of education for various professions, to cultivate more 
 methodically the use of the hands. Children and young people, in obtain- 
 ing practical familiarity with ingenious toys, tools of carpentry, games of 
 address, musical instruments, &c., are often fitting themselves for the 
 important business of their future life. 
 
 * "We have been already compelled on one or two occasions to differ from the able author 
 of this work, in relation to the practical application of some of his principles, and we must 
 be again permitted to record our dissent from his opinion that the pneumatic tractor, under 
 certain circumstances is peculiarly adapted as a substitute for the obstetric forceps. Our 
 author cannot be a practical accoucheur or he would at once perceive that the various ma- 
 noeuvres by which labour is assisted with the forceps, cannot be accomplished with the 
 tractor. That address and knowledge are requisite to apply the forceps properly, is no 
 objection to their use ; it only shows the necessity of the operator's acquiring this dexterity 
 and knowledge before attempting to apply the instruments; and these acquirments are not 
 so difficult as our author seems to think, nor do we believe that the number who possess 
 them is so very small. It is not contended even by the author that the tractor is superior to 
 the forceps ; he only recommends it as being less dangerous in the hands of the unskilful. 
 
 Now it might be supposed from this that the tractor is readily applied and cannot effect in- 
 jury, both of which are erroneous. Every instrument is dangerous in the hands of igno- 
 rance. If a person deficient in dexterity could succeed in applying the traction, (of which 
 we have strong doubts, believing it would require, in most instances, even Dr. Arnott's skill 
 and knowledge) it is quite as probable that he would produce injury as benefit. In certain 
 states of labour, the tractor may be applied to the neck of the uterus instead of the head 
 of the child, or to both, drawing out the uterus thus as well as the child ; it may be applied 
 before the uterus is sufficiently dilated, or the force may be applied in the wrong direc- 
 tion ; indeed, there are but few cases in which force could be applied in the proper direc- 
 tion with the tractor, <tc. These accidents cannot happen to the well instructed ; but in the 
 hands of such, the forceps are more effectual and equally safe. The tractor, then, requiring 
 skill for its proper application, and being a less efficient instrument than the forceps, ought 
 not, independent of many otherreasons, to be recommended. It is not to those who devise 
 imperfect substitutes for valuable instruments, or temporary palliatives for important ope- 
 rations, in order that the awkward and ignorant may imperfectly perform what the skilful 
 or instructed should only attempt, or are capable of accomplishing, that praise is to be 
 awarded. It is the just meed of those who furnish proper instructions for the use of instru- 
 ments and for performing operations, and present the means of gaining information and 
 skill. Am. Ed. 
 
v PELVIC APPARATUS. 479 
 
 While the author directs the attention of the profession to the important 
 physical considerations set forth in the preceding pages, he deems it neces- 
 sary most pointedly to remark, that in the living body mechanical principles 
 are generally associated in their operation with the more recondite principles 
 of chemistry and of life j and that the man who allows his mind to dwell 
 too exclusively on any one of the three classes, must be a very bad reasoner 
 in questions either of health or disease. It is within a very recent period, 
 however, that just views on this subject have begun to prevail, and that the 
 titles of the peculiarly mechanical physician, or chemical physician, or phy- 
 sician attending only to the influence of nerves or life, are likely to be no 
 longer justly applicable. The light of true philosophy is at last breaking in. 
 upon the very complex and difficult subjects of medical inquiry; and where 
 formerly keen penetration beheld only confusion, even common minds now 
 begin to see clear divisions and beautiful arrangement. 
 
THE 
 
 ANALYTICAL TABLE. 
 
 INTRODUCTION. 
 
 Progress of man and stationary condition of inferior animals. 
 The progress more rapid at present than ever. 
 The divisions of human knowledge. 
 Natural philosophy particularly considered. 
 SYNOPSIS. 
 
 The fundamental truths of natural philosophy explained under the terms 
 atoms, attraction, repulsion, and inertia the divisions of this work, 
 page 19. 
 
 PART I. The FOUR FUNDAMENTAL TRUTHS minutely examined 
 and used to explain the general constitution of material substances, and 
 of the motions going on among them, page 22. 
 
 SECT. I. CONSTITUTION or. MATERIAL MASSES, 22. 
 
 ATOMS minute indestructible occupying space, 22. 
 ATTRACTION of atoms is mutual, 25. 
 
 Gravitation, 26 Cohesion, 27 Capillary attraction, 28 Che- 
 mical attraction, 28 Definite proportions, 29. 
 REPULSION The influence of heat on masses, 30. 
 Solid liquid air, 31. 
 Repulsion of surfaces, 32. 
 
 Modification of Masses : 33 Crystal, 33 Porosity, 34 Den- 
 > sity, 35, Hardness, 36 Elasticity, 37 Brittleness, 38 
 
 Malleability, 38 Ductility, 38 Pliancy, 39 Tenacity, 39. 
 APPENDIX, Properties of Matter, 41. 
 
 SEQT. II. MOTIONS AMONG BODIES, 41. 
 
 Motion and Rest; 
 INERTIA of matter, 42. 
 
 Motion is naturally permanent, 46 uniform, 47 straight, 47'. 
 Centripetal and centrifugal forces, 49. 
 Quantity of motion and force, 53 momentum, 54. 
 Direction of forces and composition of motion, 55. 
 The two forces of nature are Attraction and Repulsion, 58. 
 Accelerated motion, 58. 
 Retarded motion, 60. 
 Pendulum and balance wheel, 60. 
 Projectiles, 64. 
 
 Tides, winds, currents, &c., obey attraction, 66. 
 Explosion, steam, &c., obey repulsion, 66. 
 31 
 
482 ANALYTICAL TABLE. 
 
 All great velocities are results of continued action, and are de- 
 stroyed by continued action, 67. 
 Action and reaction equal and contrary, 70. 
 APPENDIX, Definitions, 74 Laws of motion, 75 Impulsive 
 force and rectilinear motion, 75 Constant force and constantly 
 accelerated motion, 76 Of Gravity, 77 Motion produced by 
 joint forces, 77 Equilibriums, 78 Joint^ction of an impulsive 
 and constant force, 78 Laws of central forces, 80 Joint effect 
 of active and inactive forces, 80 Pendulum, 80 Impact of 
 bodies, 83. 
 
 PART II. DOCTRINES OF SOLIDS or MECHANICS, 84. 
 
 Centres of inertia and gravity, 85. 
 
 In animal bodies, 90 Sea sickness, 91. 
 
 Centres of percussion and oscillation, 93. 
 Solids moving round a centre, or so that different parts may have 
 
 different speed, 94, 
 Simple machines, 95. 
 Lever, 95 Wheel and Axle, 103 Inclined plane, 105 
 
 Wedge, 106, Screw, 107 Pulley, 108 Engine of oblique 
 
 action, 109. 
 Fly wheels, 111. 
 Complex machines, 113. 
 
 Friction, 114. 
 
 Wheel carriages, 115 Rail- ways, 116. 
 Strength of materials, 119. 
 Influence of form Arches, &c., 122. 
 
 PART III. DOCTRINES OF FLUIDS or HYDRODYNAMICS, 126. 
 SECT. I. HYDROSTATICS, or fluids in repose, 126. 
 
 Pressure in a fluid extends equally through the whole, 127. 
 
 Hydrostatic press, &c., 129. 
 
 Pressure in a> fluid increases with the depth, 129. 
 
 Compressibility of water, &c., 131. 
 Not influenced by shape of vessel, 132. 
 Level surface of fluids, 133. 
 
 Spirit level, 133 Canals, 134 Running streams, 135 
 gradual change of the earth's surface produced by running 
 water, 136. 
 
 Same level in communicating vessels, 139. 
 City water works, 140. 
 Springs and wells, 141. 
 Support of bodies floating in fluids, 143. 
 Specific gravities, 144. 
 Floating bodies, 148. 
 
 Swimming of man and inferior animals, 149. 
 Ballast, 152. 
 Fluids of different density, 153. 
 
ANALYTICAL TABLE. 483 
 
 SECT. II. PNEUMATICS, or phenomena of air, 156. 
 
 Lightness, 158. 
 Elasticity, 158. 
 
 Air-pumps, 159 Diving-bell, 162 Water-balloon, 163 
 
 Hiero's fountain, lb'4. 
 Pressure in all directions, 165. 
 Pressure as the depth, 165. 
 
 Weight of the atmosphere, 167. 
 Atmospheric pressure on solids, 167. 
 Magdeburgh hemispheres, 168. 
 Pneumatic tractor, 168. 
 Atmospheric pressure on liquids, 169. 
 
 Pumps, 171 Syphon, 172 Intermitting fountains, 173 
 
 Bird-glass, 174 Vent-plugs, 174. 
 Atmospheric pressure on animal body, 175. 
 Cupping, &c., 176. 
 The barometer, 176. 
 Atmospheric pressure determines the liquid or aeriform state of 
 
 aeriform substances, 182. 
 Boiling, 182. 
 
 at different heights, 183. 
 in vacuo and distilling, 184. 
 Elastic force of steam, 186. 
 Steam-engines, 187. 
 Explosions, 190, 193. 
 Atmospheric pressure affecting combinations of bodies, 194. 
 
 Effervescence sparkling liquids, 194. 
 Atmospheric pressure affecting the density and temperature of the 
 
 air, 195. 
 
 Climate depending on elevation, 196. 
 
 Atmospheric pressure affecting the humidity of the air, 196. 
 Rain, mist, snow, hail, dew, 197 Hygrometer, 198. 
 Rain and clouds among mountains, 199. 
 Fluid support of floating in air, 200. 
 Balloons, 200. 
 
 Ascent of flame and smoke, 202. 
 Chimneys, 203. 
 Warming and ventilating of houses, 205. 
 
 Apartments for consumptive patients, 207. 
 Winds, 208. Trade winds, 208. 
 Land and sea breezes, 209. 
 Monsoons, 210. 
 Pneumatic trough, 210. 
 Gasometer, 211. 
 Pneumatic chemistry, 212. 
 
 SECT. III. HYDRAULICS, or fluids in motion, 213. 
 
 Fluids moving in channels or issuing from vessels, 213. 
 
 Aqueducts, 216. 
 
 Fountains and jets, 216. 
 Waves, 216. 
 Momentum and resistance of fluids, 220. 
 
484 ANALYTICAL TABLE. 
 
 Resistance to ships, &c., increases much more rapidly than the 
 velocity, 220. 
 
 Steam-boats, 221. 
 
 Paddle-wheels, 221. 
 Resistance to bodies in air, 222. 
 Fluid resistance limits many velocities, 222. 
 is influenced by shape of solid, 223. 
 
 Water-wheels, 224. 
 Fluid resistance proportioned to surface of contact, and not to 
 
 quantity of matter, 224. 
 Projectiles, 225 -levigating, 225 Winnowing, 226 Washing 
 
 gold dust, 226. 
 Oblique action of fluids, 226. 
 
 Navigation Sails, 226 Rudder, 227. 
 
 Windmills, 229 Feathered arrows, 229 Paper kites, 230. 
 Lifting fluids, 231. 
 Buckets Pumps Wheels Water-screw, 231 Water-ram, 232. 
 
 SECT. IV. ACOUSTICS, or doctrines of sound, 234. 
 
 Nature of simple sound, 235. 
 
 Continued sound or tone, 235 Grave and sharp sounds, 238. 
 Musical sounds, 238. 
 
 Musical scale, 239. 
 
 Melody Harmony Accompaniment Time, 241. 
 
 Tuning-forks, 242. 
 
 Musical ear, 245. 
 Spreading of sound in solid and fluid, 246 Stethoscope, 247. 
 
 Velocity of sound, 248 Many examples, 248. 
 Reflection of sound, 248. 
 
 Echo whispering galleries Ear trumpets Speaking-trumpets, 
 249. 
 
 Animal ear, 252. 
 
 PART IV. Doctrines of imponderable substance, 254. 
 HEAT. 256. 
 
 SECT. 1 HEAT, phenomena which it produces, 257. 
 
 ' Properties, 25$ Means of measuring quantity in bodies, 259. 
 
 Diffuses itself equally, 259. 
 Cold, 260. 
 
 Conducting power of bodies, 260 Natural covering of animals, 
 262 Clothing 262. 
 Mode in which heat spreads, 266. 
 
 By the motion of the particles in fluids, 266. 
 By radiation, 269. 
 Expansion of bodies by heat, 274. 
 
 Capacity of bodies for heat, 275 Influence -of bulk on this, 
 277 Of density, 280. 
 
ANALYTICAL TABLE. 485 
 
 Each substance expands in a degree peculiar to itself, 281 
 Expansion of solids, 282 Expansion of fluids, 284 Expan- 
 sion of gases, 284. 
 
 Latent heat, 290. 
 
 Boiling, 295. 
 
 Distillation, 295. 
 
 Evaporation, 295. 
 
 Table of temperature, 300 -Thermometer, 300. 
 
 Absolute quantity of heat, 304. 
 
 Influence of heat on chemical combinations, 305. 
 
 Influence of heat on vegetables and animals, 307. 
 
 Sources of heat, 309 The sun, 309 Electricity, 311 Com- 
 bustion, 311 Fuel, 317 Condensation and friction, 322 
 Functions of animal life, 323. 
 
 SEC. II LIGHT, 325 Sources of, 327. 
 
 Becomes less intense as it spreads, 327 Falling on bodies ren- 
 ders them visible, 328. 
 Shadows, 329, 331> 
 Velocity of light, 329. 
 Direction of light, 330. 
 Transmission of bodies, 332. 
 Refraction, 333. 
 Lenses, 339. 
 
 Camera obscura, solar microscope, magic lantern, 341. 
 Eye, 346. 
 Vision, 3 ; 48, 352. 
 Distinct vision, 350. 
 Short-sightedness, 350 
 Long-sightedness, 350. 
 Visual angle, 355. 
 Apparent size of objects, 357. 
 Foreshortening, 360. 
 Perspective, 362, 373. 
 Intensity of light, shade and colour, 369. 
 Divergence of rays of light, 374. 
 Convergence of axes of the eyes, 375. 
 Cosmorama, 377. 
 
 Painting representing motion, 378. 
 Art of painting, 379. 
 Telescope. 381. 
 
 Mirrors, straight, 386 Curved, 390. 
 Comparison of light and sound, 392. 
 Perfection of the eye, 395. 
 
 PART V. ANIMAL AND MEDICAL PHYSICS. 
 SECT. I. ANIMAL MECHANICS, 399. 
 
 Skull, &c., 399. 
 
 Spine and its distortions, 400. 
 
 Limbs and mechanical surgery, 401. 
 
486 ANALYTICAL TABLE. 
 
 Living force, 409 Treadmill, 410. 
 Surgical instruments, 410. 
 
 SECT. II. ANIMAL HYDROSTATICS AND HYDRAULICS, or Fluidity in 
 relation to animals, 414. 
 
 1. Circulation of blood. 
 
 In arteries, 416. 
 
 In capillaries, 421. 
 
 In veins, 423. 
 Force of the heart, 430. 
 Hydrostatic or floating bed, 431. 
 Velocity of blood, 435. 
 The pulse, 436. 
 Circulation in the head, 440. 
 Effects of position on the circulation, 441. 
 Fainting from diminished arterial tension, 442. 
 Diffused pressure, 444. 
 Mercurial bath, 445. 
 Transfusion of blood, 445. 
 
 2. Respiration and voice, 445. 
 
 Action of chest, 446. 
 
 Wounds of chest, 447. 
 
 Hemoptysis, 448. 
 
 Coughing, 448 Sneezing Hiccup, &c., 449. 
 
 Suffocation, 449. 
 
 Humane Society's apparatus, 450. 
 
 Artificial respiration, 450. 
 
 Speech, 451. 
 
 Modifications of voice, 452. 
 
 Table of articulations, 455. 
 
 Stuttering, 456. . 
 
 Ventriloquism, 461. 
 
 3. Digestion, 463. 
 
 Mechanism of the organs, 463. 
 Effects of abdominal pressure, 465. 
 Vomiting, 466. 
 Stomach pump, &c., 466. 
 Enema funnel, 468. 
 
 4. Secretion of the kidneys, 469. 
 
 The apparatus. 
 Obstructions in urethra, 470. 
 New instruments and means 'for treatment, 471. 
 Stone in the bladder, 474. 
 -New instruments and means, 475. 
 
 5. Uterine phenomena. 
 
 Protection of foetus by the liquor amnii, 477. 
 
 Position of ditto, 477. 
 
 Importance of physical knowledge and manual dexteirfcy, 478 
 
 THE END. 
 
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 MEDICAL AND SURGICAL PUBLICATIONS, 
 
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 &jT The attention of physicians is especially solicited to the following important new works 
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 Druitt's Surgery, " 10 
 
 Dalton's Human Physiology, " 11 
 
 Dunglison's Medical Dictionary, " \- 
 
 Erichsen's System of Surgery, " 14 
 
 Flint on the Heart, " 14 
 
 Fownes' Manual of Chemistry, ........." 15 
 
 Gross's System of Surgery, " 16 
 
 Gray's Anatomy, Descriptive and Surgical, " 17 
 
 Habershon on Alimentary Canal, " 18 
 
 Hamilton on Fractures and Dislocations, . 
 
 Hodge on Diseases of Women, 
 
 Meigs on Diseases of Women, 
 
 Parrish's Practical Pharmacy, 
 
 Stille's Therapeutics and Materia Medica, 
 
 Simpson on Diseases of Women, 
 
 Todd's Clinical Lectures, 
 
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 28 
 29 
 30 
 
 Toynbee on the Ear. 
 Watson's Practice of Physic, . 
 
 Walshe on the Lungs, " 30 
 
 Wilson's Human Anatomy, .......... u 31 
 
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 West on Diseases of Children , " 32 
 
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 BUSHNAN'S PHYSIOLOGY OF ANIMAL AND 
 VEGETABLE LIFE; a Popular Treatise on the 
 Functions and Phenomena of Organic Life. In 
 one handsome royal 12mo. volume, extra cloth, 
 with over 100 illustrations, pp.234. 80 cents. 
 
 BUCKLER ON THE ETIOLOGY, PATHOLOGY, 
 AND TREATMENT OF FIBRO-BRONCHI- 
 TIS AND RHEUMATIC PNEUMONIA. In 
 
 one 8vo. volume, extra cloth, pp. 150. SI 25. 
 
 BLOOD AND URINE (MANUALS ON). BY 
 JOHN WILLIAM GRIFFITH, G. OWEN 
 REESE, AND ALFRED MARKWICK. One 
 thick volume, royal 12mo., extra cloth, with 
 plates, pp. 460. $1 25. 
 
 BRODIE'S CLINICAL LECTURES ON SUR- 
 GERY. 1 vol. 8vo. cloth. 350pp. $125. 
 
AND SCIENTIFIC PUBLICATIONS. 
 
 BARCLAY (A. W.), M. D., 
 
 Assistant Physician to St. George's Hospital, &c. 
 
 A MANUAL OF MEDICAL DIAGNOSIS; being an Analysis of the Signs 
 
 and Symptoms of Disease. In one neat octavo volume, extra cloth, of 424 pages. $200. (Lately 
 
 issued.} 
 
 Of works exclusively devoted to this important | The task of composing such a work is neither an 
 branch, our profession has at command, cornpara- easy nor a light one ; but Dr. Barclay has performed 
 tively, but few, and, therefore, in the publication of j it in a manner which meets our most unqualified 
 
 the present work, Messrs. Blanchard & Lea have 
 conferred a great favor upon us. Dr. Barclay, from 
 having occupied, for a long period, the position of 
 Medical Registrar at St. George's Hospital, pos- 
 sessed advantages for correct observation and reli- 
 able conclusions, as to the significance of symptoms, 
 which have fallen to the lot of but few, either in 
 his own or any other country. He has carefully 
 systematized the results of his observation of over 
 twelve thousand patients, and by his diligence and 
 judicious classification, the profession has been 
 presented with the most convenient and reliable 
 work on the subject of Diagnosis that it has been 
 our good fortune ever to examine; we can, there- 
 fore, say of Dr. Barclay's work, that, from his sys- 
 tematic manner of arrangement, his work is one of 
 the best works " for reference" in the daily emer- 
 gencies of the practitioner, with which we are ac- 
 quainted ; but, at the same time, we would recom- 
 mend our readers, especially the younger ones, to 
 read thoroughly and study diligently the whole work, 
 and the "emergencies" will not occur so often. 
 Southern Med. and Surg. Journ., March, 1858. 
 
 To give this information, to supply this admitted 
 deficiency, is the object of Dr. Barclay's Manual. 
 
 approbation. He is no mere theorist; he knows his 
 work thoroughly, and in attempting to perform it, 
 has not exceeded his powers. British Med. Journal, 
 Dec. 5, 1857. 
 
 We venture to predict that the work will be de- 
 servedly popular, and soon become, like Watson's 
 Practice, an indispensable necessity to the practi- 
 tioner. N. A. Med. Journal, April, 1858. 
 
 An inestimable work of reference for the young 
 practitioner and student. Nashville Med. Journal, 
 May, 1858. 
 
 We hope the volume will have an extensive cir- 
 culation, not among students of medicine only, but 
 practitioners also. They will never regret a faith- 
 ful study of its pages. Cincinnati Lancet, Mar. '58. 
 
 An important acquisition to medical literature. 
 It is a work of high merit s both from the vast im- 
 portance of the subject upon which it treats, and 
 also from the real ability displayed in its elabora- 
 tion. In conclusion, let us bespeak for this volume 
 that attention of every student of our art which it 
 so richly deserves that place in every medical 
 library which it can so well adorn. Peninsular 
 Medical Journal, Sept. 1858. 
 
 BARLOW (GEORGE H.), M. D, 
 
 Physician to Guy's Hospital, London, &c. 
 
 A MANUAL OF THE PRACTICE OF MEDICINE. With Additions by D. 
 
 F. CONDIE, M. D., author of " A Practical Treatise on Diseases of Children," &c. In one hend- 
 some octavo volume, leather, of over 600 pages. $2 75. 
 
 will be found hardly less useful to the experienced 
 
 We recommend Dr . Barlow' s Manual in the warm- 
 est manner as a most valuable vade-mecum. We 
 have had frequent occasion to consult it, and have 
 found it clear, concise, practical, and sound. It is 
 eminently a practical work, containing all that is 
 essential, and avoiding useless theoretical discus- 
 sion. The work supplies what has been for some 
 time wanting, a manual of practice based upon mo- 
 dern discoveries in pathology and rational views of 
 treatment of disease. It is especially intended for 
 the use of students and junior practitioners, but it 
 
 physician. The American editor has added to the 
 work three chapters on Cholera Infantum, Yellow 
 Fever, and Cerebro-spinal Meningitis. These addi- 
 tions, the two first of which are indispensable to a 
 work on practice destined for the profession in this 
 country, are executed with great judgment and fi- 
 delity, by Dr. Condie, who has also succeeded hap- 
 pily in imitating the conciseness and clearness of 
 style which are such agreeable characteristics of 
 the original book. Boston Med. and Surg. Journal. 
 
 BARTLETT (ELISHA), M. D. 
 THE HISTORY, DIAGNOSIS, AND TREATMENT OF THE FEVERS 
 
 OF THE UNITED STATES. A new and revised edition. By ALONZO CLARK, M. D., Prof. 
 
 of Pathology and Practical Medicine in the N. Y. College of Physicians and Surgeons, &c. In 
 
 one octavo volume, of six hundred pages, extra cloth. Price $3 00. 
 
 logy. His annotations add much to the interest of 
 the work, and have brought it well up to the condi- 
 tion of the science as it exists at the present day 
 in regard to this class of diseases. Southern Med. 
 and Surg. Journal, Mar. 1857. 
 
 It is a work of great practical value and interest, 
 containing much that is new relative to the several 
 diseases of which it treats, and, with the additions 
 of the editor, is fully up to the times. The distinct- 
 ive features of the different forms of fever are plainly 
 and forcibly portrayed, and the lines of demarcation 
 carefully and accurately drawn, and to the Ameri- 
 can practitioner is a more valuable and safe guide 
 than any work on fever extant. Ohio Med. and 
 Surg. Journal, May, 1857. 
 
 It is the best work on fevers which has emanated 
 from the American press, and the present editor has 
 carefully availed himself of all information exist- 
 ing upon the subject in the Old and New World, so 
 that the doctrines advanced are brought down to the 
 latest date in the progress of this department of 
 Medical Science. London Med. Times and Gazette, 
 May 2, 1857. 
 
 This excellent monograph on febrile disease, has 
 stood deservedly high since its first publication. It 
 will be seen that it has now reached its fourth edi- 
 tion under the supervision of Prof. A. Clark, a gen- 
 tleman who, from the nature of his studies and pur- 
 suits, is well calculated to appreciate and discuss 
 the many intricate and difficult questions in patho- 
 
 BROWN (ISAAC BAKER), 
 
 Surgeon- Accoucheur to St. Mary's Hospital, &c. 
 
 ON SOME DISEASES OF WOMEN ADMITTING OF SURGICAL TREAT- 
 
 MENT. With handsome illustrations. One vol. 8vo., extra cloth, pp. 276. f 1 60. 
 
 Mr. Brown has earned for himself a high reputa- [ and merit the careful attention of every surgeon- 
 tion in the operative treatment of sundry diseases accoucheur. Association Journal. 
 and injuries to which females are peculiarly subject. 
 We can truly say of his work that it is an important 
 addition to obstetrical literature. The operative 
 suggestions and contrivances which Mr. Brown de- 
 scribes, exhibit much practical sagacity and skill, 
 
 We have no hesitation in recommending this book 
 to tlie careful attention of all surgeons who make 
 female compiaints a partof their study and prsctioa. 
 Dublin Quarterly Journal. 
 
6 
 
 BLANCHARD & LEA'S MEDICAL 
 
 CARPENTER (WILLIAM B.), M. D., F. R. S., &c., 
 
 Examiner in Physiology and Comparative Anatomy in the University of London. 
 
 PRINCIPLES OF HUMAN PHYSIOLOGY; with their chief applications to 
 
 Psychology, Pathology, Therapeutics, Hygiene, and Forensic Medicine. A new American, from 
 the last and revised London edition. With nearly three hundred illustrations. Edited, with addi- 
 tions, by FRANCIS GTJRNEY SMITH, M. D., Professor of the Institutes of Medicine in the Pennsyl- 
 vania Medical College, &c. In one very large and beautiful octavo volume, of about nine hundred 
 large pages, handsomely printed and strongly bound in leather, with raised bands. $4 25. 
 
 In the preparation of this new edition, the author ha* spared no labor to render it, as heretofore, 
 a complete and lucid exposition of the most advanced condition of its important subject. The 
 amount of the additions required to effect this object thoroughly, joined to the former large size of 
 the volume, presenting objections arising from the unwieldy bulk of the work, he has omitted all 
 those portions not bearing directly upon HUMAN PHYSIOLOGY, designing to incorporate them in 
 his forthcoming Treatise on GENERAL PHYSIOLOGY. As a full and accurate text-book on the Phy- 
 siology of Man, the work in its present condition therefore presents even greater claims upon 
 the student and physician than those which have heretofore won for it the very wide and distin- 
 guished favor which it has so long enjoyed. The additions of Prof. Smith will be found to supply 
 whatever may have been wanting to the American student, while the introduction of many new 
 illustrations, and the most careful mechanical execution, render the volume one of the most at- 
 tractive as yet issued. 
 
 For upwards of thirteen years Dr. Carpenter's 
 work has been considered by the profession gene- 
 rally, both in this country and England, as the most 
 valuable compendium on the subject of physiology 
 in our language. This distinction it owes to the high 
 attainments and unwearied industry of its accom- 
 plished author. The present edition (which, like the 
 last American one, was prepared by the author him- 
 self)} is the result of such extensive revision, that it. 
 may almost be considered a new work. We need 
 hardly say, in concluding this brief notice, that while 
 the work is indispensable to every student of medi- 
 eine in this country, it will amply repay the practi- 
 tioner for its perusal by the interest and value of its 
 contents. Boston Med. and Surg. Journal. 
 
 This is a standard work the text-book used by all 
 medical students who read the English language. 
 It has passed through several editions in order to 
 keep pace with the rapidly growing science of Phy- 
 siology. Nothing need be said in its praise, for its 
 merits are universally known ; we have nothing to 
 say of its defects, for they only appear where the 
 science of which it treats is incomplete. Western 
 Lancet. 
 
 The most complete exposition of physiology which 
 any language can at present give. Brit . and For. 
 Med.-Chirurg. Review. 
 
 The greatest, the most reliable, and the best book 
 on the subject which we know of in the English 
 language. Stethoscope . 
 
 To eulogize this great work would be superfluous. 
 
 We should observe, however, that in this edition 
 
 i the author has remodelled a large portion of the 
 
 I former, and the editor has added much matter of in- 
 
 j terest, especially in the form of illustrations. We 
 
 I may confidently recommend it as the most complete 
 
 I work on Human Physiology in our language. 
 
 Southern Med. and Surg. Journal. 
 
 The most complete work on the science in o-ur 
 language. Am. Med. Journal. 
 
 The most complete work now extant in our lan- 
 guage. N. O. Med. Register. 
 
 The best text-book in the language on this ex- 
 tensive subject. London Med. Times. 
 
 A complete cyclopaedia of this branch of science. 
 N. Y. Med. Times. 
 
 The profession of this country, and perhaps also 
 of Europe, have anxiously and for some time awaited 
 the announcement of this new edition of Carpenter's 
 Human Physiology. His former editions have for 
 many years been almost the only text-book on Phy- 
 siology in all our medical schools, and its circula- 
 tion among the profession has been unsurpassed by 
 any work in any department of medical science. 
 
 It is quite unnecessary for us to speak of this 
 work as its merits would justify. The mere an- 
 nouncement of its appearance will afford the highest 
 pleasure to every student of Physiology, while its 
 perusal will be of infinite service in advancing 
 physiological science. Ohio Med. and Surg. Jour*. 
 
 BY THE SAME AUTHOR. 
 
 PRINCIPLES OF COMPARATIVE PHYSIOLOGY. New American, from 
 
 the Fourth and Revised London edition. In one large and handsome octavo volume, with over 
 three hundred beautiful illustrations, pp.752. Extra cloth, $4 80 ; leather, raised bands, $5 25. 
 
 The delay which has existed in the appearance of this work has been caused by the very thorough 
 revision and remodelling which it has undergone at the hands of the author, and the large number 
 of new illustrations which have been prepared for it. It will, therefore, be found almost a new 
 work, and fully up to the day in every department of the subject, rendering it a reliable text-book 
 for all students engaged in this branch of science. Every effort has been made to render its typo- 
 graphical finish and mechanical execution worthy of its exalted reputation, and creditable to the 
 mechanical arts of this country. 
 
 This book should not only be read but thoroughly 
 studied by every member of the profession. None 
 are too wise or old, to be benefited thereby. But 
 especially to the younger class would we cordially 
 commend it as best fitted of any t work in the English 
 language to qualify them for the reception and com- 
 prehension of those truths which are daily being de- 
 veloped in physiology. Medical Counsellor. 
 
 Without pretending to it, it is an encyclopedia of 
 the subject, accurate and complete in all respects 
 a truthful reflection of the advanced state at which 
 the science has now arrived. Dublin Quarterly 
 Journal of Medical Science. 
 
 A truly magnificent work in itself a perfect phy- 
 siological study. Ranking' s Abstract. 
 
 This work stands without its fellow. It in one 
 few men in Europe could have undertaken ; it is one 
 
 no man, we believe, could have brought to so suc- 
 cessful an issue as Dr. Carpenter. It required for 
 its production a physiologist at once deeply read in 
 the labors of others, capable of taking a general, 
 critical, and unprejudiced view of those labors, and 
 of combining the varied, heterogeneous materials at 
 his disposal, so as to form an harmonious whole. 
 We feel that this abstract can give the reader a very 
 imperfect idea of the fulness of this work, and no 
 idea of its unity, of the admirable manner in which 
 material has been brought, from the most various 
 sources, to conduce to its completeness, of the lucid- 
 ity of the reasoning it contains, or of the clearness 
 of language in which the whole is clothed. Not the 
 profession only, but the scientific world at large, 
 must feel deeply indebted to Dr. Carpenter for this 
 great work. It must, indeed, add largely even to 
 his high reputation. Medical Times. 
 
AND SCIENTIFIC PUBLICATIONS 
 
 CARPENTER (WILLIAM B.), M. D., F. R. S., 
 
 Examiner in Physiology and Comparative Anatomy in the University of London. 
 
 DHE MICROSCOPE AND ITS REVELATIONS. With an Appendix con- 
 
 taining the Applications of the Microscope to Clinical Medicine, &c. By F. G. SMITH, M. D. 
 Illustrated by four hundred and thirty-four beautiful engravings on wood. In one large and verp 
 handsome octavo volume, of 724 pages, extra cloth, $4 00 ; leather, $4 50. 
 
 Dr. Carpenter's position as a microscopist and physiologist, and his great experience as a teacher, 
 jminently qualify him to produce what has long been wanted a good text-book on the practical 
 jse of the microscope. In the present volume his object has been, as stated in his Preface, " to 
 jombine, within a moderate compass, that information with regard to the use of his ' tools,' which 
 .s most essential to the working microscopist, with such an account of the objects best fitted for 
 lis study, as might qualify him to comprehend what he observes, and might thus prepare him to 
 jeneh't science, whilst expanding and refreshing his own mind " That he has succeeded in accom- 
 jlishing this, no one acquainted with his previous labors can doubt. 
 
 The great importance of the microscope as a means of diagnosis, and the number of microsco- 
 jists who are also physicians, have induced the American publishers, with the author's approval, to 
 idd an Appendix, carefully prepared by Professor Smith, on the applications of the instrument to 
 clinical medicine, together with an account of American Microscopes, their modifications and 
 iccessories. This portion of the work is illustrated with nearly one hundred wood-cuts, and, it is 
 loped, will adapt the volume more particularly to the use of the American student. 
 
 Every care has been taken in the mechanical execution of the work, which is confidently pre- 
 sented as in no respect inferior to the choicest productions of the London press. 
 
 The mode in which the author has executed his intentions may be gathered from the following 
 sondensed synopsis of the 
 
 CONTENTS. 
 
 [INTRODUCTION History of the Microscope. CHAP. I. Optical Principles of the Microscope. 
 CHAP. II. Construction of the Microscope. CHAP. III. Accessory Apparatus. CHAP. IV. 
 Management of the Microscope CHAP. V. Preparation, Mounting, and Collection of Objects. 
 CHAP. VI. Microscopic Forms of Vegetable Life Protophytes. CHAP. VII. Higher Cryptoga- 
 mia. CHAP. VIII. Phanerogamic Plants. CHAP. IX. Microscopic Forms of Animal Life Pro- 
 tozoa Animalcules. CHAP. X. Foraminifera, Polycystina, and Sponges. CHAP. XL Zoophytes. 
 CHAP. XII. Echinodermata. CHAP. XIII. Polyzoa and Compound Tunicata. CHAP. XIV. 
 Molluscous Animals Generally. CHAP. XV. Annulosa. CHAP. XVI. Crustacea. CHAP. XVII. 
 Insects and Arachnida. CHAP. XVIII. Vertebrated Animals. CHAP. XIX. Applications of the 
 Microscope to Geology. CHAP. XX. Inorganic or Mineral Kingdom Polarization. APPENDIX. 
 Microscope as a means of Diagnosis Injections Microscopes of American Manufacture. 
 
 Those who are acquainted with Dr. Carpenter's 
 previous writings on Animal and Vegetable Physio- 
 fogy, will fully understand how vast a store of know- 
 ledge he is able to bring to bear upon so comprehen- 
 sive a subject as the revelations of the microscope ; 
 and even those who have no previous acquaintance 
 with the construction or uses of this instrument, 
 will find abundance of information conveyed in clear 
 and simple language. Med. Times and Gazette. 
 
 Although originally not intended as a strictly 
 
 medical work, the additions by Prof. Smith give it 
 a positive claim upon the profession, for which we 
 doubt not he will receive their sincere thanks. In- 
 deed, we know not where the student of medicine 
 will find such a complete and satisfactory collection 
 of microscopic facts bearing upon physiology and 
 practical medicine as is contained in Prof. Smith's 
 appendix; and this of itself, it seems to us, is fully 
 worth the cost of the volume. Louisville Medical 
 Review, Nov. 1856. 
 
 BY THE SAME AUTHOR. 
 
 ELEMENTS (OR MANUAL) OF PHYSIOLOGY, INCLUDING PHYSIO- 
 LOGICAL ANATOMY. Second American, from a new and revised London edition. With 
 one hundred and ninety illustrations. In one very handsome octavo volume, leather, pp. 566. 
 $3 00. 
 
 In publishing the first edition of this work, its title was altered from that of the London volume, 
 by the substitution of the word " Elements" for that of " Manual," and with the author's sanction 
 tae title of " Elements" is still retained as being more expressive of the scope of the treatise. 
 
 To say that it is the best manual of Physiology 
 now before the public, would not do sufficient justice 
 to the author. Buffalo Medical Journal. 
 
 In his former works it would seem that he had 
 exhausted the subject of Physiology. In the present, 
 he gives the essence, as it were, of the whole. N. Y. 
 Journal of Medicine . 
 
 Those who have occasion for an elementary trea- 
 tise on Physiology, cannot do better than to possess 
 themselves of the manual of Dr. Carpenter. Medical 
 Examiner. 
 
 The best and most complete expose" of modern 
 Physiology, in one volume, extant in the English 
 language. St. Louis Medical Journal. 
 
 BY THE SAME AUTHOR. (Preparing.) 
 
 PRINCIPLES OF GENERAL PHYSIOLOGY, INCLUDING ORGANIC 
 
 CHEMISTRY AND HISTOLOGY. With a General Sketch of the Vegetable and Animal 
 Kingdom. In one large and very handsome octavo volume, with several hundred illustrations. 
 The subject of general physiology having been omitted in the last editions ot the author's " Com- 
 parative Physiology" and "Human Physiology," he has undertaken to prepare a volume which 
 shall present it more thoroughly and fully than has yet been attempted, and which may be regarded 
 as an introduction to his other works. 
 
 BY THE SAME AUTHOR. 
 
 A PRIZE ESSAY ON THE USE OF ALCOHOLIC LIQUORS IN HEALTH 
 
 AND DISEASE. New edition, with a Preface by D. F. CONDIE, M. D., and explanations of 
 Hp.ip.ntifif. words. Tn ons neat 12rno. vnlnme. extra eloth. tm. 178. 50 cents. 
 
8 
 
 BLANCHAKD & LEA'S MEDICAL 
 
 CONDIE (D. F.), M. D., &c. 
 A PRACTICAL TREATISE ON THE DISEASES OF CHILDREN. Fifth 
 
 edition, revised and augmented. In one large volume, 8vo., leather, of over 750 pages. $3 25. 
 
 (Just Issued, 1859.) 
 
 In presenting a new and revised edition of this favorite work, the publishers have only to state 
 that the author has endeavored to render it in every respect "a complete and faithful exposition of 
 the pathology and therapeutics of the maladies incident to the earlier stages of existence a full 
 and exact account of the diseases of infancy and childhood.." To accomplish this he has subjected 
 the whole work to a careful and thorough revision, rewriting a considerable portion, and adding 
 several new chapters. In this manner it is hoped that any deficiencies which may have previously 
 existed have been supplied, that the recent labors of practitioners and observers have been tho- 
 roughly incorporated, and that in every point the work will be found to maintain the high reputation 
 it has enjoyed as a complete and thoroughly practical book of reference in infantile affections. 
 
 A few notices of previous editions are subjoined. 
 
 Dr. Condie's scholarship, acumen, industry, and 
 practical sense are manifested in this, as in all his 
 numerous contributions to science. Dr. Holmes's 
 Report to the American Medical Association. 
 
 Taken as a whole, in our judgment, Dr. Condie's 
 Treatise is the one from the perusal of which the 
 practitioner in this country will rise with the great- 
 est satisfaction. Western Journal of Medicine and 
 Surgery. 
 
 One of the best works upon the Diseases of Chil- 
 dren in the English language. Western Lancet. 
 
 We feel assured from actual experience that no 
 physician's library can be complete without a copy 
 of this work. N. Y. Journal of Medicine. 
 
 A veritable psediatric encyclopaedia, and an honor 
 to American medical literature. Ohio Medical and 
 SurgicalJournal. 
 
 We feel persuaded that the American medical pro- 
 fession will soon regard it not only as a very good, 
 but as the VERY BEST " Practical Treatise on the 
 Diseases of Children." American Medical Journal. 
 
 In the department of infantile therapeutics, the 
 work of Dr. Condie is considered one of the best 
 which has been published in the English language. 
 The Stethoscope. 
 
 We pronounced the first edition to be the best 
 work on the diseases of children in the English 
 language, and, notwithstanding all that has been 
 
 we still regard it in that light. Medical 
 
 anguage, 
 published, 
 Examiner. 
 
 The value of works by native authors on the dis- 
 eases which the physician is called upon to combat, 
 will be appreciated by all ; and the work of Dr. Con- 
 die has gained for itself the character of a safe guide 
 for students, and a useful work for consultation by 
 those engaged in practice. N. Y. Med. Times. 
 
 This is the fourth edition of this deservedly popu- 
 lar treatise. During the interval since the last edi- 
 tion, it has been subjected to a thorough revision 
 by the author; and all new observations in the 
 pathology and therapeutics of children have been 
 included in the present volume. As we said before, 
 we do not know of a better book on diseases of chil- 
 dren, and to a large part of its recommend.itions we 
 yield an unhesitating concurrence. Buffalo Med. 
 Journal. 
 
 Perhaps the most full and complete work now be- 
 fore the profession of the United States ; indeed, we 
 may say in the English language. It is vastly supe- 
 rior to most of its predecessors. Transylvania Med. 
 Journal . 
 
 CHRISTISON (ROBERT), M. D., V. P< R. S. E., &c. 
 A DISPENSATORY; or. Commentary on the Pharmacopeias of Great Britain 
 
 and the United States ; comprising the Natural History, Description, Chemistry, Pharmacy, Ac- 
 tions, Uses, and Doses of the Articles of the Materia Medica. Second edition, revised and im- 
 proved, with a Supplement containing the most important New Remedies. With copious Addi- 
 tions, and two hundred and thirteen large wood -engravings. By R. EGLESFELD GRIFFITH, M. D. 
 In one very large and handsome octavo volume, leather, raised bands, of over 1000 pages. $3 50. 
 
 COOPER (BRANSBY B.), F. R. S. 
 LECTURES ON THE PRINCIPLES AND PRACTICE OF SURGERY. 
 
 in one very large octavo volume, extra cloth, of 750 pages. $300. 
 
 COOPER ON DISLOCATIONS AND FRAC- 
 TURES OF THE JOINTS. Edited by BRANSBY 
 B. COOPER, F. R. S., &c. With additional Ob- 
 servations by Prof. J. C. WARREN. A new Ame- 
 rican edition. In one handsome octavo volume, 
 extra cloth, of about 500 pages, with numerous 
 illustrations on wood. $3 25. 
 
 COOPER ON THE ANATOMY AND DISEASES 
 OF THE BREAST, with twenty-five Miscellane- 
 ous and Surgical Papers. One large volume, im- 
 perial 8vo., extra cloth, with 252 figures, on 36 
 plates. $2 50. 
 
 COOPER ON THE STRUCTURE AND DIS- 
 EASES OF THE TESTIS, AND ON THE 
 THYMUS GLAND. One vol. imperial 8vo., ex- 
 tra cloth, with 177 figures on 29 plates. $2 00. 
 
 COPLAND ON THE CAUSES, NATURE, AND 
 TREATMENT OF PALSY AND APOPLEXY. 
 In one volume, royal 12mo., extra cloth, pp. 328. 
 80 cents. 
 
 CLYMER ON FEVERS; THEIR DIAGNOSIS, 
 PATHOLOGY, AND TREATMENT In one 
 octavo volume, leather, of 600 pages, f 1 50. 
 
 COLOMBAT DE L'ISERE ON THE DISEASES 
 
 OF FEMALES, and on the special Hygiene of 
 their Sex. Translated, with many Notes and Ad- 
 ditions, by C. D. MEIGS, M. D. Second edition, 
 revised and improved. In one large volume, oc- 
 tavo, leather, with numerous wood-cuts, pp. 720. 
 $350. 
 
 CARSON (JOSEPH), M. D., 
 
 Professor of Materia Medica and Pharmacy in the University of Pennsylvania. 
 
 SYNOPSIS OF THE COURSE OF LECTURES ON MATERIA MEDICA 
 
 AND PHARMACY, delivered in the University of Pennsylvania, Second and revised edi- 
 tion. In one very neat octavo volume, extra cloth, of 208 pages. $1 50. 
 
 CURLING (T. B.), F.R.S., 
 
 Surgeon to the London Hospital, President of the Hunterian Society, &c. 
 
 A PRACTICAL TREATISE ON DISEASES OF THE TESTIS, SPERMA- 
 TIC CORD, AND SCROTUM. Second American, from the second and enlarged English edi- 
 tion. In one handsome octavo volume, extra cloth, with numerous illustrations, pp. 420. $2 00. 
 
AND SCIENTIFIC PUBLICATIONS 
 
 CHURCHILL (FLEET WOOD), M. D., M. R. I. A. 
 ON THE THEORY AND PRACTICE OF MIDWIFERY. A new American 
 
 from the fourth revised and enlarged London edition. With Notes and Additions, by D. FRANCIS 
 CONDIE, M. D., author of a "Practical Treatise on the Diseases of Children," &c. With 194 
 illustrations. Tn one very handsome octavo volume, leather, of nearly 700 large pages. $3 50. 
 (Now Ready, October, I860.) 
 
 This work has been so long an established favorite, both as a text-book for the learner and as a 
 reliable aid in consultation for the practitioner, that in presenting a new edition it is only necessary 
 to call attention to the very extended improvements which it has received. Having had the benefit 
 of two revisions by the author since the last American reprint, it has been materially enlarged, and 
 Dr. Churchill's well-known conscientious industry is a guarantee that every portion has been tho- 
 roughly brought up with the latest results of European investigation in all departments of the sci- 
 ence and art of obstetrics. The recent date of the last Dublin edition has not left much of novelty 
 for the American editor to introduce, but he has endeavored to insert whatever has since appeared, 
 together with such matters as his experience has shown him would be desirable for the American 
 student, including a large number of illustrations. With the sanction of the author he has added 
 in the form of an appendix, some chapters from a little "Mmiual for Mid wives and Nurses," re- 
 cently issued by Dr. Churchill, believing that the details there presented can hardly fail to prove of 
 advantage to the junior practitioner. The result of all these additions is that the work now con- 
 tains fully one-half more matter than the last American edition, with nearly one-half more illus- 
 trations, so that notwithstanding the use of a smaller type, the volume contains almost two hundred 
 pages more than before. 
 
 No effort has been spared to secure an improvement in the mechanical execution of the work 
 equal to that which the text has received, and the volume is confidently presented as one of the 
 handsomest that has thus far been laid before the American profession; while the very low price 
 at which it is offered should secure for it a place in every lecture-room and on every office table. 
 
 The most popular work on midwifery ever issued 
 
 A better book in which to learn these important 
 points we have not met than Dr. Churchill's. Every 
 page of it is full of instruction; the opinion of all 
 writers of authority is given on questions of diffi- 
 culty, as well as the directions and advice of the 
 learned author himself, to which he adds the result 
 of statistical inquiry, putting statistics in their pro- 
 per place and giving them their due weight, and no 
 more. We have never read a book more free from 
 professional jealousy than Dr. Churchill's. It ap- 
 pears to be written with the true design of a book on 
 medicine, viz : to give all that is known on the sub- 
 ject of which he treats, both theoretically and prac- 
 tically, and to advance such opinions of his own as 
 he believes will benefit medical science, and insure 
 the safety of the patient. We have said enough to 
 convey to the profession that this book of Dr. Chur- 
 chill's is admirably suited for a book of reference 
 for the practitioner, as well as a text-book for the 
 student, and we hope it may be extensively pur- 
 chased amongst our readers. To "them we most 
 strongly recommend it. Dublin Medical Press , 
 June 20, 1860. 
 
 To bestow praise on a book that has received such 
 marked approbation would be superfluous. We need 
 only say, therefore, that if the first edition was 
 thought worthy of a favorable reception by the 
 medical public, we can confidently affirm that this 
 will be found much more so. The lecturer, the 
 practitioner, and the student, may all have recourse 
 to its pages, and derive from their perusal much in- 
 terest and instruction in everything relating to theo- 
 retical and practical midwifery. Dublin Quarterly 
 Journal of Medical Science. 
 
 A work of very great merit, and such as we can 
 confidently recommend to the study of every obste- 
 tric practitioner. London Medical Gazette. 
 
 This is certainly the most perfect system extant. 
 It is the best adapted for the purposes of .a teixt- 
 book, and that which he whose necessities confine 
 him to one book, should select in preference to all 
 
 from the American press. Charleston Med. Journal. 
 
 Were we reduced to the necessity of having but 
 sne work on midwifery, and permitted to choose, 
 we would unhesitatingly take Churchill. Western 
 Med. and Surg. Journal. 
 
 It is impossible to conceive a more useful anil 
 jlegant manual than Dr. Churchill's Practice of 
 Midwifery. Provincial Medical Journal. 
 
 Certainly, in our opinion, the very best work on 
 he subject which exists. IV. Y. Annalist. 
 
 No work holds a higher position, or is more de- 
 serving of being placed in the hands of the tyro, 
 the advanced student, or the practitioner. Medical 
 Examiner. 
 
 Previous editions, under the editorial supervision 
 of Prof R. M. Huston, have been received with 
 marked favor, and they deserved it; but this, re- 
 printed from a very late Dublin edition, carefully 
 revised and brought up by the author to the present 
 time, does present an unusually accurate and able 
 exposition of every important particular embraced 
 in the department of midwifery. # * The clearness^ 
 directness, and precision of its teachings, together 
 with the great amount of statistical research which 
 its text exhibits, have served to place it already in 
 the foremost rank of works in this department of re- 
 medial science. N. O. Med. and Surg. Journal. 
 
 In our opinion, it forms one of the best if not the 
 very best text-book and epitome of obstetric science 
 which we at present possess in the English lan- 
 guage. Monthly Journal of Medical Science. 
 
 The clearness and precision of style in which it is 
 written, and the greatamount of statistical research 
 which it contains, have served to place it in the first 
 rank of works in this departmentof medical science. 
 N. Y. Journal of Medicine. 
 
 Few treatises will be found better adapted as a 
 text-book for the student, or as a manual for the 
 frequent consultation of the young practitioner. 
 American Medical Journal. 
 
 others. Southern Medical and Surgical Journal. 
 
 BY THE SAME AUTHOR. (Lately Published.) 
 
 ON THE DISEASES OF INFANTS AND CHILDREN. Second American 
 
 Edition, revised and enlarged by the author. Edited, with Notes, by W. V. KEATING, M. D. In 
 one large and handsome volume, extra cloth, of over 700 pages. $3 00, or in leather, $3 25. 
 In preparing this work a second time for the American profession, the author has spared no 
 labor in giving it a very thorough revision, introducing several new chapters, and rewriting others, 
 while every portion of the volume has been subjected to a severe scrutiny. The efforts of the 
 American editor have been directed to supplying such information relative to matters peculiar 
 to this country as might have escaped the attention of the author, and the whole may, there- 
 fore, be safely pronounced one of the most complete works on the subject accessible to the Ame- 
 rican Profession. By an alteration in the size of the page, these very extensive additions have 
 been accommodated without unduly increasing the size of the work. 
 
 BY THE SAME AUTHOR. 
 
 ESSAYS ON THE PUERPERAL FEVER, AND OTHER DISEASES PE- 
 CULIAR TO WOMEN. Selected from the writings of British Authors previous to the close of 
 the Eighteenth Century. In one neat octavo volume, extra cloth, of about 450 pages. $2 50. 
 
10 BLANCHARD & LEA'S MEDICAL 
 
 CHURCHILL (FLEETWOOD), M. D., M . R. I. A., &c 
 ON THE DISEASES OF WOMEN; including those of Pregnancy and Child- 
 bed. A new American edition, revised by the Author. With Notes and Additions, by D. FRAN- 
 CIS CONDIE, M. D., author ot " A Practical Treatise on the Diseases of Children." With nume- 
 rous illustrations. In one large and handsome octavo volume, leather, of 768 pages. $3 00. 
 This edition of Dr. Churchill's very popular treatise may almost be termed a new work, so 
 thoroughly has he revised it in every portion. It will be found greatly enlarged, and completely 
 brought up to the most recent condition of the subject, while the very handsome series of illustra- 
 tions introduced, representing such pathological conditions as can be accurately portrayed, present 
 a novel feature, and afford valuable assistance to the young practitioner. Such additions as ap- 
 peared desirable for the American student have been made by the editor, Dr. Condie, while a 
 marked improvement in the mechanical execution keeps pace with the advance in all other respects 
 which the volume has undergone, while the price has been kept at the former very moderate rate. 
 It comprises, unquestionably, one of the most ex- I extent that Dr. Churchill does. His, indeed, is the 
 act and comprehensive expositions of the present | only thorough treatise we know of on the subject; 
 state of medical knowledge in respect to the diseases j and it may be commended to practitioners and stu- 
 dents as a masterpiece in its particular department. 
 
 of women that has yet been published. Am. Journ. 
 Med. Sciences, July, 1857. 
 
 This work is the most reliable which we possess 
 on this subject; and is deservedly popular with the 
 profession. Charleston Med. Journal, July, 1857. 
 
 We know of no author who deserves that appro- 
 bation, on "the diseases of females," to the same 
 
 Tht Western Journal of Medicine and Surgery. 
 
 As a comprehensive manual for students, or a 
 work of reference for practitioners, it surpasses any 
 other that has ever issued on the same subject from 
 the British press. Dublin Quart. Journal. 
 
 DICKSON (S. H.), M. D., 
 
 Professor of Practice of Medicine in the Jefferson Medical College, Philadelphia. 
 
 ELEMENTS OF MEDICINE; a Compendious View of Pathology and Thera- 
 peutics, or the History and Treatment of Diseases. Second edition, revised. In one large and 
 handsome octavo volume ; of 750 pages, leather. $3 75. (Just Issued.) 
 
 The steady demand which has so soon exhausted the first edition of this work, sufficiently shows 
 that the author was not mistaken in supposing that a volume of this character was needed an 
 elementary manual of practice, which should present the leading principles of medicine with the 
 practical results, in a condensed and perspicuous manner. Disencumbered of unnecessary detail 
 and fruitless speculations, it embodies what is most requisite for the student to learn, and at the 
 same time what the active practitioner wants when obliged, in the daily calls of his profession, to 
 refresh his memory on special points. The clear and attractive style of the author renders the 
 whole ea^y of comprehension, while his long experience gives to his teachings an authority every- 
 where acknowledged. Few physicians, indeed, have had wider opportunities for observation and 
 experience, and few, perhaps, have used them to better purpose. As the result of a long life de- 
 voted to study and practice, the present edition, revised and brought up to the date of publication, 
 will doubtless maintain the reputation already acquired as a condensed and convenient American 
 text-book on the Practice of Medicine. 
 
 DRUITT (ROBERT), M.R. C.S., &c. 
 THE PRINCIPLES AND PRACTICE OF MODERN SURGERY. A new 
 
 and revised American from the eighth enlarged and improved London edition. Illustrated with 
 four hundred and thirty-two wood-engravings. In one very handsomely printed octavo volume, 
 leather, of nearly 700 large pages. $3 50. (Now Ready, October, 1860.) 
 
 A work which like DRUITT'S SURGERY has for so many years maintained the position of a lead- 
 ing favorite with all classes of the profession, needs no special recommendation to attract attention 
 to a revised edition. It is only necessary to state that the author has spared no paiijs to keep the 
 work up to its well earned reputation of presenting in a small and convenient compass the latest 
 condition of every department of surgery, considered both as a science and as an art; and that the 
 services of a competent American editor have been employed to introduce whatever novelties may 
 have escaped the author's attention, or may prove of service to the American practitioner. As 
 several editions have appeared in London since the issue of the last American reprint, the volume 
 has had the benefit of repeated revisions by the author, resulting in a very thorough alteration and 
 improvement. The extent of these additions may be estimated from the fact that it now contains 
 about one-third more matter than the previous American edition, and that notwithstanding the 
 adoption of a smaller type, the pages have been increased by about one hundred, while nearly two 
 hundred and fifty wood-cuts have been added to the former list of illustrations. 
 
 A marked improvement will also be perceived in the mechanical and artisJical execution of the 
 work, which, printed in the best style, on new type, and fine paper, leaves little to be desired as 
 regards external finish ; while at the very low price affixed it will be found one of the cheapest 
 volumes accessible to the profession. 
 
 This popular volume, now a most comprehensive j nothing of real practical importance has been omit- 
 work on surgery, has undergone many corrections, ted ; it presents a faithful epitome of everything re- 
 improvements, and additions, and the principles and j lating t ) surgery up to the present hour. It is de- 
 the practice of the art have been brought down to j servedly a popular manual, both with the student 
 the latest record and observation. Of the operations and practitioner. London Lancet, Nov. 19, 1859. 
 
 SK^ 
 
 trations so accurate and numerous, that the student J ial ' v " ? ver T thls 8t > com P/ ehensiv 
 
 can have no difliculty, with instrument in hand, and hand-book. It must prove a vast assistance, not 
 
 book by his side, over the dead body, in obtaining onl y ! . the stud . ent of surgery, but also to the busy 
 
 a propfr knowledge and sufficient Jc in this much practi loner whc may not have ^f^ ? V !! 
 
 neglected department of medical education. Briti&h 
 and Foreign Mtdico-Chirurg. Review, Jan. 1960. 
 
 In the present edition the author has entirely re- 
 written many of the chapters, and has incorporated 
 the various improvements and additions in modern 
 surgery. On carefully going over it, we find that 
 
 himself to the study of more lengthy volumes. 
 London Med. Times and Gazette, Oct 22, 1859. 
 
 In a word, this eighth edition of Dr. Druitt' 
 Manual 
 or practiti 
 Journal of Med. Sciences, Nov. 
 
 of Surgery is all that the surgical student 
 titioner could desire. Dublin Quarterly 
 
AND SCIENTIFIC PUBLICATIONS. 
 
 11 
 
 DALTON, JR. (J. C.), M. D. 
 
 Professor of Physiology in the College of Physicians, New York. 
 
 A TREATISE ON HUMAN PHYSIOLOGY, designed for the use of Students 
 
 and Practitioners of Medicine. With two hundred and fifty-four illustrations on wood. In one 
 very beautiful octavo volume, of over 600 pages, extra cloth, $4 00 ; leather, raised bands, $4 25. 
 (Just Issued.) 
 
 This system of Physiology, both from the ex- 
 cellence of the arrangement studiously observed 
 throughout every page, and the clear, lucid, and in- 
 structive manner in which each subject is treated, 
 promises to form one of the most generally received 
 class-books in the English language. It is, in fact, 
 a most admirable epitome of ail the really important 
 discoveries that have always been received as incon- 
 testable truths, as well as of those which have been 
 recently added to our stock of knowledge on this sub- 
 ject. We will, however, proceed to give a few ex- 
 tracts from the book itself, as a specimen of its style 
 and composition, and this, we conceive, will be quite 
 sufficient to awaken a general interest in a work 
 which isimmeasurabl} superior in its details to the 
 majority of those of the same class t^ which it he- 
 longs. In its purity of style and elegance of com- 
 position it may safely take its place with the very 
 best of our English classics; while in accuracy of 
 description it is impossible that it could be surpass 
 ed. In every line is beautifully shadowed forth the 
 emanations of the polished scholar, whose reflec- 
 tions are clothed in a garb as interesting as they are 
 impressive; with the one predominant feeling ap- 
 pearing to pervade the whole an anxious desire to 
 please and at the same time to instruct. Dublin 
 Quarterly Journ. of Med. Sciences, Nov. 1859. 
 
 The work before us, however, in our humble judg- 
 ment, is precisely what it purports to be, and will 
 answer admirably the purpose for which it is in- 
 tended. It is par excellence,* text-book; and the 
 best text-book in this department that we have ever 
 seen. We have carefully read the book, and speak 
 of its merits from a more than cursory perusal. 
 Looking back upon the work we have just finished, 
 we must say a word concerning the excellence of its 
 illustrations. No department is so dependent upon 
 good illustrations, and those which keep pace with 
 our knowledge of the subject, as that of physiology. 
 The wood- cuts in the work before us are the best 
 we have ever seen, and, being original, serve to 
 illustrate precisely what is desired Buffalo Med. 
 Journal, March, 1859. 
 
 A book of genuine merit like this deserves hearty 
 praise before subjecting it tu any minute criticism. 
 We are not prepared to find any fault with its design 
 until we have had more time to appreciate its merits 
 as a manual for daily consultation, and to weigh 
 its statements and conclusions more deliberately. 
 Its excellences we are sure of; its defects we have 
 yet to discover. It is a work highly honorable to 
 
 its author ; to his talents, his industry, his training ; 
 to the institution with whi^h he is connected, and 
 to American science. Boston Med. and Surgical 
 Journal, Feb. 24, 1859. 
 
 A NEW book and a first rate one ; an original book, 
 and one which cannot be too highly appreciated, 
 and which we are proud to see emanating from our 
 country's press. It is by an author who, though 
 young, is considerably famous for physiological re- 
 search, and who in this work has erected for him- 
 self an enduring monument, a token at once of his 
 labor and his success. Nashville Medical Journal, 
 March, 1859. 
 
 Throughout the entire work, the definitions are 
 clear and precise, the arrangement admirable, the 
 argument briefly and well stated, and the style 
 nervous, simple, and concise. Section third, treat- 
 ing of Reproduction is a monograph of unap- 
 proached excellence, upon this subject, in the Eng- 
 lish tongue. For precision, elegance and force of 
 style, exhaustive method and extent of treatment, 
 fulness of illustration and weight of personal re- 
 search, we know of no American contribution to 
 medical science which surpasses it, and the day is 
 far distant when its claims to the respectful atten- 
 tion of even the best informed scholars will not be 
 cheerfully conceded by all acquainted with its range 
 anu depth. Charleston Med. Journal, May, 1859. 
 
 A new elementary work on Human Physiology 
 lifting up its voice in the presence of late and sturdy 
 editions of Kirke's, Carpenter's, Todd and Bow- 
 
 i have something superior in the matter or the 
 jr of its utterance in order to win for itself 
 
 man's, to say nothing of Durglison's and Draper's, 
 should '- 
 manner 
 
 deserved attention and a name. That matter and 
 that manner, after a candid perusal, we think dis- 
 tinguish this work, and we are proud to welcome it 
 not merely for its nativity's sake, but for its own 
 intrinsic excellence. Its language we find to be 
 plain, direct, unambitious, and falling with a just 
 conciseness on hypothetical or unsettled questions, 
 and yet with sufficient fulness on those living topics 
 already understood, or the path to whose solution 
 is definitely marked out. It does not speak exhaust- 
 i/ely upon every subject that it notices, but it does 
 speak suggestively, experimentally, and to their 
 main utilities. Into the subject of Reproduction 
 our author plunges with a kind of loving spirit. 
 Throughout this interesting and obscure department 
 he is a clear and admirable teacher, sometimes a 
 brilliant leader. Am. Med. Monthly, May, 1859. 
 
 DUNGLISON, FORBES, TWEEDIE, AND CONOLLY. 
 THE CYCLOPAEDIA OF PRACTICAL MEDICINE: comprising Treatises on 
 
 the Nature and Treatment of Diseases, Materia Medica, and Therapeutics, Diseases of Women 
 and Children, Medical Jurisprudence, &c. &c. In four large super-royal octavo volumes, of 
 3254 double-columned pages, strongly and handsomely bound, with raised bands. $12 00. 
 *^* This work contains no less than four hundred and eighteen distinct treatises, contributed by 
 
 sixty-eight distinguished physicians, rendering it a complete library of reference for the country 
 
 practitioner. 
 The most complete work on Practical Medicine I titioner. This estimate of it has not been formed 
 
 extant; or, at least, in our language. Buffalo 
 Medical and Surgical Journal. 
 
 For reference, it is above all price to every prac- 
 titioner. Western Lancet. 
 
 One of the most valuable medical publications of 
 the day as a work of reference it is invaluable. 
 Western Journal of Medicine and Surgery. 
 
 It has been to us, both as learner and teacher, a 
 work for ready and frequent reference, one in which 
 modern English medicine is exhibited in the most 
 advantageous light. Medical Examiner. 
 
 We rejoice that this work is to be placed within 
 the reach of the profession in this country, it bein- 
 unquestionably one of very great value to the prac- 
 
 from a hasty examination, but after an intimate ac- 
 quaintance derived from frequent consultation of it 
 during the past nine or ten years. The editors are 
 practitioners of established reputation, and the list 
 of contributors embraces many of the most eminent 
 professors and teachers of London, Edinburgh, Dub- 
 lin, and Glasgow. It is, indeed, the great merit of 
 this work that the principal articles have been fur- 
 nished by practitioners who have not only devoted 
 especial attention to the diseases about which they 
 have written, but have also enjoyed opportunities 
 for an extensive practical acquaintance with them, 
 and whose reputation carries the assurance of their 
 competency justly to appreciate the opinions of 
 others, while it stamps their own doctrines with 
 high and just authority. American Medical Journ. 
 
 DEWEES'S COMPREHENSIVE SYSTEM OF 
 MIDWIFERY. Illustrated by occasional cases 
 and many engravings. Twelfth edition, with the 
 author's last improvements and corrections In 
 one octavo volume, extra cloth, of 600 pages. $320. 
 
 DEWEES'S TREATISE ON THE PHYSICAL 
 
 AND MEDICAL TREATMENT OF CHILD- 
 REN. The last edition. In one volume, octavo, 
 extra cloth, 548 pages. $2 80 
 
 DEWEES'S TREATISE ON THE DISEASES 
 OF FEMALES. Tenth edition. In one volume, 
 octavo extra cloth, 532 pages, with plates. $3 QO 
 
12 
 
 BLANCHARD & LEA'S MEDICAL 
 
 DUNGLISON (ROBLEY), M.D., 
 
 Professor of Institutes of Medicine in the Jefferson Medical College, Philadelphia. 
 
 NEW AND ENLARGED EDITION. 
 MEDICAL LEXICON; a Dictionary of Medical Science, containing a concise 
 
 Explanation of the various Subjects and Terms of Anatomy, Physiology, Pathology, Hygiene, 
 Therapeutics; Pharmacology, Pharmacy, Surgery, Obstetrics, Medical Jurisprudence, Dentistry, 
 Sec. Notices of Climate and of Mineral Waters; Formulae for Officinal, Empirical, and Dietetic 
 Preparations, &c. With French and other Synonymes. Revised and very greatly enlarged. 
 In one very large and handsome octavo volume, of 992 double-columned pages, ia small type; 
 strongly bound in leather, with raised bands. Price $4 00. 
 
 Especial care has been devoted in the preparation of this edition to render it in every respect 
 worthy a continuance of the very remarkable favor which it has hitherto enjoyed. The rapid 
 sale of FIFTEEN large editions, and the constantly increasing demand, show that it is regarded by 
 the profession as the standard authority. Stimulated by this fact, the author has endeavored in the 
 present revision to introduce whatever might be necessary " to make it a satisfactory and desira- 
 ble if not indispensable lexicon, in which the student may search without disappointment for 
 every term that has been legitimated in the nomenclature of the science." To accomplish this, 
 large additions have been found requisite, and the extent of the author's labors may be estimated 
 from the fact that about Six THOUSAND subjects and terms have been introduced throughout, ren- 
 dering the whole number of definitions about SIXTY THOUSAND, to accommodate which, the num- 
 ber of pages has been increased by nearly a hundred, notwithstanding an enlargement in the size 
 of the page. The medical press, both in this country and in England, has pronounced the work in- 
 dispensable to all medical students and practitioners, and the present improved edition will not lose 
 that enviable reputation. 
 
 The publishers have endeavored to render the mechanical execution worthy of a volume of such 
 universal use in daily reference. The greatest care hns been exercised to obtain the typographical 
 accuracy so necessary in a work of the kind. By the small but exceedingly clear type employed, 
 an immense amount of matter is condensed in its thousand ample pages, while the binding will be 
 found strong and durable. With all these improvements and enlargements, the price has been kept 
 at the former very moderate rate, placing it within the reach of all. 
 
 This work, the appearance of the fifteenth edition 
 of which, it has become our duty and pleasure to 
 announce, is perhaps the most stupendous monument 
 of labor and erudition in medical literature. One 
 would hardly suppose after constant use of the pre- 
 ceding editions, where we have never failed to find 
 a sufficiently full explanation of ever) medical term, 
 that in this edition "about six thousand subjects 
 and terms have been added," with a careful revision 
 and correction of the entire work. It is only neces- 
 sary to announce the advent of this edition to make 
 it occupy the place of the preceding one on the table 
 of every medical man, as it is without doubt the best 
 and most comprehensive work of the kind which has 
 ever appeared. Buffalo Med. Journ., Jan. 1858. 
 
 The work is a monument of patient research, 
 skilful judgment, and vast physical labor, that will 
 perpetuate the name of the author more effectually 
 than any possible device of stone or metal. Dr. 
 Dunglison deserves the thanks not only of the Ame- 
 rican profession, but of the whole medical world. 
 North Am. Medico-Chir. Review, Jan. 1858. 
 
 A Medical Dictionary better adapted for the wants 
 of the profession than any other with which we are 
 acquainted, and of a character which places it far 
 above comparison and competition. Am. Journ. 
 Med. Sciences, Jan. 1858. 
 
 We need only say, that the addition of 6,000 new 
 terms, with their accompanying definitions, may be 
 said to constitute a new work, by itself. We have 
 examined the Dictionary attentively, and are most 
 happy to pronounce it unrivalled of its kind. The 
 erudition displayed, and the extraordinary industry 
 which must have been demanded, in its preparation 
 and perfection, redound to the lasting credit of its 
 author, and have furnished us with a volume indis- \ 
 jiensable at the present day, to all who would find i 
 themselves au niveau with the highest standards of j 
 medical information. Boston Medical and Surgical 
 Journal, Dec. 31, 1857. 
 
 Good lexicons and encyclopedic works generally, 
 are the most labor-saving contrivances which lite- 
 rary men enjoy ; and the labor which is required to 
 produce them in the perfect manner of this example 
 is something appalling to contemplate. The author 
 
 tells us in his preface that he has added about six 
 thousand terms and subjects to this edition, which, 
 before, was considered universally as the best work 
 of the kind in any language. Silliman's Journal, 
 March, 1858. 
 
 He has razed his gigantic structure to the founda- 
 tions, and remodelled and reconstructed the entire 
 pile. No less than six thousand additional subjects 
 and terms are illustrated and analyzed in this new 
 edition, swelling the grand aggregate to beyond 
 sixty thousand ! Thus is placed before the profes- 
 sion a complete and thorough exponent of medical 
 terminology, without rival or possibility of rivalry. 
 Nashville Journ. of Med. and Surg., Jan. 1858. 
 
 It is universally acknowledged, we believe, that 
 this work is incomparably the best and most com- 
 plete Medical Lexicon in the English, language. 
 The amount of labor which the distinguished author 
 has bestowed upon it is truly wonderful, and the 
 learning and research displayed in its preparation 
 are equally remarkable. Comment and commenda- 
 tion are unnecessary, as no one at the present day 
 thinks of purchasing any other Medical Dictionary 
 than this. St. Louis Med. and Surg. Journ.. Jan. 
 1858. 
 
 It is the foundation stone of a good medical libra- 
 ry, and should always be included in the first list of 
 books purchased by the medical student. Am. Med. 
 Monthly, Jan. 1858. 
 
 A very perfect work of the kind, undoubtedly the 
 most perfect in the English language. Med. and 
 Surg. Reporter, Jan. 1858. 
 
 It is now emphatically the Medical Dictionary of 
 the English language, and for it there is no substi- 
 tute. N. H. Med. Journ., Jan. 1858. 
 
 It is scarcely necessary to remark that any medi- 
 cal library wanting a copy of Dunglison's Lexicon 
 must be imperfect. Cin. Lancet, Jan. 1858. 
 
 We have ever considered it the best authority pub- 
 lished, and the present edition we may safely say has 
 no equal in the world. Peninsular Med. Journal, 
 Jan. 1858. 
 
 The most complete authority on the subject to be 
 foundin any language. Va. Med. Journal, Feb. '58. 
 
 BY THE SAME AUTHOR. 
 
 THE PRACTICE OF MEDICINE. A Treatise OH Special Pathology and The- 
 rapeutics. Third Edition. In two large octavo volumes, leather, of 1,500 pages. $635. 
 
AHD SCIENTIFIC PUBLICATIONS. 
 
 DUNGLISON (ROBLEY, M. D., 
 
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 BLANCHARD & LEA'S MEDICAL 
 
 ERICHSEN (JOHN), 
 
 Professor of Surgery in University College, London, &c. 
 
 THE SCIENCE AND ART OF SURGERY; BEING A TREATISE ON SURGICAL 
 
 INJURIES, DISEASES, AND OPERATIONS. New and improved American, from the second enlarged 
 
 and carefully revised London edition. Illustrated with over four hundred engravings on wood. 
 
 In one large and handsome octavo volume, of one thousand closely printed pages, leather, 
 
 raised bands. -$4 50. (Just Issued.) 
 
 The very distinguished favor with which this work has been received on both sides of the Atlan- 
 tic has stimulated the author to render it even more worthy of the position which it has so rapidly 
 attained as a standard authority. Every portion has been carefully revised, numerous additions 
 have been made, and the most watchful care has been exercised to render it a complete exponent 
 of the most advanced condition of surgical science. In this manner the work has been enlarged by 
 about a hundred pages, while the series of engravings has been increased by more than a hundred, 
 rendering it one of the most thoroughly illustrated volumes before the profession. The additions oJ 
 the author having rendered unnecessary most of the notes of the former American editor, but little 
 has been added in this country; some few notes and occasional illustrations have, however, been 
 introduced to elucidate American modes of practice. 
 
 It is, in our humble judgment, decidedly the best step of the operation, and not deserting him until the 
 
 book of the kind in the English language. Strange 
 that just such books are notoftener produced by pub- 
 lic teachers of surgery in this country and Great 
 Britain Indeed, it is a matter of great astonishment, 
 but no less true than astonishing, that of the many 
 works on surgery republished in this country within 
 the last fifteen or twenty years a? text-books for 
 medical students, this is the only one that even ap- 
 proximates to the fulfilment of the peculiar wants of 
 young men just entering upon the study of this branch 
 of the profession. Western Jour, of Med. and Surgery. 
 
 Its value is greatly enhanced by a very copious 
 well-arranged index. We regard this as one of the 
 most valuable contributions to modern surgery. To 
 one entering his novitiate of practice, we regard it 
 the mot serviceable guide which he can consult. He 
 will find a fulness of detail leadinghim through every 
 
 final issue of the case is decided. Sethoscope. 
 
 Embracing, as will be perceived, the whole surgi- 
 cal domain, and each division of itself almost com- 
 plete and perfect, each chapterfull and explicit, each 
 subject faithfully exhibited, we can only express ouj 
 estimate of it in the aggregate. We consider it an 
 excellent contribution to surgery, as probably the 
 best single volume now extant on the subject, and 
 with great pleasure we add it to onr text-books. 
 Nashville Journal of Medicine and Surgery. 
 
 Prof. Erichsen's work, for its size, has not beeK 
 surpassed; his nine hundred and eight pages, pro- 
 fusely illustrated, are rich in physiological, patholo- 
 gical, and operative suggestions, doctrines, details, 
 and processes ; and will prove a reliable resource 
 for information, both to physician and sursreon, in the 
 hour of peril. N. 0. Med. and Surg. Journal. 
 
 FLINT (AUSTIN), M . D., 
 
 Professor of the Theory and Practice of Medicine in the University of Louisville, Ac. 
 
 PHYSICAL EXPLORATION AND DIAGNOSIS OP DISEASES AFFECT- 
 ING THE RESPIRATORY ORGANS. In one large and handsome octavo volume, extra 
 cloth, 636 pages. $3 00. 
 We regard it, in point both of arrangement and of 
 
 the marked ability of its treatment of the subjects, 
 
 as destined to take the first rank in works of this 
 
 class. So far as our information extends, it has at 
 present no equal. To the practitioner, as well as 
 the student, it will be invaluable in clearing up the 
 diagnosis of doubtful cases, and in shedding light 
 upon difficult phenomena. Buffalo Med. Journal. 
 
 A work of original observation of the highest merit. 
 We recommend the treatise to every one who wishes 
 
 to become a correct auscultator. Based to a very 
 large extent upon cases numerically examined, it 
 carries the evidence of careful study and discrimina- 
 tion upon every page. It does credit to the author, 
 and, through him, to the profession in this country. 
 It is, what we cannot call every book upon auscul- 
 tation, a readable book. Am. Jour. Med. Sciences, 
 
 BY THE SAME AUTHOR. (Now Ready.) 
 
 A PRACTICAL TREATISE ON THE DIAGNOSIS, PATHOLOGY, AND 
 
 TREATMENT OF DISEASES OF THE HEART. 
 
 500 pages, extra cloth. $2 75. 
 
 We do no* know that Dr. Flint has written any- 
 thing which is not first rate ; but this, his latest con- 
 tribution to medical literature, in our opinion, sur- 
 
 passes all the others. The work is most comprehen- 
 sive in its scope, and most sound in the views it enun- 
 ciates. The descriptions are clear and methodical ; 
 the statements are substantiated by facts, ai.d are 
 made with such simplicity and sincerity, that with- 
 out them they would carry conviction. The style 
 is admirably clear, direct, and free from dryness 
 With Dr. Walshe's excellent treatise before us, we 
 have no hesitation in saying that Dr. Flint's book is 
 the best work on the heart in the English language. 
 Boston Med. and Surg. Journal, Dec. 15, 1S59. 
 
 We have thus endeavored to present our readers 
 with a fair analysis of tiiis remarkable work. Pre- 
 ferring to employ the very words of thedisr.inguished 
 author, wherever it was possible, we have essayed 
 to condense into the briefest spacea general view of 
 his observations and suggestions, and to direct the 
 attention of our brethren to the abounding stores of 
 valuable matter here collected and arranged for their 
 use and instruction. No medical library will here 
 after be considered complete without this volume; 
 and we trust it will promptly find its way into the 
 hands of every Amoican student and physician. 
 N. Am. Med. Chir. Review, Jan 1860. 
 
 This last work of Prof. Flint will add much to 
 his previous well-earned celebrity, as a writer of 
 great force and beauty, and, with his previous work, 
 places him ut the head of American writers upon 
 
 In one neat octavo volume, of about 
 
 diseases of the chest. We have adopted his work 
 upon the heart as a t^xt-book, believing it to be 
 more valuable for that purpose than any work of the 
 
 kind that has yet appeared. Nashville Med.Journ.. 
 Dec. 1859. 
 
 With more than pleasure do we hail the advent of 
 this work, for it fills a wide gap on the list nf text- 
 books for our schools, and is, Jor the practitioner, 
 the most valuable practical work of its kind. JV. O. 
 Med. Newt, Nov. 1859. 
 
 In regard to the merits of the work, we have no 
 hesitation in pronouncing it full, accurate, and ju- 
 dicious. Considering the present state of science, 
 such a work was mucb needed. It should be in the 
 hands of every practitioner. Chicago Med. Journal, 
 April, 1860. 
 
 Bat these are very trivial spots, and in nowise 
 prevent us from declaring our most hearty approval 
 of the author's ability, industry, and conscientious- 
 ness. Dublin Quarterly Journal of Med. Sciences, 
 Feb. 1860. 
 
 He has labored on with the same industry and care, 
 and his place among the first authors of our country 
 is becoming fully established. To this end, the work 
 whose title is given above, contributes in no small 
 degree. Our spa^e will not admit of tn extended 
 analysis, and we will close this orief noice by 
 commending it without reserve to every class of 
 readers in the profession Peninsular Med. Journ., 
 Feb. 1860. 
 
AND SCIENTIFIC PUBLICATIONS. 
 
 ir, 
 
 FOWNES (GEORGE), PH. D., &c. 
 A MANUAL OF ELEMENTARY CHEMISTRY; Theoretical and Practical. 
 
 From the seventh revised and corrected London edition. With one hundred and ninety-seven 
 
 illustrations. Edited by ROBERT BRIDGES, M. D. In one large royal 12mo. volume, of 600 
 
 pages. In leather, $1 65 ; extra cloth, $1 50. (Just Issued.) 
 
 The death of the author having placed the editorial care of this work in the practised hands of 
 Drs. Bence Jones and A. W. Hoffman, everything has been done in its revision which experience 
 eould suggest to keep it on a level with the rapid advance of chemical science. The additions 
 requisite to this purpose have necessitated an enlargement of the page, notwithstanding which the 
 work has been increased by about fifty pages. At the same time every care has been used to 
 maintain its distinctive character as a condensed manual for the student, divested of all unnecessary 
 detail or mere theoretical speculation. The additions have, of course, been mainly in the depart- 
 ment of Organic Chemistry, which has made such rapid progress within the last few years, but 
 yet equal attention has been bestowed on the other branches of the subject Chemical Physics and 
 Inorganic Chemistry to present all investigations and discoveries of importance, and to keep up 
 the reputation of the volume as a complete manual of the whole science, admirably adapted for the 
 learner. By the use of a small but exceedingly clear type the matter of a large octavo is compressed 
 within the convenient and portable limits of a'moderate sized duodecimo, and at the very low price 
 affixed, it is offered as one of the cheapest volumes before the profession. 
 
 Dr. Fownes' excellent work has been universally 
 recognized every where in his own and this country, 
 as the best elementary treatise on chemistry in the 
 English tongue, and is very generally adopted, we 
 believe, as the standard text- book in all< ur colleges, 
 both literary and scientific. Charleston Med. Journ. 
 and Review, Sept. 1859. 
 
 A standard manual, which has long enjoyed the 
 reputation of embodying much knowledge in a small 
 space. The author has achieved the difficult task of 
 condensation with masterly tact. His book is con- 
 cise without being dry, and brief without being too 
 dogmatical or general. Virginia Med. and Surgical 
 Journal. 
 
 The work of Dr. Fownes has long been before 
 the public, and its merits have been fully appreci- 
 ated as the best text-book on chemistry now in 
 existence. We do not, of course, place it in a rank 
 superior to the works of Brande, Graham, Turner, 
 
 Gregory 
 
 for students, it is preferable to any of them 
 
 or Gmelin, but we say that, as a work 
 
 Lon- 
 
 don Journal of Medicine. 
 
 A work well adapted to the wants of the student 
 It is an excellent exposition of the chief doctrines 
 and facts of modern chemistry . The size of the work, 
 and still more the condensed yet perspicuous style 
 in which it is written, absolve it from the charges 
 very properly urged against most manuals termed 
 popular. Edinburgh Journal of Medical Science. 
 
 FISKE FUND PRIZE ESSAYS THE EF- 
 FECTS OF CLIMATE ON TUBERCULOUS 
 
 EDWARD WARREN, M. D , of Edenton, N. C. To- 
 gether in one neat 8vo. volume, extra cloth. $1 00. 
 
 DISEASE. By EDWIN LEE, M.R.C.S , London, | FRICK ON RENAL AFFECTIONS; their Diag 
 
 and THE INFLUENCE OF PREGNANCY ON 
 THE DEVELOPMENT OF TUBERCLES By 
 
 nosis and Pathology. With illustrations. One 
 volume, royal I2mo., extra cloth. 75 cents 
 
 FERGUSSON (WILLIAM), F. R. S., 
 
 Professor of Surgery in King's College, London, &c. 
 
 A SYSTEM OF PRACTICAL SURGERY. Fourth American, from the third 
 
 and enlarged London edition. In one large and beautifully printed octavo volume, of about 700 
 pages, with 393 handsome illustrations, leather. $3 00. 
 
 GRAHAM (THOMAS), F. R. S. 
 THE ELEMENTS OF INORGANIC CHEMISTRY, including the Applica- 
 
 tions of the Science in the Arts. New and much enlarged edition, by HENRY WATTS and ROBERT 
 BRIDGES, M. D. Complete in one large and handsome octavo volume, of over 800 very large 
 pages, with two hundred and thirty-two wood-cuts, extra cloth. $4 00. 
 
 j<*%. Part II., completing the work from p. 431 to end, with Index, Title Matter, &c., may be 
 had separate, cloth backs and paper sides. Price $2 50. 
 
 From Prof. E. N. Horsford, Harvard College. 
 
 It has, in its earlier and less perfect editions, been 
 familiar to me, and the excellence of its plan and 
 the clearness and completeness of its discussions, 
 have long been my admiration. 
 
 No reader of English works on this science can 
 
 afford to be without this edition of Prof. Graham's 
 Elements. Silliman's Journal, March, 1858. 
 
 From Prof. Wolcott Gibbs, N. Y. Free Academy. 
 
 The work is an admirable one in all respects, and 
 its republication here cannot fail to exert a positive 
 influence upon the progress of science in this country. 
 
 GRIFFITH (ROBERT E.), M. D., &c. 
 
 A UNIVERSAL FORMULARY, containing the methods of Preparing and Ad- 
 ministering Officinal and other Medicines. The whole adapted to Physicians and Pharmaceu- 
 lists. SECOND EDITION, thoroughly revised, with numerous additions", by ROBERT P. THOMAS, 
 M. D., Professor of Materia Medica in the Philadelphia College of Pharmacy. In one large and 
 handsome octavo volume, extra cloth, of 650 pages, double columns. $3 00; or in sheep, $3 25. 
 
 It was a work requiring much perseverance, and 
 when published was looked upon as by far the best 
 work of its kind that had issued from the American 
 press. Prof Thomas has certainly "improved," as 
 well as added to this Formulary, and lias rendered il 
 additionally deserving of the confidence of pharma- 
 ceutists and physicians. Am. Journal of Pharmacy. 
 
 We are happy to announce a new and improved 
 edition of this, one of the most valuable and useful 
 works that have emanated from an American pen. 
 Et would do credit to any country, and will be found 
 of daily usefulness to practitioners of medicine; it is 
 better adapted to their purposes than the dispensato 
 ries. Southern Med. and Surg. Journal. 
 
 It is one of the most useful books a country practi- 
 tioner can possibly have. Medica I Chronicle. 
 
 This is a work of six hundred and fifty one pages, 
 imbracing all on the subject of preparing and admi- 
 
 medicines that can be desired by the physi- 
 pharmaceutist. Western Lancet. 
 
 nisterin 
 
 an an 
 
 The amountof useful, every-day matter. for a prac- 
 ticing physician, is really immense. Boston Med. 
 and Surg. Journal. 
 
 This edition has been greatly improved by the re- 
 vision and ample additions of Dr Thomas, and is 
 now, we believe, one of the mosi complete works 
 of its kind iu any language. The addition? amount 
 to about seventy pages, and no effort has been spared 
 to include in them all the recent improvements. A 
 work of this kind appears to us indispensable to the 
 physician, and there is none we can more cordially 
 recommend. N. Y. Journal of Medicine. 
 
Itj 
 
 BLANCHARD & LEA'S MEDICAL 
 
 GROSS (SAMUEL D.), M. D., 
 
 Professor of Surgery in the Jefferson Medical College of Philadelphia, &c. 
 Just Issued. 
 
 A SYSTEM OF SURGERY : Pathological, Diagnostic, Therapeutic, and Opera- 
 tive. Illustrated by NINE HUNDRED AND THIRTY-SIX ENGRAVINGS. In two large and beautifully 
 printed octavo volumes, of nearly twenty-four hundred pages; strongly bound in leather, with 
 raised bands. Price $12. 
 
 FROM THE AUTHOR'S PREFACE. 
 
 " The object of this work is to furnish a systematic and comprehensive treatise on the science and 
 practice of surgery, considered in the broadest sense; one that shall serve the practitioner as a 
 faithful and available guide in his daily routine of duty. It has been too much the custom of mod- 
 ern writers on this department of the healing art to omit certain topics altogether, and to speak of 
 others at undue length, evidently assuming that their readers could readily supply the deficiencies 
 from other sources, or that what has been thus slighted is of no particular practical value. My aim 
 has been to embrace the whole domain of surgery, and to allot to every subject its legitimate claim 
 to notice in the great family of external diseases and accidents. How far this object has been accom- 
 plished, it is not for me to determine. It may safely be affirmed, however, that there is no topic, 
 properly appertaining to surgery, that will not be found to be discussed, to a greater or less extent, 
 in these volumes. If a larger space than is customary has been devoted to the consideration of 
 inflammation and its results, or the great principles of surgery, it is because of the conviction, 
 grounded upon long and close observation, that there are no subjects so little understood by the 
 general practitioner. Special attention has also been bestowed upon the discrimination of diseases; 
 and an elaborate chapter has been introduced on general diagnosis." 
 
 That these intentions have been carried out in the fullest and most elaborate manner is sufficiently 
 shown by the great extent of the work, and the length of time during which the author has been 
 concentrating on the task his studies and his experience, guided by the knowledge which twenty 
 years of lecturing on surgical topics have given him of the wants of the profession. 
 
 Of Dr. Gross's treatise on Surgery we can say 
 no more than that it is the most elaborate and com- 
 
 Elete work on this branch of the healing art which 
 as ever been published in any country. A sys- 
 tematic work, it admits of no analytical review; 
 but, did our space permit, we should gladly give 
 some extracts from it, to enable our readers to judge 
 of the c'assical style of the author, and the exhaust- 
 ing way in which each subject is treated. Dublin 
 Quarterly Journal of Med. Science, Nov. 1859. 
 
 The work is so superior to its predecessors in 
 matter and extent, as well as in illustrations and 
 style of publication, that we can honestly recom- 
 mend it as the best work of the kind to be taken 
 home by the young practitioner. Am. Med. Journ., 
 Jan. I860. 
 
 The treatise of Prof. Gross is not, therefore, a 
 mere text-book for undergraduates, but a systema- 
 tic record of more than thirty years' experience,, 
 reading, and reflection by a man of observation, 
 sound judgment, and i are practical tact, and as such 
 deserves to take rank with the renowned produc- 
 tions of a similar character, by Vidal and Boyer, of 
 France, or those of Chelius, Blasius, and Langen- 
 beck, of Germany. Henee, we do not hesitate to 
 express the opinion that it will speedily take the 
 same elevated position in regard to surgery that has 
 been given by common consent to the masterly work 
 of Percira in Materia Medica, or to Todd and Bow- 
 man in Physiology. N. O. Med. and Surg. Journal, 
 Jan. 1860. 
 
 At present, however, our object is not to review 
 the work (this we purpose doing hereafter), but 
 simply to announce its appeal ance, that in the 
 meantime our readers may procure and examine it 
 for themselves. But even this much we cannot do 
 without expressing the opinion that, in putting forth 
 these two volumes, Dr. Gross has reared for him- 
 self a lasting monument to his skill as a surgeon, 
 and to his industry and learning as an author. St. 
 Louis Med. and Surg. Journal, Nov. 1859. 
 
 With pleasure we record the completion of this 
 long-anticipeted work. The reputation which the 
 author has for many years sustained, both as a sur- 
 geon and as a writer, had prepared us to expect a 
 treatise of great excellence and originality; but we 
 confess we were by no means prepared tor the work 
 which is before us the most complete treatise upon 
 surgery ever published, either in this or any othtr 
 country, and we might, perhaps, safely say, the 
 most original. There is no subject belonging pro- 
 perly to surgery which has not received from the 
 author a due share of attention. Dr. Grots has sup- 
 plied a want in surgical literature which has long 
 been felt by practitioners; he has furnished us with 
 a complete practical treatise upon surgery in all its 
 departments As Anaericins, we are proud of the 
 achievement; as surgeons, we are most sincerely 
 thankful to him for his extraord nary labors in our 
 behalf. N. Y. Monthly Review and Buffalo Med. 
 Journal, Oct. 1850. 
 
 BY THE SAME AUTHOR. 
 
 ELEMENTS OF PATHOLOGICAL ANATOMY. Third edition, thoroughly 
 
 revised and greatly improved. In one large and very handsome octavo volume, with about three 
 hundred and fifty beautiful illustrations, of which a large number are from original drawings. 
 Price in extra cloth, $4 75; leather, raised bands, $5 25. (Lately Published.) 
 The very rapid advances in the Science of Pathological Anatomy during the last few years have 
 rendered essential a thorough modification of this work, with a view of making it a correct expo- 
 nent of the present state of the subject. The very careful manner in which this task has been 
 executed, and the amount of alteration which it has undergone, have enabled the author to say that 
 " with the many changes and improvements now introduced, the work may be regarded almost as 
 a new treatise," while the efforts of the author have been seconded as regards the mechanical 
 execution of the volume, rendering it one of the handsomest productions of the American press. 
 
 We most sincerely congratulate the author on the 
 successful manner in which he has accomplished his 
 proposed object. His book is most admirably cal- 
 culated to fill up a blank which has long been felt to 
 exist in this department of medical literature, and 
 us such must become very widely circulated amongst 
 all classes of the profession. Dublin Quarterly 
 Journ. of Med. Science, Nov. 1857. 
 
 We have been favorably impressed with the gene- 
 ral manner in which Dr. Gross has executed his task 
 of affording a comprehensive digest of the present 
 state of the literature of Pathological Anatomy, and 
 have much pleasure in recommending his work to 
 our readers, as we believe one well deserving of 
 
 diligent 
 Chiron., 
 
 ;rusal and careful study, 
 ept. 1857. 
 
 Montreal Med. 
 
 BY THE SAME AUTHOR. 
 
 A PRACTICAL TREATISE ON FOREIGN BODIES IN THE AIR-PAS- 
 
 SAGES. In one handsome octavo volume, extra cloth, with illustrations, pp. 468. $2 75. 
 
AND SCIENTIFIC PUBLICATIONS. 
 
 17 
 
 GROSS (SAMUEL D.), M . D., 
 
 Professor of Surgery in the Jefferson Medical College of Philadelphia, &c. 
 
 A PRACTICAL TREATISE ON THE DISEASES, INJURIES, AND 
 
 MALFORMATIONS OF THE URINARY BLADDER, THE PROSTATE GLAND, AND 
 THE URETHRA. Second Edition, revised and much enlarged, with one hundred and eighty- 
 four illustrations. In one large and very handsome octavo volume, of over nine hundred pages. 
 In leather, raised bands, $5 25 ; extra cloth, $4 75. 
 
 Philosophical in its design, methodical in its ar- 
 rangement, ample and sound in its practical details, 
 it may in truth be said to leave scarcely anything to 
 be desired on so important a subject. Boston Med. 
 and Surg. Journal. 
 
 Whoever will peruse the vast amount of valuable 
 practical information it contains, will, we think, 
 
 agree with us, that there is no work in the English 
 language which can make any just pretensions to 
 be its equal. N. Y. Journal of Medicine. 
 
 A volume replete with truths and principles of the 
 almost value in the investigation of these diseases. - 
 American Medical Journal . 
 
 70 1 1.-. ,>;>. yJoV GY/ii'JO 6fli vbaeil V"f, thrill?! *>'X, 
 
 GRAY (HENRY), F. R. S., 
 
 Lecturer on Anatomy at St. George's Hospital, London, &. 
 
 ANATOMY, DESCRIPTIVE AND SURGICAL. The Drawings by H. V. 
 
 CARTER, M. D., late Demonstrator on Anatomy at St. George's Hospital ; the Dissections jointly 
 by the AUTHOR and Dr. CARTER. In one magnificent imperial oclavo volume, of nearly 800 
 pages, with 363 large and elaborate engravings on wood. Price in extra cloth, $6 25; leather 
 raised bands, $7 00. (Just Issued.) 
 
 The author has endeavored in this work to cover a more extended range of subjects than is 
 customary in the ordinary text-books, by giving not only the details necessary for the student, but 
 also the application of those details in the practice of medicine and surgery, thus rendering it both 
 a grade for the learner, and an admirable work of reference lor the active practitioner. The 
 engravings form a special feature in the work, many of them being the size of nature, nearly all 
 original, and having the names of the various parts printed on the body of the cut, in place of figures 
 of reference with descriptions at the foot. They thus form a complete and splendid series, which 
 will greatly assist the student ia obtaining a clear idea of Anatomy, and will also serve to refresh 
 the memory of those who may find in the exigencies of practice the necessity of recalling the details 
 of the dissecting room ; while combining, as it does, a complete Atlas of Anatomy, with a thorough 
 treatise on systematic, descriptive, and applied Anatomy, the work will be found of essential use 
 to all physicians who receive students in their offices, relieving both preceptor and pupil of much 
 labor in laying the groundwork of a thorough medical education. 
 
 to exist in this country. Mr. Gray writes through- 
 out with both branches of his subject in view. His 
 
 The work before us is one entitled to the highest 
 praise, and we accordingly welcome it as a valu- 
 able addition to medical literature. Intermediate 
 in fulness of detail between the treatises of S.iar 
 pey and of Wilson, its characteristic merit lies in 
 the number and excellence of the engravings it 
 contains. Most of these are original, of much 
 larger than ordinary size, and admirably executed. 
 The various parts are also lettered after the plan 
 adopted in Holden's Osteology. It would be diffi- 
 cult to over-estimate the advantages offered by this 
 mode of pictorial illusiration. Bones, ligaments, 
 muscles, bloodvessels, and nerves are each in turn 
 figured, and marked with their appropriate names; 
 thus enabling the student to comprehend, at a glance, 
 what would otherwise often be ignored, or at any 
 rate, acquired only by prolonged and irksome ap- 
 plication. In conclusion, we heartily commend the 
 work of Mr. Gray to the attention of the medical 
 profession, feeling certain that it should be regarded 
 
 description of each particular part is followed by a 
 notice of its relations to tiie parts with which it is 
 connected, and this, too, sufficiently ample for all 
 the purposes of the operative surgeon. After de- 
 scribing the bones and muscles, he gives a concise 
 statement of the fractures to which the bones of 
 the extremities are most liable, together with the 
 amount and direction of the displacement to which 
 the fragments are subjected by muscular action. 
 The section on arteries is remarkably full and ac- 
 curate. Not only is the surgical anatomy given to 
 every important vessel, with directions for its liga- 
 tion, but at the end of the description of each arte- 
 rial trunk we have a useful summary of the irregu- 
 larities which may occur in its origin, course, and 
 termination. N. A. Med. Chir. Review, Mar. 1359. 
 
 Mr. Gray's book, in excellency of arrangement 
 
 as one of the most valuable contributions eve? made and completeness of execution, exceeds any work 
 
 on anatomy hitherto published in the English lan- 
 
 to educational literature. N. Y. Monthly Review. 
 Dec. 1859. 
 
 In this view, we regard the work of Mr. Gray as 
 far belter adapted to the wants of the profession, 
 and especially of the student, than any treatise on 
 anatomy yet published in this country. 1 1 is destined, 
 we believe, to supersede all others, both as a manual 
 of dissections, and a standard of reference to the 
 etudent of general or relative anatomy. N. Y. 
 Journal of Medicine, Nov. 1859. 
 
 This is by all comparison the most excellent work 
 on Anatomy extant. It is just the thing that has 
 been long desired by the profession. With such a 
 guide as this, the student of anatomy, the practi- 
 tioner of medicine, and the surgical devotee have 
 all a newer, clearer, and more radiant light thrown 
 upon the intricacies and mysteries of this wonder- 
 ful science, and are thus enabled to accomplish re- 
 sults which hitherto seemed possible only to the 
 specialist. The plates, which are copied from re- 
 cent dissections, are so well executed, that the most 
 superficial observer cannot fail to perceive the posi- 
 tions, relations, and distinctive features of the vari- 
 ous parts, and to take in more of anatomy at a glance, 
 than by many long hours of diligent study over the 
 most erudite treatise, or, perhaps, at the dissecting 
 table itself. Med. Journ. of N. Carolina, Oct. 1859. 
 
 For this truly admirable work the profession is 
 indebted to the distinguished author of " Gray on 
 the Spleen." The vacancy it fills has been long felt 
 
 guage, affording a complete view of the structure of 
 the human body, with especial reference to practical 
 surgery. Thus the volume constitutes a perfect book 
 of reference for the practitioner, demanding a place 
 in even the most limited library of the physician or 
 surgeon, and a work of necessity for the student to 
 fix in his mind what he has learned by the dissecting 
 knife from the book of nature. The Dublin Quar- 
 terly Journal of Med. Sciences, Nov. 1858. 
 
 In our judgment, the mode of illustration adopted 
 in the present volume cannot but present many ad- 
 vantages to the student of anatomy. To the zealous 
 disciple of Vesalius, earnestly desirous of real im- 
 provement, the book will certainly be of immense 
 value; but, at the same time, we must also confess 
 that to those simply desirous of "cramming" it 
 will be an undoubted godsend. The peculiar value 
 of Mr. Gray's mode of illustration is nowhere more 
 markedly evident than in the chapter on osteology, 
 and especially in those portions which treat of the 
 bones of the head and of thsir development. The 
 study of these parts is thus made one of comparative 
 ease, if not of positive pleasure ; and those bugbears 
 of the student, the temporal and sphenoid bones, are 
 shorn of half their terrors. It is, in our estimation, 
 an admirable and complete text-book for the student, 
 and a useful work of reference for the practitioner; 
 its pictorial character forming a novel element, to 
 which we have already sufficiently alluded. Am. 
 Journ. Med. Sci., July, 1859. 
 
IS 
 
 BLANCHARD & LEA'S MEDICAL 
 
 GIBSON'S INSTITUTES AND PRACTICE OF 
 SURGERY. Eighth edition, improved and al- 
 tered. With thirty- four plates. In two handsome 
 octavo volumes, containing about 1,000 pages, 
 leather, raised bandi. $6 50. 
 
 GARDNER'S MEDICAL CHEMISTRY, for the 
 use of Students and the Profession. In one royal 
 l'2mo. vol., cloth, pp. 396, with wood-cuts. 81. 
 
 GLUGE'S ATLAS OF PATHOLOGICAL HIS- 
 TOLOGY. Translated, with Notes and Addi- 
 
 tions, by JOSEPH LEIDY, M. D. In one volume, 
 very bvrge imperial quarto, extra cloth, witi 320 
 copper- plate figures, plain and colored, $5 00. 
 
 HUGHES' INTRODUCTION TO THE PRAC- 
 TICE OF AUSCULTATION AND OTHER 
 MODES OF PHYSICAL DIAGNOSIS IN DIS- 
 EASES OF THE LUNGS AND HEART. Se- 
 cond edition 1 vol. royal 12mo., ex. cloth, pp. 
 304. $1 00. 
 
 HAMILTON (FRANK H.), M. D., 
 
 Professor of Surgery in the University of Buffalo, &c. 
 
 A PRACTICAL TREATISE ON FRACTURES AND DISLOCATIONS. In 
 
 one large and handsome octavo volume, of over 750 pages, with 289 illustrations. $4 25. (Now 
 
 Ready, January, 1860.) 
 
 This is a valuable contribution to the surgery of [ illustrated, which will be a desideratum for those 
 most important affections, and is the more welcome, i practitioners who cannot conveniently see the mo- 
 inasmuch as at the present time we do not possess j dels applied. New York Med. Press, Feb. 4, 1860. 
 a single complete treatise on Fractures and Dislo- j We re?ard thig work ag an h(mor m)t on , to its 
 cations in the English language. It has remained for | author, but to the profession of our country. Were 
 our American brother to produce a complete treatise j we to r ' ev iewit thoroughly, we could not convey to 
 upon the subject, and bring together in a convenient j the mind of lhe reade r more forcibly our ho ^ est 
 form those alterations and improvements that have ; O p mion f X p re ssed in the few words we think it the 
 been made from time to time in the treatment of these j best book & its kmd extant . Every man inte rested 
 affections. One great and valuable feature in the 1 in Burgery will 8Oon nave this work on his desk, 
 work before us is the fact that it comprises all the He wl f o d { 5ea not will be the io8er ._ New OrUans 
 improvements introduced into the practice of both 
 English and American surgery, and though far from 
 omitting mention of our continental neighbors, the 
 
 .author by no means encourages the notion but too 
 
 Medical News, March. 1860. 
 
 Now that it is before us, we feel bound to say that 
 much as was expected from it, and onerous as was 
 
 prevalent in some quarters- that nothing is good ! the undertaking, it has surpassed expectation, and 
 unless imported from France or Germany. Tne j achieved more than was pledged in its behalf; for 
 latter half of the work is devoted to the considera- lts tltle does not express in full the richness of its 
 tion of the various dislocations and their appropri- contents. On the whole we are prouder of this 
 ate treatment, and its merit is fully equal to that of ! w <>rk than of any which has for years emanated 
 the preceding portion.-TAe London Lancet, May 5, from th K e American medical press ; its sale will cer- 
 - 1 tamly be very large in this country, and we antici- 
 
 pate its eliciting much attention in Europe. Nash- 
 ville Medical Record, Mar. 1860. 
 
 1860. 
 
 It is emphatically the book upon the subjects of 
 
 Every surgeon, young and old, should possess 
 
 which it treats, and we cannot doubt that it will 
 
 continue so to be for an indefinite period of time. , ..- , ..- ,- .- 
 
 When we say, however, that we believe it will at j himself of it, and give it a careful perusal, in doing 
 
 once take its place as the best book for consultation wh , lc c h he Wl11 be , r hly ?*%&* St " L Ul$ Med ' 
 
 by the practitioner; and that it will form the most j and Sur S- Journal, March, I860. 
 
 complete, available, and reliable guide in emergen- I Dr. Hamilton is fortunate in having succeeded in 
 
 ciesof every nature connected with its subjects; and i filling the void, so long felt, with what cannot fail 
 
 also that the student of surgery may make it his text- i to be at once accepted as a model monograph in some 
 
 book wi Hi entire confidence, and with pleasure also, ! respects, and a work of classic*! authority. We 
 
 from its agreeable and easy style wefhink our own j sincerely congratulate the profession of the United 
 
 opinion may be gathered as to its value. Boston 
 Medical and Surgical Journal, March 1, 1860. 
 
 The work is concise, judicious, and accurate, and 
 adapted to the wants of the student, practitioner, 
 and investigator, honorable to the author and to the 
 profession. Chicago Med. Journal, March, 1860. 
 
 States on the appearance of such a publication from 
 one of their number. We have reason to be proud 
 of it as an original work, both in a literary and sci- 
 entific point of view, and to esteem it as a valuable 
 guide in a most difficult and important branch of 
 study and practice. On every account, therefore, 
 
 , we hope that it may soon be widely known abroad 
 We venture to say that this is not alone the only ; as an evideaee of genuine progress on this side of 
 complete treatise on the subject in the language, j the Atlantic, and further, that it may be still more 
 but the best and most practical we have ever read, j widely known at home as an authoritative teacher 
 The arrangement is simple and systematic, the die- i from which every one may profitably learn, and as 
 tion clear and graphic, and the illustrations nume- j affording an example of honest, well-directed, and 
 rous and remarkable for accuracy of delineation. UL tiring industry in authorship which every surgeon 
 The various mechanical appliances are faithfully may emulate.- Am. Med. Journal, April, 1860. 
 
 HOBLYN (RICHARD D.), M. D. 
 A DICTIONARY OF THE TERMS USED IN MEDICINE AND THE 
 
 COLLATERAL SCIENCES. A new American edition. Revised, with numerous Additions, 
 by ISAAC HAYS, M. D., editor of the " American Journal of the Medical Sciences." In one large 
 royal 12mo. volume, leather, of over 500 double columned pages. $1 50. 
 To both practitioner and student, we recommend use ; embracing every department of medical science 
 
 this dictionary as being convenient in size, accurate 
 in definition, and sufficiently full and complete for 
 ordinary consultation. Charleston Med. Journ. 
 
 We know of no dictionary better arranged and 
 adapted. It is not encumbered with the obsolete terms 
 of a bygone age, but it contains all that are now in 
 
 down to the very latest date. Western Lancet. 
 
 Hoblyn's Dictionary has long been a favorite with 
 us. It is the best book of definitions we have, and 
 ought always to be upon the student's table. 
 Southern Med. and Surg. Journal. 
 
 HOLLAND'S MEDICAL NOTES AND RE- 
 FLECTIONS. From the third London edition. 
 In one handsome octavo volume, extra cloth. S3. 
 
 HORNER'S SPECIAL ANATOMY AND HIS- 
 
 TOLOGY. Eighth edition. Extensively revised 
 and modified. In two large octavo volumes, ex- 
 tra cloth, of more than 1000 pages, with over 300 
 illustrations. $6 00. 
 
 HABERSHON (S. O.), M. D., 
 
 Assistant Physician to and Lecturer on Materia Medica and Therapeutics at Guy's Hospital, &c. 
 
 .PATHOLOGICAL AND PRACTICAL OBSERVATIONS ON DISEASES 
 
 OF THE ALIMENTARY CANAL, CESOPHAGUS, STOMACH, CAECUM, AND INTES- 
 TINES. With illustrations on wood. In one handsome octavo volume of 312 page*, extra 
 cloth $1 75. (Now Ready.} 
 
AND SCIENTIFIC PUBLICATIONS. 
 
 10 
 
 HODGE (HUGH L.), M. D., 
 
 Professor of Midwifery and the Diseases of Women and Children in the University of Pennsylvania, &c. 
 
 ON DISEASES PECULIAR TO WOMEN, including Displacements of the 
 Uterus. With original illustrations. In one beautifully printed octavo volume, of nearly 500 
 pages. (Now Ready.) 
 
 The profession will look with much interest on a volume embodying the long and extensive ex- 
 perience of Professor Hodge on an important branch of practice in which his opportunities for 
 investigation have been so extensive. A short summary of the contents will show the scope of 
 the work, and the manner in which the subject is presented. It will be seen that, with the excep- 
 tion of Displacements of the Uterus, he divides the Diseases peculiar, to Women into iwo great 
 constitutional class-es those arising from irritation, and those arising from sedation. 
 
 CONTENTS. 
 
 PART I. DISEASES OF IRRITATION. CHAPTER I. Nervous Irritation, and its Consequences II. 
 Irritable Uterus. III. Local Symptoms of Irritable Uterus: Menorrhagia and Hgemorrhagia; 
 Leucorrhoea ; Dysmenorrhoea IV. Local Symptoms of Irritable Uterus ; Complications. V. 
 General Symptoms of Irritable Uterus : Cerebro-spinal Irritations. VI. General Symptoms of 
 Irritable Uterus. VII. Progress and Results of Irritable Uterus. VIII. Causes and Pathology 
 of Irritable Diseases IX. Treatment of Irritable Uterus; Removal or Palliation of the Cause. 
 X. Treatment of Irritable Uterus: To Diminish or Destroy the Morbid Irritability X[. 
 Treatment of the Complications of Irritable Uterus. XII, Treatment of the Complications of 
 Irritable Uterus. 
 
 PART II. DISPLACEMENTS OF THE UTERUS. CHAPTER I. Natural Position and Supports of the 
 Uterus. II. Varieties of Displacements of the Uterus, and their Causes. III. Symptoms of 
 Displacements of the Uterus. IV. Treatment of Displacements of the Uterus. V. Treatment 
 of Displacements; Internal Supports. VI. Treatment of Displacements ; Lever Pessaries. 
 VII. Treatment of the Varieties of Displacements. VIII. Treatment of Complications of Dis- 
 placements of the Uterus. IX. Treatment of Enlargements and Displacements of the Ovaries, &c. 
 
 PART III. DISEASES OF SEDATION. CHAPTER I. Sedation and its Consequences: Organic and 
 Nervous Sedation; Passive Congestion; Reaction; Treatment II. Sedation of the Uterus; 
 Amenorrhoea: Sedation of the Uterus from Moral Causes; Sedation of the Uterus from Physical 
 Causes. III. Diagnosis and Treatment of Sedation, of the Uterus. 
 The illustrations, which are all original, are drawn to a uniform scale of one-half the natural size. 
 
 JONES (T. WHARTON), F. R. S., 
 
 Professor of Ophthalmic Medicine and Surgery in University College, London, &c. 
 
 THE PRINCIPLES AND PRACTICE OF OPHTHALMIC MEDICINE 
 
 AND SURGERY. With one hundred and ten illustrations. Second American from the second 
 and revised London edition, with additions by EDWARD HARTSHORNE, M. D., Surgeon to Wills' 
 Hospital, &c. In one large, handsome royal 12mo. volume, extra cloth, of 500 pages. $1 50. 
 
 JONES (C. HAN DPI ELD), F. R. S., &. EDWARD H. SIEVEKING, M.D., 
 
 Assistant Physicians and Lecturers in St. Mary's Hospital, London. 
 
 A MANUAL OF PATHOLOGICAL ANATOMY. First American Edition, 
 
 Revised. With three hundred and ninety-seven handsome wood engravings. In one large and 
 beautiful octavo volume of nearly 750 pages, leather. $3 75. 
 
 As a concise text-book, containing, in a condensed obliged to glean from a great number of monographs, 
 
 form, a complete outline of what is known in the and the field was so extensive that but few cultivated 
 
 domain of Pathological Anatomy, it is perhaps the it with any degree of success. As a simple work 
 
 best work in the English language. Its great merit of reference, therefore, it is of great value to the 
 
 consists in its completeness and brevity, and in this student of pathological anatomy, and should be in 
 
 respect it supplies a great, desideratum in our lite- every physician's library. Western Lancet. 
 rature. Heretofore the student of pathology was 
 
 KIRKES (WILLIAM SENHOUSE), M.D., 
 
 Demonstrator of Morbid Anatomy at St. Bartholomew's Hospital, &c. 
 
 A MANUAL OF PHYSIOLOGY. A new American, from the third and 
 improved London edition. With two hundred illustrations. In one large and -handsome royal 
 12mo. volume, leather, pp. 586. $2 00. (Lately Published.) 
 
 This is a new and very much improved edition of 
 Dr. Kirkes' well-known Handbook of Physiology. 
 It combines conciseness with completeness, and is, 
 therefore, admirably adapted for consultation by the 
 busy practitioner. Dublin Quarterly Journal. 
 
 Its excellence is in its compactness, its clearness, 
 and its carefully cited authorities. It is the most 
 convenient of text-books. These gentlemen, Messrs. 
 Kirkes and Paget, have really an immense talent for 
 silence, which is not so common or so cheap as prat- 
 ing people fancy. They have the gift of telling us 
 what we want to know, without thinking it neces- 
 sary to tell us all they know. 5oszo Med and 
 Surg. Journal. 
 
 One of the very best handbooks of Physiology we 
 possess presenting just such an outline of the sci- 
 ence as the student requires during his attendance 
 upon a course of lectures, or for reference whilst 
 preparing for examination. Am. Medical Journal. 
 
 For the student beginning this study, and the 
 practitioner who has but leisure to refresh his 
 memory, this book is invaluable, as it contains all 
 that it is important to know, without special details, 
 which are read with interest only by those who 
 would make a specialty, or desire to possess a criti- 
 cal knowledge of the subject. Charleston Med. 
 Journal. 
 
20 BLANCHARD & LEA'S MEDICAL 
 
 KNAPP'S TECHNOLOGY ; or, Chemistry applied 
 to the Arts and to Manufactures. Edited by Dr. 
 RONALDS, Dr. RICHARDSON, and Prof. XV. R. 
 
 LAYCOCK'S LECTURES ON THE PRINCI- 
 PLES AND METHODS OF MEDICAL OB- 
 SERVATION AND RESEARCH. For the Use 
 
 JOHNSON. In two handsome Svo.vols., with about I of Advanced Students and Junior Practitioners. 
 500 wood- engravings. $600. In one royal 12mo. volume, extra cloth. Price $1. 
 
 LALLEMAND AND WILSON. 
 A PRACTICAL TREATISE ON THE CAUSES, SYMPTOMS, AND 
 
 TREATMENT OF SPERMATORRHOEA. By M. LALLEMAND. Translated and edited bv 
 
 HENRY J McDouGALL. Third American edition. To which is added ON DISEASES 
 
 OF THE VESICUL^; SEMINALES; AND THEIR ASSOCIATED ORGANS. With special refer- 
 ence to the Morbid Secretions of the Prostatic and Urethra! Mucous Membrane. By MARRIS 
 WILSON, M. D. In one neat octavo volume, of about 400 pp., extra cloth. $2 00. (Just Issued.} 
 
 LA ROCHE (R.), M. D., &c. 
 
 YELLOW FEVER, considered in its Historical, Pathological, Etiological, and 
 Therapeutical Relations. Including a Sketch of the Disease as it has occurred in Philadelphia 
 from 1699 to 1854, with an examination of the connections between it and the fevers known under 
 the same name in other parts of temperate as well as in tropical regions. In two large and 
 handsome octavo volumes of nearly 1500 pages, extra cloth. $7 00. 
 
 From Professor S. H. Dickson, Charleston, S. C., 
 September 18, 1855. 
 
 A monument of intelligent and well applied re- 
 search, almost without example. It is, indeed, in 
 itself, a large library, and is destined to constitute 
 the special resort as a book of reference, in the 
 subject of which it treats, to all future time. 
 
 We have not time at present, engaged as we are, 
 by da,y and by night, in the work of combating this 
 very disease, now prevailing in our city, to do more 
 than give this cursory notice of what we consider 
 as undoubtedly the most able and erudite medical 
 publication our country has yet produced. But in 
 
 nant and unmanageable disease of modern times, 
 has for several years been prevailing in our country 
 to a greater extent than ever before; that it is no 
 longer confined to either large or small cities, but 
 penetrates country villages, plantations, and farm- 
 houses ; that it is treated with scarcely better suc- 
 cess now than thirty or forty years ago ; that there 
 is vast mischief done by ignorant pretenders to know- 
 ledge in regard to the disease, and in view of the pro- 
 bability that a majority of southern physicians will 
 be called upon to treat the disease, we trust that this 
 able and comprehensive treatise will he very gene- 
 rally read in the south. Memphis Med. Recorder. 
 
 view of the startling fact, that this, the most malig- 
 
 BY THE SAME AUTHOR. 
 
 PNEUMONIA ; its Supposed Connection, Pathological and Etiological, with Au- 
 tumnal Fevers, including an Inquiry into the Existence and Morbid Agency of Malaria. In one 
 handsome octavo volume, extra cloth, of 500 pages. $3 00. 
 
 LUDLOW (J. L.), M. D. 
 A MANUAL OP EXAMINATIONS upon Anatomy, Physiology, Surgery, 
 
 Practice of Medicine, Obstetrics, Materia Medica, Chemistry, Pharmacy, and Therapeutics. To 
 which is added a Medical Formulary. Third edition, thoroughly revised and greatly extended 
 and enlarged. With 370 illustrations. In one handsome royal 12mo. volume, leather, of 816 
 large pages. $2 50. 
 
 The great popularity of this volume, and t he numerous demands for it during the two years in which 
 it has been out of print, have induced the author in its revision to spare no pains to render it a 
 correct and accurate digest of the most recent condition of all the branches of medical science. In 
 many respects it may, therefore, be regarded rather as a new book than a new edition, an entire 
 section on Physiology having been added, as also one on Organic Chemistry, and many portions 
 having been rewritten. A very complete series of illustrations has been introduced, and every 
 care has been taken in the mechanical execution to render it a convenient and satisfactory book for 
 study or reference. The arrangement of the volume in the form of question and answer renders it 
 especially suited for the office examination of students and for those preparing for graduation. 
 
 We know of no better companion for the student I crammed into his head by the various professors to 
 during the hours spent in the lecture room, or to re- whom he is compelled to listen. Western Lancet, 
 fresh, at a glance, his memory of the various topics | May, 1857. 
 
 LEHMANN (C. G.) 
 
 PHYSIOLOGICAL CHEMISTRY. Translated from the second edition by 
 GEORGE E. DAY, M. D., F. R. S., &c., edited by R. E. ROGERS, M. D., Professor of Chemistry 
 in the Medical Department of the University of Pennsylvania, with illustrations selected from 
 Funke's Atlas of Physiological Chemistry, and an Appendix of plates. Complete in two large 
 and handsome octavo volumes, extra cloth, containing 1200 pages, with nearly two hundred illus- 
 trations. $6 00. 
 
 The most important contribution as yet made to 
 
 The work of Lehmann stands unrivalled as the 
 most comprehensive book of reference and informa- 
 tion extant on every branch of the subject on which 
 
 Physiological Chemistry. Am. Journal Med. Sci- 
 ences, Jan. 1856. 
 
 it treats. Edinburgh Journal of Medical Science. 
 
 BY THE SAME AUTHOR. (Lately Published.} 
 
 MANUAL OF CHEMICAL PHYSIOLOGY. Translated from the German, 
 
 with Notes and Additions, by J. CHESTON MORRIS, M. D., with an Introductory Essay on Vital 
 Force, by Professor SAMUEL JACKSON, M. D., of the University of Pennsylvania. With illus- 
 trations on wood. In one very handsome octavo volume, extra cloth, of 336 pages. $2 25. 
 
 From Prof. Jackson's Introductory Essay. 
 
 In adopting the handbook of Dr. Lehmann as a manual of Organic Chemistry for the use of the 
 students of the University, and in recommending his original work of PHYSIOLOGICAL CHEMISTRY 
 for their more mature studies, the high value of his researches, and the great weight of his autho- 
 rity in that important department of medical science are fully recognized. 
 
AND SCIENTIFIC PUBLICATIONS. 
 
 21 
 
 LAWRENCE (W.), F. R. S., &c. 
 A TREATISE ON DISEASES OF THE EYE. A new edition, edited, 
 
 with numerous additions, and 243 illustrations, by ISAAC HAYS, M. D., Surgeon to Will's Hospi- 
 tal, &c. In one very large and handsome octavo volume, of 950 pages, strongly bound in leather 
 with raised bands. $5 00. 
 
 MEIGS (CHARLES D.), M. D., 
 
 Professor of Obstetrics, &c. in the Jefferson Medical College, Philadelphia. 
 
 OBSTETRICS : THE SCIENCE AND THE ART. Third edition, revised 
 
 and improved. With one hundred and twenty-nine illustrations. In one beautifully printed octavo 
 
 volume, leather, of seven hundred and fifty-two large pages. $3 75. 
 
 The rapid demand for another edition of this work is a sufficient expression of the favorable 
 verdict of the profession. In thus preparing it a third time for the presS, the author has endeavored 
 to render it in every respect worthy of the favor which it has received. To accomplish this he 
 has thoroughly revised it in every part. Some portions have been rewritten, others added, new 
 illustrations have been in many instances substituted for such as were not deemed satisfactory, 
 while, by an alteration in the typographical arrangement, the size of the work has not been increased, 
 and the price remains unaltered. In its present improved form, it is, therefore, hoped that the work 
 will continue to meet the wants of the American profession as a sound, practical, and extended 
 SYSTEM OF MIDWIFERY. 
 
 Though the work has received only five pages of 
 enlargement, its chapters throughout wear the im- 
 press of careful revision. Expunging and rewriting, 
 remodelling its sentences, with occasional new ma- 
 terial, all evince a lively desire that it shall deserve 
 to be regarded as improved in manner as well as 
 matter. In the matter, every stroke of the pen has 
 
 increased the value of the book, both in expungings 
 and additions Western Lancet, Jan. 1857. 
 
 The best American work on Midwifery that is 
 accessible to the student and practitioner N. W. 
 Med. and Surg. Journal, Jan. 1857. 
 
 This is a standard work by a great American Ob- 
 stetrician. It is the third and last edition, and, in 
 the language of the preface, the author has "brought 
 the subject up to the latest dates of real improve- 
 
 ment in our art and Science." Nashville Journ. of 
 Med. and Surg., May, 1857. 
 
 BY THE SAME AUTHOR. (Just Issued.} 
 
 WOMAN: HER DISEASES AND THEIR REMEDIES. A Series of Lec- 
 tures to his Class. Fourth and Improved edition. In one large and beautifully printed octavo 
 volume, leather, of over 700 pages. $3 60. 
 In other respects, in our estimation, too much can- 
 not be said in praise of this work. It abounds with 
 beautiful passages, and for conciseness, for origin- 
 
 ality, and for all that is commendable in a work on 
 the diseases of females, it is not excelled, and pro- 
 bably not equalled in the English language. On the 
 whole, we know of no worit on the diseases of wo- 
 men which we can so cordially commend to the 
 student and practitioner as the one before us. Ohio 
 Med. and Surg. Journal. 
 
 The body of the book is worthy of attentive con- 
 sideration, and is evidently the production of a 
 clever, thoughtful, and sagacious physician. Dr. 
 Meigs's letters on the diseases of the external or- 
 gans, contain many interesting and rare cases, and 
 many instructive observations. We take our leave 
 of Dr. Meigs, with a high opinion of his talents and 
 originality. The British and Foreign Medico-Chi- 
 rurgical Review. 
 
 Every chapter is replete with practical instruc- 
 tion, and bears the impress of being the composition 
 of an acute and experienced mind. There is a terse- 
 ness, and at the same time an accuracy in his de- 
 scription of symptoms, and in the rules for diagnosis, 
 which cannot fail to recommend the volume to the 
 attention of the reader. Ranking^ Abstract. 
 
 It contains a vast amount of practical knowledge, 
 by one who has accurately observed and retained 
 
 the experience of many years. 
 Journal. 
 
 -Dublin Quarterly 
 
 Full of important matter, conveyed in a ready and 
 agreeable manner. St. Louis Med. and Surg. Jour. 
 
 There is an off-hand fervor, a glow, and a warm- 
 heartedness infecting the effort of Dr. Meigs, which 
 is entirely captivating, and which absolutely hur- 
 ries the reader through from beginning to end. Be- 
 sides, the book teems with solid instruction, and 
 it shows the very highest evidence of ability, viz., 
 the clearness with which the information is pre- 
 sented. We know of no better test of one's under- 
 standing a subject than the evidence of the power 
 of lucidly explaining it. The most elementary, as 
 well as the obscurest subjects, under the pencil of 
 Prof. Meigs, are isolated and made to stand out in 
 such bold relief, as to produce distinct impressions 
 upon the mind and memory of the reader. Tht 
 Charleston Med. Journal. 
 
 Professor Meigs has enlarged and amended this 
 great work, for such it unquestionably is, having 
 passed the ordeal of criticism at home and abroad, 
 but been improved thereby ; for in this new edition 
 the author has introduced real improvements, and 
 increased the value and utility of the book im- 
 measurably. It presents so many novel, bright, 
 and sparkling thoughts; such an exuberance of new 
 ideas on almost every page, that we confess our- 
 selves to have become enamored with the book 
 and its author ; and cannot withhold our congratu- 
 lations from our Philadelphia confreres, that such a 
 teacher is in their service. N. Y. Med. Gazette. 
 
 BY THE SAME AUTHOR. 
 
 ON THE NATURE, SIGNS, AND TREATMENT OF CHILDBED 
 
 FEVER. In a Series of Letters addressed to the Students of his Class. In one handsome 
 
 octavo volume, extra cloth, of 365 pages. $2 50. 
 
 lectable book. * * * This treatise upon child- 
 bed fevers will have an extensive sale, being des- 
 tined, as it deserves, to find a place in the library 
 
 The instructive and interesting author of this 
 work, whose previous labors have placed his coun- 
 trymen under deep and abiding obligations, again 
 challenges their admiration in the fresh and vigor- 
 ous, attractive and racy pages before us. It is a de- 
 
 of every practitioner who scorns to lag in the rear. 
 Nashville Journal of Medicine and Surgery. 
 
 BY THE SAME AUTHOR J WITH COLORED PLATES. 
 
 A TREATISE ON ACUTE AND CHRONIC DISEASES OF THE NECK 
 
 OF THE UTERUS. With numerous plates, drawn and colored from nature in the highest 
 style oi art. In one handsome octavo volume, extra cloth. $4 50. 
 
 MAYNE'S DISPENSATORY AND THERA- 
 PEUTICAL REMEMBRANCER. With every 
 Practical Formula contained in the three British 
 Pharmacopoeias. Edited, with the addition of the 
 Farmulse of the U.S. Pharmacopoeia, by R. E. 
 GRIFFITH, M.D. 112mo.vol.ex.cl.,300pp. 75 c. 
 
 MALGAIGNE'S OPERATIVE SURGERY, based 
 on Normal and Pathological Anatomy. Trans- 
 lated from the French by FREDERICK BRITTAN, 
 A . B . , M . D . Wi th numerous illustrations on wood . 
 In one handsome octavo volume, extra cloth, of 
 nearly six hundred pages. $2 25. 
 
BLANCHARD & LEA'S MEDICAL 
 
 MACLISE (JOSEPH), SURGEON. 
 
 SURGICAL ANATOMY. Forming one volume, very large imperial quarto. 
 With sixty-eight large and splendid Plates, drawn in the best style and beautifully colored. Con- 
 taining one hundred and ninety Figures, many of them the size of life. Together with copious 
 and explanatory letter-press. Strongly and handsomely bound in extra cloth, being one of the 
 cheapest and best executed Surgical works as yet issued in this country. $11 00. 
 *,* The size of this work prevents its transmission through the post-office as a whole, but those 
 
 who desire to have copies forwarded by mail, can receive them in five parts, done up in stout 
 
 wrappers. Price $9 00. 
 
 One of the greatest artistic triumphs of the age 
 in Surgical Anatomy. British American Medical 
 Journal. 
 
 No practitioner whose meas will admit should 
 fail to possess it. Ranking's Abstract. 
 
 A work which has no parallel in point of accu- 
 racy and cheapness in the English language. N. Y. 
 Journal of Medicine. 
 
 We are extremely gratified to announce to the 
 profession the completion of this truly magnificent 
 
 Too much cannot be said in its praise; indeed, I work which ag - whol certainly 'stands unri- 
 we have not language to do it justice. Ohio Medi- ' > .- '- ,._...__ -- 
 
 cal and Surgical Journal. 
 
 The most accurately engraved and beautifully 
 colored plates we have ever seen in an American 
 book one of the best and cheapest surgical works 
 ever published. Buffalo Medical Journal. 
 
 It is very rare that so elegantly printed, so well 
 illustrated, and so useful a work, is offered at BO 
 moderate a price. Charleston Medical Journal. 
 
 Its plates can boast a superiority which places 
 them almost beyond the reach of competition. Medi- 
 cal Examiner. 
 
 Country practitioners will find these plates of im- 
 mense value. N. Y. Medical Gazette. 
 
 vailed, both for accuracy of drawing, beauty of 
 coloring, and all the requisite explanations of the 
 subject in hand. Tht Neu> Orleans Medical and 
 Surgical Journal. 
 
 This is by far the ablest work on Surgical Ana- 
 tomy that has come under our observation. We 
 know of no other work that would justify a stu- 
 dent, in any degree, for neglect of actual dissec- 
 tion. In those sudden emergencies that so often 
 arise, and which require the instantaneous command 
 of minute anatomical knowledge, a work of this kind 
 keeps the details of the dissecting-room perpetually 
 fresh in the memory. The Western Journal of Medi- 
 cine and Surgery. 
 
 MILLER (HENRY), M. D., 
 
 Professor of Obstetrics and Diseases of Women and Children in the University of Louisville. 
 
 PRINCIPLES AND PRACTICE OF OBSTETRICS, &o. ; including the Treat- 
 
 ment of Chronic Inflammation of the Cervix and Body of the Uterus considered as a frequent 
 cause of Abortion. With about one hundred illustrations on wood. In one very handsome oc- 
 tavo volume, of over 600 pages. (Lately Published.) $3 75. 
 
 The reputation of Dr. Miller as an obstetrician is too widely spread to require the attention oi 
 the profession to be specially called to a volume containing the experience of his long and extensive 
 practice. The very favorable reception accorded to his " Treatise on Human Parturition," issued 
 some years since, is an earnest that the present work will fulfil the author's intention of providing 
 within a moderate compass a complete and trustworthy text-book for the student, and book of re- 
 ference for the practitioner. 
 
 tion to which its merits justly entitle it. The style 
 is such that the descriptions are clear, and each sub- 
 ject is discussed and elucidated with due regard to 
 its practical bearings, which cannot fail to make it 
 acceptable and valuable to both students and prac- 
 titioners. We cannot, however, close this brief 
 notice without congratulating the author and the 
 profession on the production of such an excellent 
 treatise. The author is a western man of whom we 
 
 We congratulate the author that the task is done. 
 We congratulate him that he has given to the medi- 
 cal public a work which will secure for him a high 
 and permanent position among the standard autho- 
 rities on the principles and practice of obstetrics. 
 Congratulations are not less due to the medical pro- 
 fession of this country, on the acquisition of a trea- 
 tise embodying the results of the studies, reflections, 
 and experience of Prof. Miller. Few men, if any, 
 in this country, are more competent than he to write 
 
 on this department of medicine. Engaged for thirty- 
 five years in an extended practice of obstetrics, for 
 many years a teacher of this branch of instruction 
 in one of the largest of our institutions, a diligent 
 student as well as a careful observer, an original and 
 independent thinker, wedded to no hobbies, ever 
 ready to consider without prejudice new views, and 
 to adopt innovations if they are really improvements, 
 and withal a clear, agreeable writer, a practical 
 treatise from his pen could not fail to possess great 
 value. Buffalo Med Journal, Mar. 1858. 
 
 In fact, this volume must take its place among the 
 standard systematic treatises on obstetrics ; a posi- 
 
 feel proud, and we cannot but think that his book 
 will find many readers and warm admirers wherever 
 obstetrics is taught and studied as a science and an 
 art. The Cincinnati Lancet and Observer, Feb. 1858. 
 A most respectable and valuable addition to our 
 home medical literature, and one reflecting credit 
 alike on the author and the institution to which he 
 is attached. The student will find in this work a 
 most useful guide to his studies ; the country prac- 
 titioner, rusty in his reading, can obtain from its 
 pages a fair resume of the modern literature of the 
 science; and we hope to see this American produc- 
 tion generally consulted by the profession. Va. 
 Med. Journal, Feb. 1858. 
 
 MACKENZIE (W.), M. D., 
 
 Surgeon Oculist in Scotland in ordinary to Her Majesty, &c. &c. 
 
 A PRACTICAL TREATISE ON DISEASES AND INJURIES OF THE 
 
 EYE. To which is prefixed an Anatomical Introduction explanatory of a Horizontal Section of 
 the Human Eyeball, by THOMAS WHARTON JONES, F. R. S. From the Fourth Revised and En- 
 larged London Edition. With Notes and Additions by ADDINELL HEWSON, M. D., Surgeon to 
 Wills Hospital, &c. &c. In one very large and handsome octavo volume, leather, raised bands, with 
 plates and numerous wood-cuts. $5 25. 
 The treatise of Dr. Mackenzie indisputably holds 
 
 spi 
 
 the first place, and forms, in respect of learning and 
 research, an Encyclopaedia unequalled in extent by 
 any other work of the kind, either English or foreign. 
 Dixon on Diseases of the Eye. 
 
 Few modern books on any department of medicine 
 or surgery have met with such extended circulation, 
 or have procured for their authors a like amount of 
 European celebrity. The immense research which 
 it displayed, the thorough acquaintance with the 
 ubiect, oracticallv as well as iU ^"nlly.and the 
 
 able manner in which the author's stores of learning 
 and experience were rendered available for general 
 use, at once procured for the first edition, as well on 
 the continent as in this country, that high position 
 as a standard work which each successive edition 
 has more firmly established. We consider it the 
 duty of ever 
 and the w 
 
 self familiar with this the most complete work in 
 the English language upon the diseases of the eye. 
 Med. Times and Gazette. 
 
 ery one who has the love of his profession 
 elfare of his patient at heart, to make him- 
 
AND SCIENTIFIC PUBLICATIONS. 
 
 MILLER (JAMES), F. R. S. E., 
 
 Professor of Surgery in the University of Edinburgh, &c. 
 
 PRINCIPLES OF SURGERY. Fourth American, from the third and revised 
 Edinburgh edition. In one large and very beautiful volume, leather, of 700 pages, with two 
 hundred and forty illustrations on wood. $3 75. 
 
 The work of Mr. Miller is too well and too favor- 
 ably known among us, as one of our best text-books, 
 to render any further notice of it necessary than the 
 announcement of a new edition, the fourth in our 
 country, a proof of its extensive circulation among 
 us. As a concise and reliable exposition of the sci- 
 ence of modern surgery, it stands deservedly high 
 we know not its superior. Boston Med. and Surg. 
 Journal . 
 
 The work takes rank with Watson's Practice of 
 Physic; it certainly does not fall behind that great 
 work in soundness of principle or depth of reason- 
 ing and research. No physiciaji who values his re- 
 putation, or seeks the interests of his clients, can 
 acquit himself before his God and the world without 
 making himself familiar with the sound and philo- 
 sophical views developed in the foregoing book. 
 New Orleans Med. and Surg. Journal. 
 
 BY THE SAME AUTHOR. (Just Issued.) 
 
 THE PRACTICE OF SURGERY. Fourth American from the last Edin- 
 burgh edition. Revised by the American editor. Illustrated by three hundred and sixty- four 
 engravings on wood. In one large octavo volume, leather, of nearly 700 pages. $3 75. 
 
 No encomium of ours could add to the popularity 
 of Miller's Surgery. Its reputation in this country 
 is unsurpassed by that of any other work, and, when 
 taken in connection with the author's Principles of 
 Surgery, constitutes a whole, without reference to 
 to which no conscientious surgeon would be willing 
 practice his art. Southern Med. and Surg. Journal. 
 
 It is seldom that two volumes have ever made so 
 profound an impression in so short a time as the 
 "Principles" and the " Practice" of Surgery by 
 Mr. Miller or so richly merited the reputation they 
 have acquired. The author is an eminently sensi- 
 ble, practical, and well-informed man, who knows 
 exactly what he is talking about and exactly how to 
 talk it. Kentucky Medical Recorder. 
 
 By the almost unanimous voice of the profession, 
 
 his works, both on the principles and practice of 
 surgery have been assigned thehighest rank. If we 
 were limited to but one work on surgery, that one 
 should be Miller's, as we regard it as superior to all 
 others. St. Louis Med. and Surg. Journal. 
 
 The author has in this and his " Principles," pre- 
 sented to the profession one of the most complete and 
 reliable systems of Surgery extant. His style of 
 writing is original, impressive, and engaging, ener- 
 getic, concise, and lucid. Few have the faculty of 
 condensing so much in small space, and at the same 
 time so persistently holding the attention. Whether 
 as a text-book for students or a book of reference 
 for practitioners, it cannot be too strongly recom- 
 mended. Southern Journal of Med. and Physical 
 Sciences. 
 
 MORLAND (W. WJ, M. D., 
 
 Fellow of the Massachusetts Medical Society, &c. 
 
 DISEASES OF THE URINARY ORGANS; a Compendium of their Diagnosis, 
 
 Pathology, and Treatment. With illustrations. In one large and handsome octavo volume, of 
 about 600 pages, extra cloth. (Just Issued.) $3 50. 
 
 Taken as a whole, we can recommend Dr. Mor- 
 land's compendium as a very desirable addition to 
 the library of every medical or surgical practi- 
 tioner. Brit, and For. Med.-Chir. Rev., April, 1859. 
 
 Every medical practitioner whose attention has 
 been to any extent attracted towards the class of 
 diseases to which this treatise relates, must have 
 often and sorely experienced the want of some full, 
 yet concise recent compendium to which he could 
 refer. This desideratum has been supplied by Dr. 
 Morland, and it has been ably done. He has placed 
 before us a full, judicious, and reliable digest. 
 Each subject is treated with sufficient minuteness, 
 
 yet in a succinct, narrational style, such as to render 
 the work one of great interest, and one which will 
 prove in the highest degree useful to the general 
 practitioner. To the members of the profession in the 
 country it will be peculiarly valuable, on account 
 of the charasteristics which we have mentioned, 
 and the one broad aim of practical utility which is 
 kept in view, and which shines out upon every page, 
 together with the skill which is evinced in the com- 
 bination of this grand requisite with the utmost 
 brevity which a just treatment of the subjects would 
 admit. N. Y. Journ. of Medicine, Nov. 1858. 
 
 MONTGOMERY (W. F.), M. D., M. R. I. A., &c., 
 
 Professor of Midwifery in the King and Queen's College of Physicians in Ireland, &c. 
 
 AN EXPOSITION OF THE SIGNS AND SYMPTOMS OF PREGNANCY. 
 
 With some other Papers on Subjects connected with Midwifery. From the second and enlarged 
 English edition. With two exquisite colored plates, and numerous wood-cuts. In one very 
 handsome octavo volume, extra cloth, of nearly 600 pages. (Lately Published.) $3 75. 
 
 A book unusually rich in practical suggestions. 
 Am. Journal Med. Sciences, Jan. 1857. 
 
 These several subjects so interesting in them- 
 selves, and so important, every one of them, to the 
 most delicate and precious of social relations, con- 
 trolling often the honor and domestic peace of a 
 family, the legitimacy of offspring, or the life of its 
 parent, are all treated with an elegance of diction, 
 fulness of illustrations, acutenessand justice of rea- 
 soning, unparalleled in obstetrics, and unsurpassed in 
 medicine. The reader's interest can never flag, so 
 fresh, and vigorous, and classical is our author's 
 style; and one forgets, in the renewed charm of 
 every page, that it, and every line, and every word 
 
 has been weighed and reweighed through years of 
 preparation ; that this is of all others the book of 
 Obstetric Law, on each of its several topics ; on all 
 points connected with pregnancy, to be everywhere 
 received as a manual of special jurisprudence^ at 
 once announcing fact, affording argument, establish- 
 ing precedent, and governing alike the juryman, ad- 
 vocate, and judge. It is not merely in its le^al re- 
 lations that we find this work so interesting. Hardly 
 a page but that has its hints or facts important to 
 the general practitioner ; and not a chapter without 
 especial matter for the anatomist, physiologist, or 
 pathologist. N. A. Med.-Chir. Review, March, 
 1857. 
 
 MOHR (FRANCIS), PH. D., AND REDWOOD (TH EOPHI LUS). 
 PRACTICAL PHARMACY. Comprising the Arrangements, Apparatus, 
 
 and 
 
 Manipulations of the Pharmaceutical Shop and Laboratory. Edited, with extensive Additions, 
 by Prof. WILLIAM PROCTER, of the Philadelphia College of Pharmacy. In one handsomely 
 printed octavo volume, extra cloth, of 570 pages, with over 500 engravings on wood. $2 75, 
 
24 BLANCHARD & LEA'S MEDICAL 
 
 NEILL (JOHN), M. D., 
 
 Surgeon to the Pennsylvania Hospital, &c. ; and 
 
 FRANCIS GURNEY SMITH, M.D., 
 
 Professor of Institutes of Medicine in the Pennsylvania Medical College. 
 
 AN ANALYTICAL COMPENDIUM OF THE VARIOUS BRANCHES 
 
 OF MEDICAL SCIENCE ; for the Use and Examination of Students. A new edition, revised 
 and improved. In one very large and handsomely printed royal 12mo. volume, of about one 
 thousand pages, with 374 wood-cuts. Strongly bound in leather, with raised bands. $3 00. 
 The very flattering reception which has been accorded to this work, and the high estimate placed 
 upon it by the profession, as evinced by the constant and increasing demand which has rapidly ex- 
 hausted two large editions, have stimulated the authors to render the volume in its present revision 
 more worthy of the success which has attended it. It has accordingly been thoroughly examined, 
 and such errors as had on former occasions escaped observation have been corrected, and whatever 
 additions were necessary to maintain it on a level with the advance of science have been introduced. 
 The extended series of illustrations has been still further increased and much improved, while, by 
 a slight enlargement of the page, these various additions have been incorporated without increasing 
 the bulk of the volume. 
 
 The work is, therefore, again presented as eminently worthy of the favor with which it has hitherto 
 been received. As a book for daily reference by the student requiring a guide to his more elaborate 
 text-books, as a manual for preceptors desiring to stimulate their students by frequent and accurate 
 examination, or as a source from which the practitioners of older date may easily and cheaply acquire 
 a knowledge of the changes and improvement in professional science, its reputation is permanently 
 established. 
 
 the students is heavy, and review necessary for an 
 
 The best work of the kind with which we are 
 acquainted. Med. Examiner. 
 
 Having made free use of this volume in our ex- 
 aminations of pupils, we can speak from experi- 
 ence in recommending it as an admirable compend 
 for students, and as especially useful to preceptors 
 who examine their pupils. It will save the teacher 
 much labor by enabling him readily to recall all of 
 the points upon which his pupils should be ex- 
 amined. A work of this sort should be in the hands 
 of every one who takes pupils into his office with a 
 view of examining them ; and this is unquestionably 
 the best of its class. Transylvania Med. Journal. 
 
 In the rapid course of lectures, where work for 
 
 examination, a compend is not only valuable, but 
 it is almost a sine qua non. The one before us is, 
 in most of the divisions, the most unexceptionable 
 of all books of the kind that we know of. The 
 newest and soundest doctrines and the latest im- 
 provements and discoveries are explicitly, though 
 concisely, laid before the student. There is a class 
 to whom we very sincerely commend this cheap book 
 as worth its weight in silver that class is the gradu- 
 ates in medicine of more than ten years' standing, 
 who have not studied medicine since. Tliey will 
 perhaps find out from it that the science is not exactly 
 now what it was when they left it off. The Stetho- 
 scope. 
 
 NELIGAN (J. MOORE), M. D., M. R. I. A., &c. 
 
 (A splendid work. Just Issued.) 
 
 ATLAS OF CUTANEOUS DISEASES. In one beautiful quarto volume, extra 
 
 cloth, with splendid colored plates, presenting nearly one hundred elaborate representations of 
 
 disease. $4 50. 
 
 This beautiful volume is intended as a complete and accurate representation of all the varieties 
 of Diseases of the Skin. While it can be consulted in conjunction with any work on Practice, it has 
 especial reference to the author's " Treatise on Diseases of the Skin," so favorably received by the 
 profession some years since. The publishers feel justified in saying that few more beautifully exe- 
 cuted plates have ever been presented to the profession of this country. 
 
 Neligan's Atlas of Cutaneous Diseases supplies a [ give, at a coup d'ail, the remarkable peculiarities 
 long existent desideratum much felt by the largest 
 class of our profession. It presents, in quarto size, 
 16 plates, each containing from 3 to 6 figures, and 
 forming in all a total of 90 distinct representations 
 of the different species of skin affections, grouped 
 together in genera or families. The illustrations 
 have been taken from nature, and have been copied 
 with such fidelity that they present a striking picture 
 of life ; in which the reduced scale aptly serves to 
 
 BY THE SAME AUTHOR. 
 
 A PRACTICAL TREATISE ON DISEASES OF THE SKIN. Third 
 
 American edition. In one neat royal 12mo. volume, extra cloth, of 334 pages. $1 00. 
 The two volumes will be sent by mail on receipt of Five Dollars. 
 
 of each individual variety. And while thus the dis- 
 ease is rendered more definable, there is yet no loss 
 of proportion incurred by ttte necessary concentra- 
 tion. Each figure is highly colored, and so truthful 
 has the artist been that the mostfastid'ous observer 
 could not justly take exception to the correctness of 
 the execution of the pictures under his scrutiny. 
 Montreal Med. Chronicle. 
 
 OWEN ON THE DIFFERENT FORMS OF I One vol. royal 12mo., extra cloth with numerous 
 THE SKELETON, AND OF THE TEETH. | illustrations. 81 25 
 
 PIRRIE (WILLIAM), F. R. S. E., 
 
 Professor of Surgery in the University of Aberdeen. 
 
 THE PRINCIPLES AND PRACTICE OF SURGERY. Edited by JOHN 
 
 NEILL, M. D., Professor of Surgery in the Penna. Medical College, Surgeon to the Pennsylvania 
 
 Hospital, &c. In one very handsome octavo volume, leather, ot 780 pages, with 316 illustrations. 
 
 $3 75. 
 
 We know of no other surgical work of a reason- J rately discussed the principles of surgery, and a 
 able size, wherein there is so much theory and prac- j safe and effectual practice predicated upon them, 
 tice, or where subjects are more soundly or clearly j Perhaps no work upon this subject heretofore issued 
 taught. The Stethoscope. \ is so full upon the science of the art of surgery. 
 
 Prof. Pirrie, in the work before us, has elabo- | Nashville Journal of Medicine and Surgery. 
 
AND SCIENTIFIC PUBLICATIONS. 
 
 2f> 
 
 PARRISH (EDWARD), 
 
 Lecturer on Practical Pharmacy and Materia Medica in the Pennsylvania Academy of Medicine, &c. 
 
 AN INTRODUCTION TO PRACTICAL PHARMACY. Designed as a Text- 
 
 Book for the Student, and as a Guide for the Physician and Pharmaceutist. With many For- 
 mulae and Prescriptions. Second edition, greatly enlarged and improved. In one handsome 
 octavo volume of 720 pages, with several hundred Illustrations, extra cloth. $3 50. (Now 
 Ready.) 
 
 During the short time in which this work has been before the profession, it has been received 
 with very great favor, and in assuming the position of a standard authority, it has filled a vacancy 
 which had been severely felt. Stimulated by this encouragement, the author, in availing himself 
 of the opportunity of revision, has spared no pains to render it more worthy of the confidence be- 
 stowed'upon it, and his assiduous labors have made it rather a new book than a new edition, many 
 portions having been rewritten, and much new and important matter added. These alterations and 
 improvements have been rendered necessary by the rapid progress made by pharmaceutical science 
 during the last few years, and by the additional experience obtained in the practical use of the 
 volume as a text-book and work of reference. To accommodate these improvements, the size of 
 the page has been materially enlarged, and the number of pages considerably increased, presenting 
 in all nearly one- half more matter than the last edition. The work is therefore now presented as a 
 complete exponent of the subject in its most advanced condition. From the most ordinary matters 
 in the dispensing office, to the most complicated details of the vegetable alkaloids, it is hoped that 
 everything requisite to the practising physician, and to the apothecary, will be found fully and 
 clearly set forth, and that the new matter alone will be worth more than the very moderate cost of 
 the work to those who have been consulting the previous edition. 
 
 That Edward Parrish, in writing a book upon 
 practical Pharmacy some few years ago one emi- 
 nently original and unique did the medical and 
 pharmaceutical professions a great and valuable ser- 
 vice, no one, we think, who has had access to its 
 pages will deny; doubly welcome, then, is this new 
 edition, containing the added results of his recent 
 and rich experience as an observer, teacher, and 
 practicil operator in the pharmaceutical laboratory. 
 The excellent plan of the first is more thoroughly, 
 and in detail, carried out in this edition. Peninsular 
 Med. Journal, Jan. 1860. 
 
 We know of no work on the subject which would 
 be more indispensable to the physician or student 
 desiring information on the subject of which it treats. 
 With Griffith's " Medical Formulary" and this, the 
 practising physician would be supplied with nearly 
 or quite all the most useful information on the sub- 
 ject. Charleston Med. Journal and Review, Jan. 
 1860. 
 
 This edition, now much enlarged, is one of the 
 most useful works of the past year. N. O. Med. 
 and Surg. Journal, Jan. 1860. 
 
 The whole treatise is eminently practical; and 
 
 there is no production of the kind in the English 
 language so well adapted to the wants of the phar- 
 maceutist and druggist. To physicians, also, it can 
 
 phys 
 
 .ble, 
 
 not fail to be highly valuable, especially to those 
 who are obliged to prepare and compound many of 
 their own medicines. N, Am. Med. Chir. Review , 
 Jan. 1860. 
 
 Of course, all apothecaries who have not already 
 a copy of the first edition will procure one of this; 
 it is, therefore, to physicians residing in the country 
 and in small towns, who cannot avail themselves of 
 the skill of an educated pharmaceutist, that we 
 would especially commend this work. In it they 
 will find all that they desire to know, and should 
 know, but very little of which they do really Know 
 in reference to this important collateral branch of 
 their profession ; for it is a well established fact, 
 that, in the education of physicians, while the sci- 
 ence of medicine is generally well taught, very 
 little attention is paid to the art of preparing them 
 for use, and we know not how this defect can be so 
 well remedied as by procuring and consulting Dr. 
 Patrish's excellent work. St. Louis Med. Journal. 
 Jan. 1860. 
 
 PEASLEE (E. R.), M. D., 
 
 Professor of Physiology and General Pathology in the New York Medical College. 
 
 HUMAN HISTOLOGY, in its relations to Anatomy, Physiology, and Pathology; 
 for the use of Medical Students. With four hundred and thirty- four illustrations. In one hand- 
 some octavo volume, of over 600 pages. (Lately Published.} $3 75. 
 It embraces a library upon the topics discussed 
 
 within itself, and is just what the teacher and learner 
 
 need. Another advantage, by no means to be over- 
 
 UCCU. AUUM1V1 aUVClUMUCCj uy I1U JIlC'MiB (*U UC UVC1- 
 
 looked, everything of real value in the wide range 
 which it embraces, is with great skill compressed 
 into an octavo volume of but little more than six 
 hundred pages. We have not only the whole sub- 
 ject of Histology, interesting in itself, ably and fully 
 discussed, but what is of infinitely greater interest 
 to the student, because of greater practical value, 
 are its relations to Anatomy, Physiology, and Pa- 
 thology, which are here fully and satisfactorily set 
 forth. Nashville Journ. of Med. and Surgery, Dec. 
 1857. 
 
 We would recommend it to the medical student 
 and practitioner ? as containing a summary of all that 
 is known of the important subjects which it treats ; 
 of all that is contained in the great works of Simon 
 and Lehmann, and the organic chemists in general. 
 Master this one volume, we would say to the medical 
 student and practitioner master this book and you 
 know all that is known of the great fundamental 
 principles of medicine, and we have EO hesitation 
 in saying that it is aa honor to the American medi- 
 cal profession that one of its members should have 
 produced it. St. Louis Mtd. and Surg. Journal, 
 March, 1858. 
 
 PEREIRA (JONATHAN), M. D., F. R. S. f AND L. S. 
 THE ELEMENTS OF MATERIA MEDICA AND THERAPEUTICS. 
 
 Third American edition, enlarged and improved by the author ; including Notices of most of the 
 Medicinal Substances in use in the civilized world, and forming an Encyclopaedia of Materia 
 Medica. Edited, with Additions, by JOSEPH CARSON, M. D., Professor of Materia Medica and 
 Pharmacy in the University of Pennsylvania. In two very large octavo volumes of 2100 pages, 
 on small type, with about 500 illustrations on stone and wood, strongly bound in leather, with 
 raised bands. $9 00. 
 x** Vol. II. will no longer be sold separate. 
 
 PARKER (LANGSTON), 
 
 Surgeon to the Queen's Hospital, Birmingham. 
 
 THE MODERN TREATMENT OF SYPHILITIC DISEASES, BOTH PRI- 
 MARY AND SECONDARY; comprising the Treatment of Constitutional and Confirmed Syphi- 
 
 I. With numerous Cases, Formulae, and Clinical Observa- 
 
 lis, by a safe and successful method. 
 
 tions. From the Third and entirely rewritten London edition. 
 
 extra cloth, of 316 pages. $1 75. 
 
 In one neat octavo volume, 
 
BLANCHARD & LEA'S MEDICAL 
 
 RAMSBOTHAM (FRANCIS H.), M.D. 
 THE PRINCIPLES AND PRACTICE OF OBSTETRIC MEDICINE AND 
 
 SURGERY, in reference to the Process of Parturition. A new and enlarged edition, thoroughly 
 revised by the Author. With Additions by W. V. KEATING, M. D. In one large and handsome 
 imperial octavo volume, of 650 pages, strongly bound in leather, with raised bands; with sixty- 
 four beautiful Plates, and numerous Wood-cuts in the text, containing in all nearly two hundred 
 large and beautiful figures. $5 00. 
 
 From Prof. Hodge, of the University of Pa. 
 To the American public, it is most valuable, from its intrinsic undoubted excellence, and as being 
 
 the best authorized exponent of British Midwifery. Its circulation will, I trust, be extensive throughout 
 
 our country. 
 
 to the | truly elegant style in which they have brqught it 
 
 It is unnecessary to say anything in regard 
 utility of this work. It is already appreciated 
 country for the value of the matter, the clearness of 
 its style, and the fulness of its illustrations. To the 
 physician's library it is indispensable, while to the 
 student as a text-book, from which to extract the 
 material for laying the foundation of an education on 
 obstetrical science, it has no superior. Ohio Med. 
 and Surg. Journal. 
 
 The publishers have secured its success by the 
 
 out, excelling themselves in its production, espe- 
 cially in its plates. It is dedicated to Prof. Meigs, 
 
 and has the emphatic endorsement of Prof. Hod 
 as the best exponent of British Midwifery. 
 knc.w of no text-book which deserves in all respects 
 to be more highly recommended to students, and we 
 
 ge, 
 e 
 
 could wish to see it in the hands of ever 
 
 ry 
 
 for they will find it invaluable for reference 
 Gazette. 
 
 practitioner, 
 Med. 
 
 RICORD (P.), M. D. 
 A TREATISE ON THE VENEREAL DISEASE. By JOHN HUNTER, F. R. S. 
 
 With copious Additions, by PH. RICORD, M.D. Translated and Edited, with Notes, by FREEMAN 
 J. BUMSTEAD, M. D., Lecturer on Venereal at the College of Physicians and Surgeons, New York. 
 Second edition, revised, containing a resume of RICORD'S RECENT LECTURES ON CHANCRE. In 
 one handsome octavo volume, extra cloth, of 550 pages, with eight plates. $3 25. (Just Issued.} 
 In revising this work, the editor has endeavored to introduce whatever matter of interest the re- 
 cent investigations of syphilographers have added to our knowledge of the subject. The principal 
 source from which this has been derived is the volume of "Lectures on Chancre," published a few 
 months since by M. Ricord, which affords a large amount of new and instructive material on many 
 controverted points. In the previous edition, M. Ricord's additions amounted to nearly one-third 
 of the whole, and with the matter now introduced, the work may be considered to present his views 
 and experience more thoroughly and completely than any other. 
 
 Every one will recognize the attractiveness and 
 value which this work derives from thus presenting 
 the opinions of these two masters side by side. But, 
 it must be admitted, what has made the fortune of 
 the book, is the fact that it contains the " most com- 
 plete embodiment of the veritable doctrines of the 
 Hopital du Midi," which has ever been made public. 
 The doctrinal ideas of M. Ricord, ideas which, if not 
 universally adopted, are incontestably dominant, have 
 heretofore only been interpreted by more or less skilfu 1 
 
 secretaries, sometimes accredited and sometimes not. 
 In the notes to Hunter, the master substitutes him- 
 self forhis interpreters, and gives hisoriginal thoughts 
 to the world in a lucid and perfectly intelligible man- 
 ner. In conclusion we can say that this is incon- 
 testably the best treatise on syphilis with which we 
 are acquainted, and, as we do not often employ the 
 phrase, we may be excused for expressing the hope 
 that it may find a ' 
 sician 
 
 may find a place in the library of every phy- 
 . Virginia Med. and Surg. Journal. 
 
 BY THE SAME AUTHOR. 
 
 RICORD'S LETTERS ON SYPHILIS. Translated by W. P. LATTIMORE, M. D. 
 
 In one neat octavo volume, of 270 pages, extra cloth. $2 00. 
 
 ROYLE'S MATERIA MEDIC A AND THERAPEUTICS; including the 
 
 Preparations of the Pharmacopoeias of London, Edinburgh, Dublin, and of the United States. 
 With many new medicines. Edited by JOSEPH CARSON, M. D. With ninety-eight illustrations. 
 In one large octavo volume, extra cloth, of about 700 pages. $3 00. 
 
 ROKITANSKY (CARL), M..D., 
 
 Curator of the Imperial Pathological Museum, and Professor at the University of Vienna, &c. 
 
 A MANUAL OF PATHOLOGICAL ANATOMY. Four volumes, octavo, 
 
 bound in two, extra cloth, of about 1200 pages. Translated by W. E. SWAINE, EDWARD SIEVE- 
 KING, C. H. MOORE, and G. E. DAY. $5 50 
 
 The profession is too well acquainted with the re- 
 putation of Rokitansky's work to need our assur- 
 ance that this is one of the most profound, thorough, 
 and valuable books ever issued from the medical 
 press. It is sui generis, and has no standard of com- 
 parison. It is only necessary to announce that it is 
 issued in a form as cheap as is compatible with its 
 size and preservation, and its sale follows as a 
 matter of course. No library can be called com- 
 plete without it. Buffalo Med. Journal. 
 
 An attempt to give our readers any adequate idea 
 of the vast amount of instruction accumulated in 
 these volumes, would be feeble and hopeless. The 
 effort of the distinguished author to concentrate 
 in a small space his great fund of knowledge, has 
 
 so charged his text with valuable truths, that any 
 attempt of a reviewer to epitomize is at once para- 
 lyzed, and must end in a failure. Western Lancet. 
 
 As this is the highest source of knowledge upon 
 the important subject of which it treats, no real 
 student can afford to be without it. The American 
 publishers have entitled themselves to the thanks of 
 the profession of their country, for this timeous and 
 beautiful edition. Nashville Journal of Medicine. 
 
 As a book of reference, therefore, this work must 
 prove of inestimable value, and we cannot too highly 
 recommend it to the profession. Charleston Med. 
 Journal and Review. 
 
 This book is a necessity to every practitioner. 
 Am. Med. Monthly. 
 
 RIGBY (EDWARD), M. D., 
 
 Senior Physician to the General Lying-in Hospital, Sec. 
 
 A SYSTEM OF MIDWIFERY. With Notes and Additional Illustrations. 
 
 Second American Edition. One volume octavo, extra cloth, 422 pages. $2 50. 
 BY THE SAME AUTHOR. (Lately Published.} 
 
 ON THE CONSTITUTIONAL TREATMENT OF FEMALE DISEASES. 
 
 In one neat royal 12mo. volume, extra cloth, of about 250 pages. $1 00. 
 
AND SCIENTIFIC PUBLICATIONS. 
 
 27 
 
 STILLE (ALFRED), M. D. 
 THERAPEUTICS AND MATERIA MEDIC A; a Systematic Treatise on the 
 
 Action, and Uses of Medicinal Agents, including their Description and History. In two large and 
 handsome octavo volumes, of 1789 pages. (Now Ready ; 1860.) $8 00. 
 
 This work is designed especially for the student and practitioner of medicine, and treats the various 
 articles of the Materia Medica from the point of view of the bedside, and not of the shop or of the 
 lecture-room. While thus endeavoring to give all practical information likely to be useful with 
 respect to the employment of special remedies in special affections, and the results to be anticipated 
 from their administration, a copious Index of Diseases and their Remedies renders the work emi- 
 nently fitted for reference by showing at a glance ihe different means which have been employed, 
 and e'nabling the practitioner to extend his resources in difficult ca-es with all that the experience 
 of the profession has suggested. At the same time particular care has been given to the subject 
 of General Therapeutics, and at the commencement of each class of medicines there is a chapter 
 devoted to the consideration of their common influence upon morbid conditions. The action of 
 remedial agents upon the healthy economy and on animals has likewise received particular notice, 
 from the conviction that their physiological effects will afford frequent explanations of their patho- 
 logical influence, and in many cases lead to new and important suggestions as to their practical use 
 in disease. Within the scope thus designed by the auihor, no labor has been spared to accumulate 
 all the facts which have accrued from the experience of the profession in all ages and all countries ; 
 and the vast amount of recent researches recorded in the periodical literature of both hemispheres 
 has been zealously laid under contribution, resulting in a mass of practical information scarcely 
 attempted hitherto in any similar work in ihe language. 
 
 Our expectations of the value of this work were 
 based on the well-known reputation and character 
 of the author as a man of scholarly attainments, an 
 elegant writer, a candid inquirer after truth, and a 
 philosophical thinker ; we knew that the task would 
 be conscientiously performed, and that few, if any, 
 among the distinguished medical teachers in this 
 country are better qualified than fce to prepare a 
 systematic treatise on therapeutics in accordance 
 wilh the present requirements of medical science. 
 Our preliminary examination of the work has satis- 
 fied us that we were not mistaken in our anticipi- 
 tions. In congratulating the author on the comple- 
 tion of the great labor which such a work involves, 
 we are happy in expressing the conviction that its 
 merits will receive that reward which is above all 
 price the grateful appreciation of his medical bre- 
 thren. New Orleans Medical News, March, I860. 
 
 We think this work will do much to obviate the 
 reluctance to a thorough investigation of this branch 
 of scientific study, for in the wide range of medical 
 literature treasured in the English tongue, we shall 
 hardly find a work written in a style more clear and 
 simple, conveying forcibly the facts taught, and yet 
 free from turgidity and redundancy. There is a fas- 
 cination in its pages that will insure to it a wide 
 popularity and attentive perusal, and a degree of 
 usefulness not often attained through the influence 
 of a single work. The author has much enhanced 
 
 the practical utility of his book 
 
 hriefly 
 
 over the physical, botani ?al , arid commercial history 
 of medicines, and directing attention chiefly to their 
 physiological action, and their application for the 
 amelioration or cure of disease. He ignores hypothe- 
 sis and theory which are so alluring to many medical 
 writers, and so liable to lead them astray, and con- 
 fines hiimelf to such facts as have been tried in the 
 crucible of experience. Chicago Medical Journal, 
 March, 1860. 
 
 The plan pursued by the author in these very ela- 
 borate volumes is not strictly one of scientific unity 
 and precision; he has rather subordinated these to 
 practical utility. Dr. Stille has produced a work 
 which will he valuable equally to the student of 
 medicine and the busy practitioner. London Lan- 
 cet, March 10, 1860. 
 
 Wilh Pereira, Dunglison, Mitchell, and Wood be- 
 fore us, we may well ask if there was a necessity 
 for a new book on the subject. After examining this 
 work with some care, we can answer affirmatively. 
 Dr. Wood's book is well adapted for students, while 
 Dr. Stille's will be more satisfactory to the practi- 
 tioner, who desires to study the action of medicines . 
 The author needs no encomiums from us, for he is 
 well known as a ripe scholar and a man of the most 
 extensive reading in his profession. This work bears 
 
 evidence of this fact on every page. 
 Lancet, April, Is60. 
 
 -Cincinnati 
 
 SMITH (HENRY H.), M. D. 
 
 MINOR SURGERY; or, Hints on the Every-day Duties of the Surgeon. With 
 247 illustrations. Third edition. 1 vol. royal 12mo., pp. 456. In leather, $2 25 ; cloth, $2 00. 
 
 BY THE SAME AUTHOR, AND 
 
 HORNER (WILLIAM E.), M.D., 
 Late Professor of Anatomy in the University of Pennsylvania. 
 
 AN ANATOMICAL ATLAS, illustrative of the Structure of the Human Body. 
 In one volume, large imperial octavo, extra cloth, with about six hundred and fifty beautiful 
 figures. $3 00. 
 
 These figures are well selected, and present a 
 complete and accurate representation of that won- 
 derful fabric, the human body. The plan of this 
 
 convenient 
 execution, 
 have been already pointed out. We must congratu- 
 
 Atlas, which renders it so peculiarly 
 for the student, and its superb artistical 
 
 late the student upon the completion of this Atlas, 
 as it is the most convenient work of the kind that 
 has yet appeared ; and we must add, the very beau- 
 tiful manner in which it is "got up" is so creditable 
 to the country as to be flattering to our national 
 pride. American Medical Journal. 
 
 SHARPEY (WILLIAM), M. D., JONES QUAIN, M. D., AND 
 
 RICHARD QUAIN, F. R. S., &c. 
 HUMAN ANATOMY. Revised, with Notes and Additions, by JOSEPH LEIDT, 
 
 M. D., Professor of Anatomy in the University of Pennsylvania. Complete in two large octavo 
 volumes, leather, of about thirteen hundred pages. Beautifully illustrated with over five hundred 
 engravings on wood. $6 00. 
 
 SIMPSON (J. Y. , M. D. f 
 
 Professor of Midwifery, &c., in the University of Edinburgh, &c. 
 
 CLINICAL LECTURES ON THE DISEASES OF FEMALES. With numc- 
 
 rotis illustrations. 
 
 This valuable series of practical Lectures is now appearing in the "MEDICAL, NEWS AND 
 LIBRARY" for 1860, and can thus be had without cost by subscribers to the "AMERICAN JOURNAL 
 OF THE MEDICAL SCIENCES." See p. 2. 
 
28 
 
 BLANCHARD & LEA'S MEDICAL 
 
 SARGENT (F. W.), M. D. 
 
 ON BANDAGING AND OTHER OPERATIONS OF MINOR SURGERY. 
 
 Second edition, enlarged. One handsome royal 12mo. vol., of nearly 400 pages, with 182 wood- 
 cuts. Extra cloth, $1 40 ; leather, $1 50. 
 
 Sargent's Minor Surgery has always been popular, 
 and deservedly so. It furnishes that knowledge of the 
 most frequently requisite performances of surgical 
 art which cannot be entirely understood by attend- 
 ing clinical lectures. The art of bandaging, which 
 is regularly taught in Europe, is very frequently 
 overlooked by teachers in this country ; the student 
 and junior practitioner, therefore, may often require 
 that knowledge which this little volume so tersely 
 and happily supplies. Charleston Med. Journ. and 
 Review, M 
 
 arch, 1856. 
 
 A work that has been so long and favorably known 
 to the profession as Dr. Sargent's Minor Surgery, 
 needs no commendation from us. We would remark, 
 however, in this connection, that minor surgery sel- 
 dom gets that attention in our schools that its im- 
 portance deserves. Our larger works are also very 
 defective in their teaching on these small practical 
 points. This little book will supply the void which 
 all must feel who have not studied its pages. West- 
 ern Lancet, March, 1856. 
 
 SMITH (W. TYLER), M. D., 
 
 Physician Accoucheur to St. Mary's Hospital, &c. 
 
 ON PARTURITION, AND THE PRINCIPLES AND PRACTICE OF 
 
 OBSTETRICS. In one royal 12mo. volume, extra cloth, of 400 pages. $1 25. 
 
 BY THE SAME AUTHOR. 
 
 A PRACTICAL TREATISE ON THE PATHOLOGY AND TREATMENT 
 
 OF LEUCORRHCEA. With numerous illustrations. In one very handsome octavo volume, 
 extra cloth, of about 250 pages. $1 50. 
 
 SOLLY ON THE HUMAN BRAIN; its Structure, 
 Physiology, and Diseases. From the Second and 
 much enlarged London edition. In one octavo 
 volume, extra cloth, of 500 pages, with 120 wood- 
 cuts. $2 00. 
 
 SKEY'S OPERATIVE SURGERY. In one very 
 
 handsome octavo volume, extra cloth, of over 650 
 pages, with about one hundred wood-cuts. $3 '25. 
 SIMON'S GENERAL PATHOLOGY, as conduc- 
 ive to the Establishment of Rational Principles 
 for the prevention ano Cure of Disease. In one 
 octavo volume, extra cloth, of 212 pages. $1 25. 
 
 TODD (R. B.), M.D., F. R. S., &c. 
 CLINICAL LECTURES ON CERTAIN DISEASES OF THE URINARY 
 
 ORGANS AND ON DROPSIES. In one octavo volume, 284 pages. . $1 50. 
 BY THE SAME AUTHOR. (Now Ready.) 
 
 CLINICAL LECTURES ON CERTAIN ACUTE DISEASES. In one neat 
 
 octavo volume, of 320 pages, extra cloth. $1 75. 
 
 The subjects treated in this volume are RHEUMATIC FEVER, CONTINUED FEVER, ERYSIPELAS, 
 ACUTE INTERNAL INFLAMMATION, PYJEMIA, PNEUMONIA, and the THERAPEUTICAL ACTION OF ALCO- 
 HOL. The importance of these matters in the daily practice of every physician, and the sound 
 practical nature of Dr. Todd's writings, can hardly fail to attract to this work the general attention 
 that it merits. 
 
 TANNER (T. H.), M. D., 
 
 Physician to the Hospital for Women, &c. 
 
 A MANUAL OF CLINICAL MEDICINE AND PHYSICAL DIAGNOSIS. 
 
 To which is added The Code of Ethics of the American Medical Association. Second 
 American Edition. In one neat volume, small 12mo., extra cloth, 87$ cents. 
 
 TAYLOR (ALFRED S.), M. D., F. R. S., 
 
 Lecturer on Medical Jurisprudence and Chemistry in Guy's Hospital. 
 
 MEDICAL JURISPRUDENCE. Fourth American Edition. With Notes and 
 References to American Decisions, by EDWARD.HARTSHORNE, M. D. In one large octavo volume, 
 leather, of over seven hundred pages. $3 00. 
 
 No work upon the subject can be put into the 
 hands of students either of law or medicine which 
 will engage them more closely or profitably ; and 
 none could be oflered to the busy practitioner oi 
 either calling, for the purpose of casual or hasty 
 reference, that would be more likely to afford the aid 
 desired. We therefore recommend it as the best and 
 safest manual for daily use. American Journal oj 
 Medical Sciences. 
 
 BY THE SAME AUTHOR 
 
 It is not excess of praise to say that the volume 
 before us is the very best treatise extant on Medical 
 Jurisprudence. In saying this, we do not wish to 
 be understood as detracting from the merits of the 
 excellent works of Beck, Ryan, Traill, Guy, and 
 others; but in interest and value we think it must 
 be conceded that Taylor is superior to anything that 
 has preceded it. N. W. Medical and Surg. Journal. 
 
 (New Edition, just issued.) 
 
 ON POISONS, IN RELATION TO MEDICAL JURISPRUDENCE AND 
 
 MEDICINE. Second American, from a second and revised London edition. In one large 
 
 octavo volume, oi 755 pages, leather. $3 50. 
 
 Since the first appearance of this work, the rapid advance of Chemistry has introduced into 
 use many new substances which may become fatal through accident or design while at the 
 same time it has likewise designated new and more exact modes of counteracting or detecting those 
 previously treated of. Mr. Taylor's position as the leading medical jurist of England, has during 
 this period conferred on him extraordinary advantages in acquiring experience on these subjects, 
 nearly all cases of moment being referred to him for examination, as an expert whose testimony 
 is generally accepted as final. The results of his labors, therefore, as gathered together in this 
 volume, carefully weighed and sifted, and presented in the clear and intelligible style for which 
 he is noted, may be received as an acknowledged authority, and as a guide to be followed with 
 implicit confidence. 
 
AND SCIENTIFIC PUBLICATIONS. 29 
 
 TODD (ROBERT BENTLEY), M. D., F. R. S., 
 
 Professor of Physiology in King's College, London; and 
 
 WILLIAM BOWMAN, F. R. S., 
 
 Demonstrator of Anatomy in King's College, London. 
 
 THE PHYSIOLOGICAL ANATOMY AND PHYSIOLOGY OF MAN. With 
 
 about three hundred large and beautiful illustrations on wood. Complete in one large octavo 
 
 volume, of 950 pages, leather. Price $4 50. 
 
 3F Gentlemen who have received portions of this work, as published in the " MEDICAL NEWS 
 AND LIBRARY," can now complete their copies, if immediate application be made. It will be fur- 
 nished as follows, free by mail, in paper covers, with cloth backs. 
 
 PARTS I., II., III. (pp. 25 to 552), $2 50. 
 
 PART IV. (pp. 553 to end, with Title, Preface, Contents, &c.), $2 00. 
 
 Or, PART IV., SECTION II. (pp. 725 to end, with Title, Preface, Contents, &c.), $1 25. 
 
 A magnificent contribution to British medicine, 
 and the American physician who shall fail to peruse 
 it, will have failed to read one of the most instruc- 
 tive books of the nineteenth century. N. O. Med. 
 and Surg. Journal, Sept. 1857. 
 
 It is more concise than Carpenter's Principles, and 
 more modern than the accessible edition of Muller's 
 Elements; its details are brief, but sufficient; its 
 descriptions vivid ; its illustrations exact and copi- 
 ous ; and its language terse and perspicuous. 
 Charleston Med. Journal, July, 1857. 
 
 We know of no work on the subject of physiology 
 
 so well adapted to the wants of the medical student. 
 Its completion has been thus long delayed, that the 
 authors might secure accuracy by personal observa- 
 tion. St. Louis Med. and Surg. Journal, Sept. '57. 
 
 Our notice, though it conveys but a very feeble 
 and imperfect idea of the magnitude and importance 
 of the work now under consideration, already tran- 
 scends our limits ; and, with the indulgtnce of our 
 readers, and the hope that they will peruse the book 
 for themselves, as we feel we can with confidence 
 recommend it, we leave it in their hands. The 
 Northwestern Med. and Surg. Journal. 
 
 TOYNBEE (JOSEPH), F. R. S., 
 
 Aural Surgeon to, and Lecturer on Surgery at, St. Mary's Hospital. 
 
 A PRACTICAL TREATISE ON DISEASES OF THE EAR; their Diag- 
 nosis, Pathology, and Treatment. Illustrated with one hundred engravings on wood. In one 
 very handsome octavo volume, extra cloth, $3 00. (Now Ready.) 
 
 Mr. Toynbee's name is too widely known as the highest authority on all matters connected with 
 Aural Surgery and Medicine, to require special attention to be called to anything which he may 
 communicate to the profession on the subject. Twenty years' labor devoted to the present work 
 has embodied in it the results of an amount of experience and observation which perhaps no other 
 living practitioner has enjoyed. It therefore cannot fail to prove a complete and trustworthy guide 
 on all matters connected with this obscure and little known class of diseases, which so frequently 
 embarrass the general practitioner. 
 
 The volume will be found thoroughly illustrated with a large number of original wood-engrav- 
 ings, elucidating the pathology of the organs of hearing, instruments, operations, &c., and in every 
 respect it is one of the handsomest specimens of mechanical execution issued from the American 
 press. 
 
 The following condensed synopsis of the contents will show the plan adopted by the author, and 
 the completeness with which all departments of the subject are brought under consideration. 
 CHAPTER I. Introduction Mode cf Investigation Dissection. II. The External Ear Ana- 
 tomy Pathology Malformations Diseases. III. The External Meatus Its Exploration. 
 IV. The External Meatus Foreign Bodies and Accumulations of Cerumen. V. The External 
 Meatus The Dermis and its Diseases. VI. The External Meatus Polypi. VII. The External 
 Meatus Tumors. VIII. The Membrana Tympani Structure and Functions. IX. The Mem- 
 brana Tympani Diseases. X. The Membrana Tympani Diseases. XI. The Eustachian 
 Tube Obstructions. XII. The Cavity of the Tympanum Anatomy Pathology Diseases. 
 XIII. The Cavity of the Tympanum Diseases. XIV. The Mastoid Cells Diseases. XV. 
 The Diseases of the Nervous Apparatus of the Ear, producing what is commonly called " Nerv- 
 ous Deafness." XVI. The Diseases of the Nervous Apparatus, continued. XVII. Malignant 
 Disease of the Ear. XVIII. On the Deaf and Dumb. XIX. Ear-Trumpets and their uses. 
 APPENDIX. 
 
 WILLIAMS (C. J. B.), M.D., F. R. S., 
 
 Professor of Clinical Medicine in University College, London, &c. 
 
 PRINCIPLES OF MEDICINE. An Eleraentaiy View of the Causes, Nature, 
 
 Treatment, Diagnosis, and Prognosis of Disease ; with brief remarks on Hygienics, or the pre- 
 servation of health. A new American, from the third and revised London edition, in one octavo 
 volume, leather, of about 500 pages. $2 50. (Just Issued.) 
 
 We find that the deeply-interesting matter and expressed. It is a judgment of almost unqualified 
 style of this book have so far fascinated us, that we praise. London Lancet. 
 
 have unconsciously hung upon its pages 
 long, indeed, for our own profit, but longer than re- 
 viewers can be permitted to indulge. We leave the 
 further analysis to the student and practitioner. Our 
 judgment of the work has already been sufficiently 
 
 A text-book to which no other in our language is 
 comparable. Charleston Medical Journal. 
 
 No work has ever achieved or maintained a more 
 deserved reputation. Va. Med. and Surg. Journal. 
 
 WHAT TO OBSERVE 
 AT THE BEDSIDE AND AFTER DEATH, IN MEDICAL CASES. 
 
 Published under the authority of the London Society for Medical Observation. A new American, 
 
 from the second and revised Londou edition. In one very handsome volume, royal 12mo., extra 
 
 cloth. $1 00. 
 
 To the observer who prefers accuracy to blunders I One of the finest aids to a young practitioner we 
 and precision to carelessness, this little book is in- I have ever seen. Peninsular Journal of Medici**. 
 valuable. N. H, Journal of Medicint. [ 
 
30 BLANCHARD & LEA'S MEDICAL 
 
 New and much enlarged edition (Just Issued.) 
 
 WATSON (THOMAS), M.D., &c., 
 
 Late Physician to the Middlesex Hospital, &c. 
 
 LECTURES ON THE PRINCIPLES AND PRACTICE OP PHYSIC. 
 
 Delivered at King's College, London. A new American, from the last revised and enlarged 
 
 English edition, with Additions, by D. FRANCIS CONDIE, M. D., author of" A Practical Treatise 
 
 on the Diseases of Children," &c. With one hundred and eighty.five illustrations on wood. In 
 
 one very large and handsome volume, imperial octavo, of over ]200 closely printed pages in 
 
 small type ; the whole strongly bound in leather, with raised bands. Price $4 2o. 
 
 That the high reputation of this work might be fully maintained, the author has subjected it to a 
 
 thorough revision ; every portion has been examined with the aid of the most recent researches 
 
 in pathology, and the results of modern investigations in both theoretical and practical subjects 
 
 have been carefully weighed and embodied throughout its pages. The watchful scrutiny of the 
 
 editor has likewise introduced whatever possesses immediate importance to the American physician 
 
 in relation to diseases incident to our climate which are little known in England, as well as those 
 
 points in which experience here has led to different modes of practice ; and he has also added largely 
 
 to the series of illustrations, believing that in this manner valuable assistance may be conveyed to 
 
 the student in elucidating the text. The work will, therefore, be found thoroughly on a level with 
 
 the most advanced state of medical science on both sides of the Atlantic. 
 
 The additions which the work has received are shown by the fact that notwithstanding an en- 
 largement in the size of the page, more than two hundred additional pages have been necessary 
 to accommodate the two large volumes of the London edition (which sells at ten dollars), within 
 the compass of a single volume, and in its present form it contains the matter of at least three 
 ordinary octavos. Believing it to be a work which should lie on the table of every physician, and 
 be in the hands of every student, the publishers have put it at a price within the reach of all, making 
 it one of the cheapest books as yet presented to the American profession, while at the same time 
 the beauty of its mechanical execution renders it an exceedingly attractive volume. 
 
 The fourth edition now appears, so carefully re- j The lecturer's skill, his wisdom, his learning, are 
 vised, as to add considerably to the value of a book ; equalled by the ease of his graceful diction, his elo- 
 already acknowledged, wherever the English Ian- | quence, and the far higher qualities of candor, of 
 guage is read, to be beyond all comparison the best i courtesy, of modesty, and of generous appreciation 
 systematic work on the Principles and Practice of; of merit in others. May he long remain to instruct 
 Physic in the whole range of medical literature. ! us, and to enjoy, in the glorious sunset of his de- 
 Every lecture contains proof of the extreme anxiety ! clining years, the honors, the confidence and love 
 of the author to keep pace with 'he advancing know- j gained during his useful life. N. A. Med-Chir. 
 ledge of the day, and to bring the results of the Review, July, 1858. 
 labors, not only of physicians, b 
 
 )ut of chemists and 
 
 histologists, before his readers, wherever they can 
 be turned to useful account. And this is done with 
 such a cordial appreciation of the merit due to the 
 industrious observer, such a generous desire to en- 
 courage younger and rising men, and such a candid 
 acknowledgment of his own obligations to them, 
 that one scarcely knows whether to admire most the 
 pure, simple, forcible English the vast amount of 
 useful practical information condensed' into the 
 Lectures or the manly, kind-hearted, unassuming 
 character of the lecturer shining through his work. 
 London Med. Times and Gazette, Oct. 31, 1857. 
 
 Thus these admirable volumes come before the 
 profession in their fourth edition, abounding in those 
 
 Watson's unrivalled, perhaps unapproachable 
 work on Practice the copious additions made to 
 which (the fourth edition) have given it all the no- 
 velty and much of the interest of a new book. 
 Charleston Med. Journal, July, 1858. 
 
 Lecturers, practitioners, and students of medicine 
 will equally hail the reappearance of the work of 
 Dr. Watson in the form of anew a fourth edition. 
 We merely do justice to our own feelings, and, we 
 are sure, of the whole profession, if we thank him 
 for having, in the trouble and turmoil of a large 
 practice, made leisure to supply the hiatus caused 
 by the exhaustion of the publisher's stock of the 
 third edition, which has been severely felt for the 
 
 distinguished attributes of moderation, judgment, I last three years. For Dr. Watson has not merely 
 ... .__,,.= i; __ -i ------- .,_j -i -------- ..-.-^u I 
 
 erudite cultivation, clearness, and eloquence, with 
 which they were from the first invested, but yet 
 richer than before in the results of more prolonged 
 observation, and in the able appreciation of the 
 latest advances in pathology and medicine by one 
 of the most profound medical thinkers of the day. 
 London Lancet, Nov. 14, 1857. 
 
 cause( i tne lectures to be reprinted, but scattered 
 through the whole work we find additions or altera- 
 tions which prove that the author has in every way 
 sought to bring up his teaching to the level of che 
 most recent acquisitions in science. Brit, and For, 
 Medico-Chir. Review, Jan. 1858. 
 
 WALSHE (W. H.), M. D., 
 
 Professor of the Principles and Practice of Medicine in University College, London, &c. 
 
 A PRACTICAL TREATISE ON DISEASES OF THE LUNGS; including 
 
 the Principles of Physical Diagnosis. A new American, from the third revised and much en- 
 larged London edition. In one vol. octavo, of 468 pages. (Just Issued, June, 1860.) $225. 
 The present edition has been carefully revised and much enlarged, and may be said in the main 
 to be rewritten. Descriptions of several diseases, previously omitted, are now introduced; the 
 causes and mode of production of the more important affections, so far as they possess direct prac- 
 tical significance, are succinctly inquired into; an effort has been made to bring the description of 
 anatomical characters to the level of the wants of the practical physician ; and the diagnosis and 
 prognosis of each complaint are more completely considered. The sections on TREATMENT and 
 the Appendix (concerning the influence of climate on pulmonary disorders), have, especially, been 
 largely extended. Author's Preface. 
 %*% To be followed by a similar volume on Diseases of the Heart and Aorta. 
 
 WILSON (ERASMUS), F. R. S., 
 
 Lecturer on Anatomy, London. 
 
 THE DISSECTOR'S MANUAL; or, Practical and Surgical Anatomy. Third 
 
 American, from the last revised and enlarged English edition. Modified and rearranged, by 
 WILLIAM HUNT, M. D., Demonstrator of Anatomy in the University of Pennsylvania. In one 
 large and handsome royal 12mo. volume, leather, of 582 pages, with 154 illustrations. $2 00. 
 
AND SCIENTIFIC PUBLICATIONS 
 
 31 
 
 New and much enlarged edition (Just Issued.) 
 
 WILSON (ERASMUS), F. R. S. 
 
 A SYSTEM OF HUMAN ANATOMY, General and Special. A new and re- 
 vised American. from the last and enlarged English Edition. Edited by W. H.GOBRECHT, M. IX, 
 Professor of Anatomy in the Pennsylvania Medical College, &c. Illustrated with three hundred 
 and ninety-seven engravings on wood. In one large and exquisitely printed octavo volume, of 
 over 600 large pages; leather. $3 25. 
 
 The publishers trust that the well earned reputation so long enjoyed by this work will be more 
 than maintained by the present edition. Besides a very thorough revision by the author, it has been 
 mo*t carefully examined by the editor, and the efforts of both have been directed to introducing 
 everything which increased experience in its use has suggested as desirable to render .it a complete 
 text-book for those seeking to obtain or to renew an acquaintance with Human Anatomy. The 
 amount of additions which it has thus received may be estimated from the fact that the present 
 edition contains over one-fourth more matter than the last, rendering a smaller type and an enlarged 
 page requisite to keep the volume within a convenient size. The author has not only thus added 
 largely to the work, but he has also made alterations throughout, wherever there appeared the 
 opportunity of improving the arrangement or style, so as to present every fact in its most appro- 
 priate manner, and to render the whole as clear and intelligible as possible. The editor has 
 exercised the utmost caution to obtain entire accuracy in the text, and has largely increased the 
 number of illustrations, of which there are about one hundred and fifty more in this edition than 
 in the last, thus bringing distinctly before the eye of the student everything of interest or importance. 
 
 It may be recommended to the student as no less 
 distinguished by its accuracy and clearness of de- 
 scription than by its typographical elegance. The 
 wood-cuts are exquisite. Brit, and For. Medical 
 Review. 
 
 An elegant edition of one of the most useful and 
 accurate systems of anatomical science which has 
 been issued from the press The illustrations are 
 really beautiful. In its style the work is extremely 
 concise and intelligible. No one can possibly take 
 up this volume without being struck with the great 
 
 beauty of its mechanical execution, and the clear- 
 ness of the descriptions which it contains is equally 
 evident. Let students, by all means examine tne 
 claims of this work on their notice, before they pur- 
 chase a text-book of the vitally important science 
 
 which this volume so fully and easily unfolds. 
 
 Lancet. 
 
 We regard it as the best system now extant for 
 students. Western Lancet. 
 
 It therefore receives our highest commendation. 
 Southern Med. and Surg. Journal. 
 
 BY THE SAME AUTHOR. (Just Issued.) 
 
 ON DISEASES OF THE SKIN. Fourth and enlarged American, from the last 
 and improved London edition. In one large octavo volume, of 650 pages, extra cloth, $2 75. 
 
 The writings of Wilson, upon diseases of the skin, 
 are by far tne most scientific and practical that 
 have ever been presented to the medical world on 
 this subject. The presenteditionisa great improve- 
 ment on all its predecessors. To dwell upon all the 
 great merits arid high claims of the work before us, 
 seriatim, would indeed be an agreeable service; it 
 would be a mental homage which we could freely 
 offer, but we should thus occupy an undue amount 
 of space in this Journal. We will, however, look 
 
 at some of the more salient points with which it 
 abounds, and which make it incomparaoiy superior in 
 excellence to all other treatises on the subject of der- 
 matology. No mere speculative views are allowed 
 a place in this volume, which, without a doubt, will, 
 for a very long period, be acknowledged as the chief 
 standard work on dermatology. The principles of 
 an enlightened and rational therapeia are introduced 
 on every appropriate occasion. Am. Jour. Med 
 Science, Oct. 1857. 
 
 of 
 
 ALSO, NOW READY, 
 
 A SERIES OF PLATES ILLUSTRATING WILSON ON DISEASES OF 
 
 THE SKIN ; consisting of nineteen beautifully executed plates, of which twelve are exquisitely 
 colored, presenting the Normal Anatomy and Pathology of the Skin, and containing accurate re- 
 presentations of about one hundred varieties of disease, most of them the size of nature. Price 
 in cloth $4 25. 
 
 In beauty of drawing and accuracy and finish of coloring these plates will be found equal to 
 anything of the kind as yet issued in this country. 
 
 The plates by which this edition is accompanied 
 leave nothing to be desired, so far as excellence of 
 delineation and perfect accuracy of illustration are 
 concerned. Medico-Chirurgical Revieto. 
 
 Of these plates it is impossible to speak too highly 
 The representations of the various forms of cutane- 
 ous disease are singularly accurate, and the color- 
 ing exceeds almost anything we have met with in 
 point of delicacy and finish. British and Foreign 
 Medical Review. 
 
 BY THE SAME AUTHOR. 
 
 ON CONSTITUTIONAL AND HEREDITARY SYPHILIS, AND OiN 
 
 SYPHILITIC ERUPTIONS. In one small octavo volume, extra cloth, beautifully printed, with 
 four exquisite colored plates, presenting more than thirty varieties of syphilitic eruptions. $2 25, 
 
 BY THE SAME AUTHOR. 
 
 HEALTHY SKIN; A Popular Treatise on the Skin and Hair, their Preserva- 
 tion and Management. Second American, from the fourth London edition. One neat volume, 
 royal 12mo. 5 extra cloth, of about 300 pages, with numerous illustrations. $1 00 ; paper cover, 
 75 cents. 
 
 We have already expressed our high appreciation 
 Mr. Wilson's treatise on Diseases of the Skin. 
 The plates are comprised in a separate volume, 
 which we counsel all those who possess the text to 
 purchase. It is a beautiful specimen of color print- 
 ing, and the representations of the various forms of 
 skin disease are as faithful as is possible in plates 
 of the size. Boston Med. and Surg. Journal. April 
 8, 1858. 
 
 VVHITEHEAD ON THE CAUSES AND TREAT- 
 MENT OF ABORTION AND STERILITY. 
 
 Second American Edition. In one volume, octa- 
 vo extra cloth, pp. 308. SI 75. 
 
32 
 
 BLANCHARD & LEA'S MEDICAL PUBLICATIONS. 
 
 ON OBSCURE DISEASES OF THE BRAIN AND DISORDERS OF THE 
 
 MIND; their incipient Symptoms, Pathology, Diagnosis, Treatment, and Prophylaxis. la one 
 handsome octavo volume, of nearly 600 pages. (Just Issued, June, 1660.) $3 00. 
 The momentous questions discussed in this volume have perhaps not hitherto been so ably and 
 elaborately treated. Dr. Window's distinguished reputation and long experience in everything re- 
 lating to insanity invest his teachings with the highest authority, and in this carefully considered 
 volume he has drawn upon the accumulated resources of a life of observation. His deductions 
 are founded on a vast number of cases, the peculiarities of which are related in detail, rendering 
 the work not only one of sound instruction, but of lively interest; the author's main object being 
 to point out the connection between organic disease and insanity, tracing the lalter through all its 
 stages from mere eccentricity to mania, and urging the necessity of early measures of prophylaxis 
 and appropriate treatment. A subject of greater importance to society at large could scarcely be 
 named; while to the physician who may at any moment be called upon for interference in the most 
 delicate relations of life, or for an opinion in a court of justice, a work like the present Kay be con- 
 sidered indispensable. 
 
 The treatment of the subject may be gathered from the following summary of the contents : 
 CHAPTER I. Introduction. II. Morbid Phenomena of Intelligence. III. Premonitory Symptoms 
 of Insanity. IV. Confessions of Patients after Recovery. V. State of the Mind during Re- 
 covery. VI. Anomalous and Masked Affections of the Mind. VII. The Stage of Consciousness. 
 Vlil. Stage of Exaltation. IX. Stage of Mental Depression. X. Stage of Aberration. XL 
 Impairment of Mind. XII. Morbid Phenomena of Attention. XIII. Morbid Phenomena of 
 Memory XIV. Acute Disorders of Memory. XV. Chronic Affections of Memory. XVI. 
 Perversion and Exaltation of Memory. XVII. Psychology and Pathology of Memory. XVIII. 
 Morbid Phenomena of Motion. XIX. Morbid Phenomena of Speech. XX. Morbid Phenomena 
 of Sensation. XXI. Morbid Phenomena of the Special Senses. XXII. Morbid Phenomena of 
 Vision, Hearing, Taste, Touch, and Smell. XXIII. Morbid Phenomena of Sleep and Dreaming. 
 XXIV. Morbid Phenomena of Organic and Nutritive Life. XXV. General Principles of Pa- 
 thology, Diagnosis, Treatment, and Prophylaxis. 
 
 WEST (CHARLES), M. D., 
 
 Accoucheur to and Lecturer on Midwifery at St. Bartholomew's Hospital, Physician to the Hospital for 
 
 Sick Children, &c. 
 
 LECTURES ON THE DISEASES OF WOMEN. Now complete in one hand- 
 some octavo volume, extra cloth, of about 500 pages ; price $2 50. 
 Also, for sale separate, PART II, being pp. 309 to end, with Index, Title matter, 
 
 &c., 8vo., cloth, price $1. 
 
 We must now conclude this hastily written sketch 
 with the confident assurance to our readers that the 
 work will well repay perusal. The conscientious, 
 painstaking, practical physician is apparent on every 
 page. N. Y. Journal of Medicine, March, 1858. 
 
 We know of no treatise of the kind so complete 
 and yet so compact. Chicago Med. Journal, Janu- 
 
 and children is not to be found in any country. 
 Southern Med. and Surg. Journal, January 1858. 
 
 We gladly recommend his Lectures as in the high- 
 est degree instructive to all who are interested in 
 obstetric practice. London Lancet. 
 
 We have to say of it, briefly and decidedly, that 
 it is the best work on the subject in any language; 
 and that it stamps Dr. West as the facile princeps 
 of British obstetric authors. Edinb. Med. Journ. 
 
 ary, 1858. 
 
 A fairer, more honest, more earnest, and more re- 
 liable investigator of the many diseases of women 
 
 BY THE SAME AUTHOR. (Now Ready.) 
 
 LECTURES ON THE DISEASES OF INFANCY AND CHILDHOOD. 
 
 Third American, from the fourth enlarged and improved London edition. In one handsome 
 octavo volume, extra cloth, of about six hundred and fifty pages. $2 75. 
 
 The continued favor with which this work has been received has stimulated the author to ren- 
 der it in every respect more complete and more worthy the confidence of the profession. Con- 
 taining nearly two hundred pages more than the last American edition, with several additional 
 Lectures and a careful revision and enlargement of those formerly comprised in it, it can hardly 
 fail to maintain its reputation as a clear and judicious text-book for the student, and a safe and 
 reliable guide for the practitioner. The fact stated by the author that these Lectures ' now embody 
 the results of 900 observations and 288 post-mortem examinations made among nearly 30,000 
 children, who, during the past twenty-years, have come under my care," is sufficient to show their 
 practical value as the result of an amount of experience which few physicians enjoy. 
 
 The three former editions of the work now before 
 us have placed the author in tne foremost rank of 
 those physicians who have devoted special attention 
 to the diseases of early life We attempt no ana- 
 1> sis of this edition, but may refer the reader to some 
 of the chapters to which the largest additions have 
 been made those on Diphtheria, Disorders of the 
 Mind, and Idiocy, for instance as a prooi that the 
 work is really a new edition ; not a mere roprint. 
 In its preient shape it will be lound of the greatest 
 possible service in the every-day practice of nine- 
 tenths of the profession. Med. Times and Gazette, 
 London, Dec. 10, 1859. 
 
 All things considtred, this book of Dr. West is 
 by far the best treatise in our language upon such 
 modifications of morbid action and disease as are 
 witnessed when we have to deal with infancy and 
 childhood. It is true that it confines itself to such 
 disorders as come within the province of the phy- 
 sician, and even with respect to these it is unequal 
 as regards minuteness of consideration, and some 
 
 diseases it omits to notice altogether. But those 
 who know anything of the present condition of 
 paediatrics will readily admit that it would be next 
 to impossible to effect more, or effect it better, than 
 the accoucheur of St. Bartholomew's has done i-n a 
 single volume. The lecture (XVI.) upon Disorders 
 of the Mind in children is an admirable specimen of 
 the value of the later information conveyed in the 
 Lectures of Dr. Charles West. London Lancet, 
 Oct. 22, 1859. 
 
 Since the appearance of the first edition, about 
 eleven years ago, the experience of the author has 
 doubled; so that, whereas the lectures at first were 
 founded on six hundred observations, and one hun- 
 dred and eighty dissections made among nearly four- 
 teen thousand children, they now embody the results 
 of nine hundred observations, and two hundred and 
 eighty-eight post- mortem examinations made among 
 nearly thirty thousand children, who, during the 
 past twenty years, have been under his care. 
 British Med. Journal, Oct. 1, 1859. 
 
 BY THE SAME AUTHOR. 
 
 AN ENQUIRY INTO THE PATHOLOGICAL IMPORTANCE OF ULCER- 
 
 ATION OF THE OS UTERI. In one neat octavo volume, extra cloth. $1 00. 
 
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